Water

Ariane O. Pinson; Elizabeth A. Payton; Tirusew Asefa; Laura E. Condon; Lesley-Ann L. Dupigny-Giroux; Benjamin L. Harding; Julie Kiang; Deborah H. Lee; Stephanie A. McAfee; Justin Pflug; Imtiaz Rangwala; Heather J. Tanana; Daniel B. Wright

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Climate change will continue to cause profound changes in the water cycle, increasing the risk of flooding, drought, and degraded water supplies for both people and ecosystems. These impacts will disproportionately impact frontline communities. While data and tools for water resources planning are improving, water infrastructure standards and management policies have been slow to meet the new challenges.

Climate change is intensifying rainfall and floods, deepening droughts, and shifting weather patterns across the globe,¹ causing profound effects on terrestrial freshwater supplies and quality. Rising sea levels, reduced snowpacks, shrinking rivers, and declining groundwater threaten cities and rural communities and endanger forest, riverine, and other ecosystems across the United States.

Authors

Federal Coordinating Lead Author: Ariane O. Pinson, US Army Corps of Engineers
Chapter Lead Author: Elizabeth A. Payton, University of Colorado Boulder, Western Water Assessment
Chapter Authors: Tirusew Asefa, Tampa Bay Water; Laura E. Condon, University of Arizona; Lesley-Ann L. Dupigny-Giroux, University of Vermont; Benjamin L. Harding, Lynker; Julie Kiang, US Geological Survey; Deborah H. Lee, NOAA Great Lakes Environmental Research Laboratory; Stephanie A. McAfee, University of Nevada, Reno; Justin Pflug, University of Maryland, College Park, Earth System Science Interdisciplinary Center; Imtiaz Rangwala, University of Colorado Boulder, North Central Climate Adaptation Science Center; Heather J. Tanana, University of Utah, S.J. Quinney College of Law; Daniel B. Wright, University of Wisconsin–Madison

Contributors

Technical Contributors: Frances V. Davenport, Colorado State University; Andrea L. Taylor, Indian Health Service
Review Editor: Beth M. Haley, Boston University, School of Public Health
USGCRP Coordinators: Katia Kontar, US Global Change Research Program / ICF; Yishen Li, US Global Change Research Program / ICF; Allyza R. Lustig, US Global Change Research Program / ICF; Drew Story, US Global Change Research Program / ICF (through July 2022)

Recommended Citation

Payton, E.A., A.O. Pinson, T. Asefa, L.E. Condon, L.-A.L. Dupigny-Giroux, B.L. Harding, J. Kiang, D.H. Lee, S.A. McAfee, J.M. Pflug, I. Rangwala, H.J. Tanana, and D.B. Wright, 2023: Ch. 4. Water. In: Fifth National Climate Assessment. Crimmins, A.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, B.C. Stewart, and T.K. Maycock, Eds. U.S. Global Change Research Program, Washington, DC, USA. https://doi.org/10.7930/NCA5.2023.CH4

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Climate change, combined with greater exposure and vulnerability, is increasing the frequency of water-related disasters in the US (Figure 4.1).

A stacked bar chart illustrates that the annual number of water-related billion-dollar disasters occurring in the United States has increased over the period 1980 to 2022, as described in the caption and text. Y-axis values range from 0 to 25 events. Five types of water-related disasters are shown: winter storm (purple), tropical cyclone (yellow), severe storm (green), flooding (blue), and drought (brown). Although the number of disasters varies from year to year, an increasing trend over the period is evident. The number of events between 1980 and 2007 ranged from 0 to 9, while the number between 2008 and 2022 ranged from 7 to 21, and every year from 2016 to 2022 has seen more than 10 such events. In recent years, severe storms have been the most common cause of water-related billion-dollar disasters, followed by tropical cyclones.

Water-related billion-dollar disasters are increasing in the United States.

Figure 4.1. Across the US, the number of water-related disasters with damages exceeding $1 billion (adjusted for inflation) during 1980–2022 rose due to increases in exposure, or assets at risk; vulnerability, or how much damage a hazard of a given intensity causes; and climate change-driven increases in the frequency of extremes. Adapted from NCEI 2023.²

While these events are primarily related to water quantity, impacts related to water quality are increasing as well, as predicted in the Fourth National Climate Assessment, released in 2018 (NCA4).³ Temperature increases, sea level rise, and changes in precipitation are expected to continue to degrade water quality for people and ecosystems (Figure 4.2; KMs 4.2, 15.1, 15.2).⁴^,⁵^,⁶

A schematic figure illustrates how climate change degrades water quality, as described in the caption and text. The figure shows that three types of climate exposure (changes in ambient temperature, sea level rise, and changes in rainfall averages and extremes) result in or interact with climate hazards to degrade water quality. Changes in ambient temperature affect wildfire, which has impacts on sediment and turbidity as well as disinfection by-products. Changes in ambient temperature also affect water temperature, with impacts on harmful algal blooms and other pathogens. Sea level rise can lead to saltwater intrusion and land inundation, thereby increasing the salinity of groundwater. Changes in rainfall averages and extremes can produce intense rainfall and flooding, which can lead to the overflow of combined sewer systems, the transport of contaminants, and excess nutrients. Changes in rainfall averages and extremes can also involve drought and reduced rainfall, which can lead to increases in the concentration of contaminants in water.

Climate change threatens the quality of freshwater supplies.

Figure 4.2. Changes in ambient temperature, sea level, and rainfall (top) can create climate-related hazards, such as changes in water temperature and saltwater intrusion (middle) that can have negative impacts on water quality (bottom). Saltwater intrusion is an imminent threat to coastal and island communities dependent on groundwater for drinking water (KMs 30.1, 9.2); agricultural areas face risks to water supplies when fertilizers and pesticides are mobilized by flooding;⁷ higher temperatures are putting many areas at risk of exposure to harmful algal blooms (e.g., KM 22.2) and increases in fecal coliform bacteria;⁶ and treatment plants are challenged by sediments and debris from wildfires in their source waters (KM 6.1).⁸^,⁹ Adapted from Nijhawan and Howard 2022⁶ [CC BY 4.0].

Climate change is forcing a reexamination of our concepts of rare events. Extreme precipitation incidents are more intense and more frequent (KM 2.2); extended droughts in the West appear to be due in part to long-term aridification in addition to episodic drying (KM 4.2); and compound hazards are increasing as the events that combine to create them become more frequent (Focus on Compound Events).

The US is slowly adapting to these changes. Utilities are exploring ways to integrate change into planning, and communities are cooperatively seeking solutions to water shortages and flooding (KM 4.3). But barriers arise from legal and regulatory institutions that have been in place for decades or even centuries, locking in practices that hinder adaptation (KM 4.3). The Nation’s aging water infrastructure, designed under regulations and standards appropriate to an unchanging climate, is deteriorating and threatening public health, a situation little changed since it was highlighted in NCA4 (KM 4.2).³

Perhaps the most notable advance in recent years is the growing recognition of environmental injustices exacerbated by climate change (KM 1.2). Overburdened populations, including Black, Hispanic, Indigenous, Tribal and other communities, are suffering disproportionate impacts from climate-driven water quality and quantity hazards that threaten these communities’ water security (KMs 4.2, 15.1, 15.2, 16.1).

The Nation is making some progress. The tools and data needed to support water resources planning and management have become more sophisticated and widely available, though gaps remain, particularly hydrologic projections for the US Caribbean, Hawai‘i, and the US-Affiliated Pacific Islands, where water security concerns are high (KM 4.1; Box 23.2). Gaps in local projections of extreme event frequencies, magnitudes, and durations also hinder adaptation. There has been enormous growth in the availability of science-based climate information for water providers and natural resource managers, demonstrating increasing awareness and demand for solutions. These and similar efforts are the first steps toward building resilient human and natural systems in the face of climate-induced changes to the water cycle.

Climate Change Will Continue to Cause Profound Changes in the Water Cycle

Changes to the water cycle pose risks to people and nature. Alaska and northern and eastern regions of the US are seeing and expect to see more precipitation on average, while the Caribbean, Hawai‘i, and southwestern regions of the US are seeing and expect to see less precipitation . Heavier rainfall events are expected to increase across the Nation , and warming will increase evaporation and plant water use where moisture is not a limiting factor . Groundwater supplies are also threatened by warming temperatures that are expected to increase demand . Snow cover will decrease and melt earlier . Increasing aridity, declining groundwater levels, declining snow cover, and drought threaten freshwater supplies .

Freshwater availability is affected by the quantity of water in storage, the timing of water movement, how much water is used, and its quality,¹⁰ all of which are governed by the interrelated hydrologic components of the water cycle. Changes to these components are occurring across the Nation as a result of human activities as well as human-caused climate change. These changes are superimposed on natural variability, resulting in changes to both water availability and water-related hazards (KMs 2.1, 2.2).

Precipitation Changes

Climate change has already shifted precipitation patterns across the country, including increased variability and elevated likelihood of extreme rainfall events (KMs 2.2, 3.5). These trends exhibit substantial regional and seasonal variations (KM 2.2).¹¹ Projected changes in annual precipitation also exhibit large regional differences (Figure 4.3). Precipitation trends and projections are discussed in more depth in Chapters 2 and 3.

Three maps of the contiguous United States, plus one map each of Alaska and Hawaii, show projected changes in annual precipitation for 2036 to 2065 relative to 1991 to 2020 under an intermediate scenario (RCP4.5), as described in the caption and text. A legend shows the difference in inches ranging from negative 5 or less (dark brown) to positive 5 or more (dark teal). In an average of all available projections (panel a, left, contiguous US, Alaska, and Hawaii), most of the contiguous US is expected to see increases of less than 0.5 inch to 2 inches, although southern portions of the Southwest and Southern Great Plains regions as well as south Florida show decreases of an inch or less. Much of Alaska shows increases of 1 to more than 5 inches, with the largest increases in southeast parts of the state. Hawaii shows decreases of 0.5 to more than 5 inches, with larger decreases on the windward side of islands and the largest on Kauai. In an average of the wettest 20% of projections (panel b, top right, contiguous US only), almost all of the contiguous US shows increases of 2 to 5 inches, with larger increases in the eastern half of the country and smaller increases in the western half. Only small areas in southern Florida, northwestern Oregon, and southwest Washington show decreases. In an average of the driest 20% of projections (panel c, bottom right, contiguous US only), much of the contiguous US shows precipitation decreases of less than 0.5 inch up to 2 inches, with the biggest decreases in the Southern Great Plains and southern Florida. Only New England, areas of the Northwest, and a few other scattered locations show increased precipitation.

Annual precipitation projections show large regional differences and a wide range of potential differences.

Figure 4.3. Under an intermediate (RCP4.5) scenario, annual precipitation is projected to increase for much of the US (a), except for the Southwest, Hawai‘i, and the US Caribbean (not shown; see Figure 23.2, which shows rainfall reductions of about 10% by midcentury, and increases in dry days during the wet season, for Puerto Rico). The wettest and driest 20% of projections (b, c) illustrate the range of uncertainty in annual precipitation projections. This figure shows projected changes in inches. In the Southwest, a half-inch change in annual precipitation has more influence on the region’s hydrology than does a half-inch change in the Northeast (see Figure 2.10 for percent changes under different warming levels). Projections are not available for the US-Affiliated Pacific Islands. Figure credit: University of Colorado Boulder, NOAA NCEI, and CISESS NC.

Evapotranspiration Changes

Evapotranspiration is water that evaporates from soil, snow, and surface water or transpires from plants. It is a key component of the water budget and drives irrigation water demand. Increases in temperature and changes in other climate variables alter the evaporative demand (or potential evapotranspiration). In recent decades, evaporative demand has increased in much of the West, with few apparent trends in the East.¹² Actual evapotranspiration is evaporative demand limited by water availability. In the continental US, actual evapotranspiration has trended lower in the Southwest as water availability has declined, while the East and North show an increase. The greatest increase in actual evapotranspiration has been in the South from eastern Texas to northern Florida.¹¹^,¹³ These trends are largely projected to continue with climate change (Figure 4.4).

Three maps of the contiguous United States, plus one map each of Alaska and Hawaii, show projected changes in annual actual evapotranspiration for 2036 to 2065 relative to 1991 to 2020 under an intermediate scenario (RCP4.5), as described in the caption and text. A legend shows the difference in inches ranging from less than negative 5 (dark brown) to more than positive 5 (dark teal). In an average of all available projections (panel a, left, contiguous US, Alaska, and Hawaii), the southern portions of the Southern Great Plains and Southwest regions show decreases of less than 0.5 inch to 1 inch, while the rest of the country generally shows increases of less than 0.5 inch to 3 inches, with the largest increases in the Northwest, Northeast, and northern parts of the Southeast. Most of Alaska shows increases of less than 0.5 to 3 or more inches, with the largest increase in southeast parts of the state. A few areas of Alaska and Northwest show increases of 5 or more inches. Hawaii shows increases of 0 to 1 inch, with the largest increases in windward parts of the islands. In an average of the wettest 20% of projections (panel b, top right, contiguous US only), almost all of the contiguous US shows increases; changes range from less than 0.5 inch to 5 inches, with the largest increases in the Northwest and Mid-Atlantic. Decreases are projected only for some parts of California, with changes in the range of less than 0.5 inch to 1 inch. In an average of the driest 20% of projections (panel c, bottom right, contiguous US only), the Southern Great Plains, eastern parts of the Southwest, and western parts of the Midwest and Southeast show decreases of less than 0.5 inch to 5 inches, with the largest decreases in Texas and Oklahoma. The Northwest and Mid-Atlantic show increases of up to 5 inches, and the Northeast increases of up to 3 inches.

Actual evapotranspiration is projected to increase across most of the Nation but decrease in the Southern Great Plains and Southwest.

Figure 4.4. Actual evapotranspiration is the water that evaporates from soil and surface water or transpires from plants. Higher rates of evapotranspiration can reduce overall water availability even if precipitation does not change; conversely, low water availability can limit actual evapotranspiration. Under an intermediate scenario (RCP 4.5), actual evapotranspiration is expected to decrease in regions with decreasing or unchanging precipitation (a), such as the US Southwest, the Southern Great Plains, and the Caribbean (not shown; Box 23.2). Wetter regions, including the Northwest, Alaska, and the eastern half of the US, will see higher actual evapotranspiration. The wettest and driest projections (b, c) illustrate the range of uncertainty. Projections are not available for the US-Affiliated Pacific Islands. Figure credit: University of Colorado Boulder, NOAA NCEI, and CISESS NC.

Snow and Glacier Changes

Snow is a natural reservoir, storing cold-weather precipitation and later releasing water through snowmelt. With higher temperatures, the fraction of precipitation falling as rain instead of snow will increase.¹⁴^,¹⁵ Warming will also cause earlier snowmelt,¹⁴^,¹⁶ altered rates of snowmelt and evaporation directly from the snow,¹⁷^,¹⁸^,¹⁹^,²⁰ and longer snow-free periods.²¹^,²² Most historical snow-observation records already show trends toward earlier peak snowpack, smaller volumes, and decreasing snow-season duration (Figure A4.7),¹¹ particularly for warmer maritime and lower-elevation regions.²³^,²⁴^,²⁵ In areas of the West where snow is the dominant source of runoff,²⁶ total seasonal snow water volume is projected to decrease by more than 24% by 2050 under intermediate (RCP4.5; Figure 4.5) and higher scenarios, with persistent low-snow conditions emerging within the next 60 years.²⁴ These snow reductions, combined with projected increases to water demand, are expected to stress water supplies, particularly in the West (KM 28.1), where snowmelt supplies a disproportionate amount of water for municipal water supplies and agriculture.²⁷^,²⁸^,²⁹ Reductions in snow cover are also accelerating the retreat of glaciers³⁰^,³¹^,³² that are critical for summer streamflow in Alaska³³ and the Pacific Northwest (Ch. 27).³⁴

Three maps of the contiguous United States and one map of Alaska show projected changes in maximum annual snow water equivalent for 2036 to 2065 relative to 1991 to 2020 under an intermediate scenario (RCP4.5), as described in the caption and text. A legend shows the difference in inches ranging from less than negative 5 (dark brown) to more than positive 5 (dark teal). In an average of all projections (panel a, left, contiguous US and Alaska), most of the contiguous US shows little change. But mountainous areas of the Northwest show decreases of up to 5 inches or more, while smaller decreases are projected for mountainous areas of the Northern Great Plains and Southwest. The Northeast, upper Midwest, and northwestern parts of the Southeast show decreases of up to 3 inches. Northern and eastern parts of Alaska show increases of up to 3 inches, while southern parts of the state show decreases of up to 5 inches or more. In an average of the wettest 20% of projections (panel b, top right, contiguous US only), the patterns are similar, but the decreases are somewhat smaller in magnitude and affect smaller total areas of the regions. In an average of the driest 20% of projections (panel c, bottom right, contiguous US only), the patterns are similar, but the decreases are larger in magnitude and affect larger total areas of the regions.

Continued decreases in snowpack water content are projected across much of the US.

Figure 4.5. Snow water equivalent (SWE), the quantity of water stored in the snowpack, is key to regional water supplies. Under an intermediate scenario (RCP 4.5), peak SWE is projected to decline across much of the country except for some high-elevation interior locations in the contiguous United States and parts of Alaska (a). The largest snowpack declines are expected in warmer snow climates like coastal southern Alaska and the mountain ranges of California and the Northwest. The wettest (b) and driest (c) projections both show decreases in SWE, reflecting the influence of warming on future snowpack. Snow on the highest Hawaiian mountain peaks has important cultural and ecological significance, but projections at this resolution are not available. Figure credit: University of Colorado Boulder, NOAA NCEI, and CISESS NC.

Soil Moisture Changes

Soil moisture is water stored in the soil, usually close to the surface. It is a key component of the water cycle, supporting agriculture and ecosystem productivity, modifying streamflow by absorbing precipitation and snowmelt, and modulating climate.³⁵^,³⁶ A scarcity of soil moisture observations³⁷ has led to uncertainty regarding overall amounts, seasonality, and the direction of changes; however, there is consensus that soils are becoming drier in the Southwest.³⁸^,³⁹^,⁴⁰^,⁴¹

Projections suggest that summer soil moisture will decrease across most of the country (Figure 4.6), with parts of the upper Midwest and Alaska⁴² as exceptions. The Northwest, parts of the central and eastern US, and Alaska can expect seasonal changes in total soil moisture, with wetter soils in winter.⁴²^,⁴³ Summer soil moisture in the Southwest could increase if summer precipitation is higher, but there is greater confidence in decreasing annual soil moisture in the region (Figure 2.4).³⁸^,⁴³

Three maps of the contiguous United States, plus one map each of Alaska and Hawaii, show projected changes in average summer (June–August) soil moisture for 2036 to 2065 relative to 1991 to 2020 under an intermediate scenario (RCP4.5), as described in the caption and text. A legend shows the difference in inches ranging from less than minus 0.5 (dark brown) to more than plus 0.5 (dark teal). In an average of all available projections (panel a at left, contiguous US, Alaska, and Hawaii), most of the contiguous US shows decreases in summer soil moisture, with the largest decrease (up to minus 0.5) in mountainous areas of the Northern Great Plains, Northwest, and Southwest. Some areas of the Southwest show increases of up to 0.05 inch, while parts of Illinois, Iowa, and Minnesota show increases of up to 0.2 inch. Alaska shows large decreases (up to minus 0.5 inch) in the south and west and large increases (up to plus 0.5 inch) in central and northern areas. Hawaii shows decreases of up to 0.3 on the Big Island, Maui, and Kauai. In an average of the wettest 20% of projections (panel b, top right, contiguous US only), much of the Great Plains, Midwest, and western parts of the Southeast show increases, with the largest (up to 0.5 inch) in Illinois, Iowa, and Minnesota. Mountainous areas of the Northern Great Plains, Northwest, and Southwest show decreases of up to 0.5 inch. In an average of the driest 20% of projections (panel c, bottom right, contiguous US only), almost all of the contiguous US shows decreases, with the largest (up to minus 0.5 inch) in the Midwest, western parts of the Southeast, and mountainous areas of the Northwest and Northern Great Plains. A few areas in California, Nevada, Utah, and Arizona show small increases of up to 0.05 inches.

Projected decreases in summer soil moisture will have important implications for agriculture and ecosystems.

Figure 4.6. Summer soil moisture supports dryland agriculture and ecosystem functions and reduces irrigation demand and wildfire risk. Under an intermediate scenario (RCP 4.5), soil moisture is projected to decrease during the summer months (June, July, and August) for most of the country (a), with the West seeing decreases even under the wettest projections. Exceptions include portions of the Upper Midwest and Alaska. The range between the wettest (b) and driest (c) projections illustrate the uncertainty in summer soil projections. Projections are not available for the US Caribbean or US-Affiliated Pacific Islands. Figure credit: University of Colorado Boulder, NOAA NCEI, and CISESS NC.

Groundwater Changes

Groundwater is water stored below the land surface; it can be close to the surface or extend hundreds of feet deep. It is a crucial water supply for human systems and can moderate changes in temperature and precipitation.⁴⁴^,⁴⁵^,⁴⁶ NCA4 noted that groundwater depletions can increase drought risk and highlighted unsustainable groundwater usage and the likelihood of further declines in the future.³ More recent work has emphasized the hydrologic connections between surface and groundwater that make surface water systems vulnerable to declining groundwater levels.⁴⁷^,⁴⁸

Groundwater trends vary regionally and are difficult to project because the intensity of both groundwater withdrawals and recharge depends on human factors (e.g., land use, population, surface water allocations, and groundwater regulation) in addition to climate drivers.⁴⁹ Natural groundwater recharge varies from year to year but is projected to decrease slightly in the Southwest and increase slightly in the Northwest.⁵⁰^,⁵¹ Higher temperatures will increase irrigation demand (Figure 4.9), which can lead to increased groundwater pumping in areas where groundwater is the primary water supply or where surface water supplies are limited.⁵²^,⁵³^,⁵⁴ Groundwater levels have already been declining in many major aquifers due to lack of management, overpumping, and decreased recharge; increased pumping could accelerate long-term storage losses, but those impacts will depend on the regional factors noted above.⁴⁹^,⁵²^,⁵⁵^,⁵⁶ Groundwater declines caused by increased drought severity and duration in the future are a concern in many parts of the country (KMs 23.3, 24.5, 28.1; Ch. 26).

Runoff Changes

Changes to the water cycle components discussed above combine with other factors to affect runoff (surface water flow). For example, snowpack changes impact the seasonality of runoff in snowmelt-dominated areas,⁵⁷ while soil moisture affects the amount of precipitation and snowmelt that becomes runoff.⁵⁸ In addition to direct precipitation and groundwater, runoff is a primary source of water supply for people and ecosystems. Annual runoff trends for the most part have tracked annual precipitation trends. Similarly, the trend toward increasing annual runoff variability in most of the eastern half of the US is consistent with increasing extreme precipitation events there.¹¹ Increases in heavy precipitation events are projected to increase annual runoff over much of the US (Figure 4.7).⁵⁹^,⁶⁰

Three maps of the contiguous United States, plus one map each of Alaska and Hawaii, show projected changes in annual runoff for 2036 to 2065 relative to 1991 to 2020 under an intermediate scenario (RCP4.5), as described in the caption and text. A legend shows the difference in inches ranging from negative 2 or less (dark brown) to positive 2 or more (dark teal). In an average of all available projections (panel a, left, contiguous US, Alaska, and Hawaii), most of the western half of the contiguous US shows increases of up to 0.1 inch, with higher increases (up to 2 inches) along some parts of the Pacific Coast; much of the eastern half of the country shows larger increases of up to 0.5 inch. However, some areas show decreases of up to about 2 inches, mostly at higher elevations in Washington, Oregon, California, Idaho, Wyoming, and Colorado, and smaller decreases in some parts of the eastern Southwest region, the Northwest, and the Southeast. Alaska shows increases of 0.1 to 2 inches in most of the state but decreases of up to 2 inches or more in the south. Hawaii shows decreases of up to 2 inches on the windward sides of the Big Island, Maui, and Kauai. In an average of the wettest 20% of projections (panel b, top right, contiguous US only), the map is very similar, although increases in the eastern half of the country are larger, up to 1.5 inches. In an average of the driest 20% of projections (panel c, bottom right, contiguous US only), most of the contiguous US shows decreases of up to 0.5 inch, with larger decreases in western mountain areas, and slight increases in the Northeast and parts of the Southwest, Northwest, and Northern Great Plains.

Projected changes in runoff vary across the Nation due to projected changes in multiple aspects of the water cycle.

Figure 4.7. Rivers and streams aggregate runoff across watersheds, and runoff integrates climate change impacts to the water cycle (Figures 4.3, 4.4, 4.5, 4.6); as a result, impacts to runoff over a watershed are commonly used as surrogates for impacts to streamflow. Under an intermediate scenario (RCP4.5), projections of annual runoff vary geographically depending on relative changes to precipitation, evapotranspiration, snow and ice, groundwater, and soil moisture. Decreases are projected in Hawai‘i and parts of the Nation supplied by snow (a). Projections are not available for US-Affiliated Pacific Islands or the US Caribbean; however, given projected decreases in precipitation and increases in temperature in the Caribbean, annual runoff is expected to decrease. The range between the wettest (b) and driest (c) projections illustrate the uncertainty in runoff projections. Figure credit: University of Colorado Boulder, NOAA NCEI, and CISESS NC.

Extreme Events: Floods and Droughts

Inland floods are driven by complex interactions among precipitation amount and timing, soil moisture, snowpack, and land cover (see KM 9.1 for coastal flooding). However, estimates of events such as the 100-year flood typically rely on historical observations and assumptions of an unchanging climate.⁶¹ Methods that account for the added uncertainty of climate change are needed for infrastructure design, land use planning, and other purposes,⁶²^,⁶³^,⁶⁴ but future flood frequency is challenging to predict (Figure 4.8).⁶⁵^,⁶⁶ For example, some extreme precipitation events will be buffered by future reductions in soil moisture, which will allow more rainfall to be absorbed,⁶⁷^,⁶⁸^,⁶⁹ and some areas are projected to see increases in floods from rain falling on snow,⁷⁰^,⁷¹ precipitation on wildfire-disturbed land,⁷²^,⁷³ and loss of natural water storage in urban landscapes.⁷⁴

Schematic illustrates the ways that climate change affects the magnitude of floods and the damages they cause, as described in the caption and text. Atmospheric temperature increases (left of schematic) lead to three changes in flood drivers (center of schematic): increased short-duration extreme rainfall; longer drought periods and drier soils; and earlier snowmelt and decreased snowpack. These in turn can lead to either increases or decreases in flood activity (right of schematic), as follows: Increased short-duration extreme rainfall can lead to increased flooding in urban areas and increased potential for rare, high-magnitude floods, especially at small-watershed scales; but it can also lead to decreased flood magnitude due to drier soils. Longer drought periods and drier soils increase the potential for rare, high-magnitude floods, especially at small-watershed scales; but they can also lead to decreases in flood magnitude due to drier soils. Earlier snowmelt and decreased snowpack can lead to decreased magnitude and frequency of snowmelt-driven floods. Although there is good scientific understanding and high confidence regarding atmospheric temperature increases, there is limited understanding and relatively lower confidence regarding how they will affect flood drivers, and even lower confidence in how changes in flood drivers will lead to future increases or decreases in flood activity.

Climate change may cause both increases and decreases in inland flooding, depending on the location and time of year.

Figure 4.8. Inland floods result from combinations of factors, primarily extreme rainfall, soil moisture, and snowpack and snowmelt conditions. Each of these are subject to substantial variability and change across a wide range of timescales, from daily to decadal, in a warming climate. Scientific confidence in how the climate drivers of flooding will change is higher than in how those drivers will combine to affect floods in particular locations and seasons. Adapted from Yu et al. 2020⁶⁶ [CC BY-NC 4.0].

Changes in future precipitation and temperature are expected to exacerbate drought across large portions of the US.⁷⁵ Observed trends in drought (Figure A4.9) and climatic water deficit reflect these changes,¹³ as do projections, with the strongest drying signal occurring in the Southwest (Figure 4.9).⁷⁶

Three maps of the contiguous United States show projected changes in annual climatic water deficit for 2036 to 2065 relative to 1991 to 2020 under an intermediate scenario (RCP4.5), as described in the caption and text. A legend shows the difference in inches ranging from negative 5 or less (dark green) to positive 5 or more (dark brown). In an average of all projections (panel a, left), most of the western US will see increases in climatic water deficit of up to 3 inches, with a few areas with increases of 3 to 5 inches in parts of the Southern Great Plains and Southwest. Some higher-elevation parts of the Northwest will see decreases of up to 3 inches. Most of the East will see increases or decreases in water deficit of 0.5 inch or less. In an average of the wettest 20% of projections (panel b, top right), most of the contiguous US from the Great Plains to the east shows decreases in climatic water deficit of up to 3 inches. Much of the West shows increases of up to 2 inches, although parts of the Northwest show decreases of up to 3 inches. In an average of the driest 20% of projections (panel c, bottom right), nearly the entire contiguous US shows increases, with the largest (5 inches or more) in southern Texas. The only decreases are in higher-elevation parts of the Northwest, where annual climatic water deficit shows declines of up to 3 inches.

Water shortages to vegetation will increase across most of the Nation.

Figure 4.9. Climatic water deficit (CWD) is the shortfall of water necessary to fully supply vegetation requirements—CWD is zero if those needs are met, and a higher number indicates drier conditions. Vegetation water needs will increase with increases in temperature; as a result, in the absence of compensating increases in precipitation, CWD is projected to increase. Under an intermediate scenario (RCP4.5), CWD is expected to rise across much of the Nation, with the Great Plains and Southwest seeing the greatest increase (a, c). Even the wettest projections show increases in CWD in the West (b). Projections are not available for the US Caribbean, Alaska, Hawai‘i, or US-Affiliated Pacific Islands; however, given expected temperature increases and annual precipitation decreases in Hawai‘i and the US Caribbean, CWD is expected to increase in those regions, while Alaska is expected to see both increases and decreases similar to the pattern seen in the Northwest. Figure credit: University of Colorado Boulder.

Box 4.1. Washington–California 2015 Snow Drought

Snow droughts occurred across much of the western coastal mountain ranges during the 2014/15 winter. However, the climatic causes of these droughts varied. Western Oregon and Washington experienced a warm snow drought, wherein wintertime precipitation was 77%–113% of normal but elevated temperatures caused a larger proportion of that precipitation to fall as rain, which reduced snow accumulation and increased winter snowmelt.¹⁵^,⁷⁷ As a result, wintertime streamflows were normal to high, but April to August flows were lower than normal (Figure 4.10).

Two time series graphs illustrate two types of snow drought as revealed by streamflows for Ahtanum Creek, Washington (top), and Merced River, California (bottom), as described in the caption and text. On each graph, a dashed black line shows the daily median daily flow for the period 1952 to 2021; gray lines show the daily average streamflows for each year in the same period; and a black line shows the 2015 water-year streamflow. On the Ahtanum Creek graph, the y-axis shows daily average streamflow in values ranging from 0 to about 900 cubic feet per second (abbreviated C F S). The x-axis shows months from October (left) to September (right). The 2015 flow is generally shifted from summer to winter compared to the median because it was too warm for snow accumulation (a warm snow drought): the graph shows one peak of more than four times the median streamflow in February, as well as several smaller peaks above the median in the period December through March; the flow is well below median values by mid April and remains there through June. Observed daily flows during the 70-year record show peaks during the period December through June, with no peaks in August and September and few in October and November; the median is close to 0 in October, rises fairly steadily to a peak just under 200 C F S in May, and then declines to close to 0 again by mid July. On the Merced River graph, the y-axis shows values ranging from 0 to 6,000 C F S, and the x-axis shows months from October (left) to September (right). The flow for 2015 remains well below the median for nearly the entire year (a dry snow drought) because of low total precipitation. Observed daily flows during the 70-year record show peaks primarily in the period May through July. Median flow is close to 0 from October through early March, rises fairly steadily to above 1,000 C F S by early June, then declines to close to 0 by early August.

In 2015, parts of Oregon and Washington experienced a warm snow drought while the California Sierra Nevada experienced a dry snow drought.

Figure 4.10. The timelines compare the 70-year (1952–2021) median streamflow (dashed line) with the 2015 water-year (October 2014–September 2015) streamflow (black line). Annual observed streamflows are also shown for 1952–2021 (gray lines). Values are daily average streamflows in cubic feet per second. Streamflow in summer 2015 was abnormally low, resulting from reduced snowpack during a warm snow drought (Ahtanum Creek) and a dry snow drought (Merced River). Daily streamflow is in cubic feet per second for each of the years 1952–2021 (gray lines). Merced River flows are lower year-round because of low total precipitation and little snowfall; Ahtanum Creek flows are shifted from summer to winter in 2014/15 because it was too warm for snow accumulation. Figure credit: University of Maryland, College Park and Lynker.

By contrast, the California Sierra Nevada experienced a dry snow drought, resulting in the shallowest snow volume ever recorded there.¹⁵^,⁷⁸^,⁷⁹ Both the dry and warm droughts caused strain on water rights holders. In Oregon and Washington, irrigated crops—including valuable orchard crops—that depend on direct streamflow diversion water rights failed (Figure 4.11), but municipal water supplies that relied on storage rights that allow reservoirs to capture winter runoff were sufficient.⁸⁰ In California, total water supply was limited, resulting in severe or complete cutbacks to junior water rights and contract holders.⁸¹

An apple orchard in the Roza Irrigation District in Washington shows extreme drought stress in September 2015.

Figure 4.11. This apple orchard suffered the effects of a warm snow drought the previous winter. The warm winter temperatures caused much of the precipitation to fall as rain instead of snow, producing a reduced snowpack, and led to early snowmelt, resulting in low streamflows during irrigation season. Photo credit: © Sonia A. Hall.

Water Cycle Changes Will Affect All Communities, with Disproportionate Impacts for Some

Natural and human systems have evolved under the water cycle’s historical patterns, making rapid adaptation challenging. Heavier rainfall, combined with changes in land use and other factors such as soil moisture and snow, is leading to increasing flood damage . Drought impacts are also increasing , as are flood- and drought-related water quality impacts . All communities will be affected, but in particular those on the frontline of climate change—including many Black, Hispanic, Tribal, Indigenous, and socioeconomically disadvantaged communities—face growing risks from changes to water quantity and quality due to the proximity of their homes and workplaces to hazards and limited access to resources and infrastructure .

Changes to the water cycle have manifold effects beyond those described in this chapter. See the Energy (Ch. 5), Ecosystems (Ch. 8), Agriculture (Ch. 11), Built Environment (Ch. 12), Transportation (Ch. 13), and Human Health (Ch. 15) chapters for more information.

Flood Impacts

Floods have important roles in creating and maintaining aquatic habitat, in regulating the reproductive cycles of fish and other river organisms, and in replenishing soil and nutrients in floodplains. Land-cover changes have limited these positive impacts and even exacerbated some of the negative consequences of floods. Climate change–driven changes in precipitation amount and duration, snowpack/snowmelt, and soil moisture have combined with land-cover change and increasing property values to increase overall economic damages from floods (Figure 4.12).⁸²

A shaded area time series graph illustrates the annual costs of flood damages associated with precipitation change in the period 1988 to 2021, as described in the caption and text. The y-axis shows cumulative flood damages, with values ranging from $0 to $250 billion. A dark gray line and gray shading, showing cumulative historical inland flood damage, starts at zero and increases steadily until 2007; it then shows a more rapid rise, reaching $230 billion in 2021. Green shading, showing the estimated portion of this total damage caused by precipitation change, starts at zero and rises steadily, with a rapid increase between 2016 and 2017, reaching $84 billion in 2021. Error bars show the 95% confidence interval for cumulative damages in 2021 due to precipitation changes: $46 billion at the lower limit and $105 billion at the upper limit, as further explained in the caption.

A portion of observed increases in inland flood damages can be attributed to changes in precipitation.

Figure 4.12. Cumulative inland flood damages (in 2021 dollars) across the contiguous US (gray) and estimated portion due to changes in precipitation (green) are shown for 1988–2021. Over this period, heavy precipitation has increased over most of the US due to climate change (see Figure 2.8 for heavy precipitation changes over the 1958–2021 period). Error bars (in green) show the plausible range of cumulative damages in 2021, calculated using a 95% confidence level. Roughly 20%–46% of increases in observed flood damages can be attributed to increasing precipitation (assuming the same historical development patterns over the period 1988–2021). Other important contributors to flood damage include urbanization and land-use change, which can exacerbate runoff, and growth in the number and value of flood-affected buildings and other assets. Adapted from Davenport et al. 2021.⁸²

In urban settings, pavements, roofs, and compacted soils do not absorb water as effectively as natural landscapes, amplifying the effects of heavy precipitation and concentrating flooding. In rural settings, lower amounts of impervious land cover allow soils to hold more rainfall. However, intensive agriculture can reduce the infiltration and water-holding capacity of soils and increase runoff, resulting in flooding.⁸³

At major watershed scales, flooding along large river and lake systems causes numerous disruptions, including to rail, roadway, and river transportation; agricultural production; commodity deliveries; and industrial production, as seen during the Mississippi River flood of 2011 (KM 24.4).⁸⁴

Increasing flood activity threatens water quality and ecosystems (Figure 4.2). As floodwaters inundate normally dry areas, they transport debris, chemicals, bacteria, and other contaminants (KM 23.1).⁸⁵^,⁸⁶ Heavy precipitation events are overwhelming aging combined stormwater–sewer systems, leading to discharges of contaminated water and raw sewage into receiving waters.⁸⁷^,⁸⁸ The upward trajectory of urban flooding impacts will likely continue with changing rainfall patterns and intensity.⁸⁹ Groundwater-sourced drinking water is becoming contaminated from standing floodwaters over wellheads and percolation into well-fields,⁹⁰ and in farmlands high runoff is discharging fertilizer into streams and lakes, causing harmful algal blooms.⁹¹

Drought Impacts

Droughts are driven by many factors, including unsupportable societal demands for water.⁹² From a climate perspective, below-normal precipitation is a primary driver of drought, but there is growing acknowledgement that higher temperatures can cause drought to develop or become more intense than would be expected from precipitation deficits alone; higher temperatures drive increased atmospheric demand for moisture—a phenomenon known as hot drought.⁷⁵^,⁹³^,⁹⁴^,⁹⁵ Above-normal temperatures also contribute to snow drought (Box 4.1) and flash drought, which develops quickly over a few weeks.⁹⁶^,⁹⁷ Megadroughts are events of extraordinary duration and severity,⁹⁸ and many are documented in paleoclimate records.⁹⁹^,¹⁰⁰ Temperature’s contribution to drought makes it clear that warming associated with climate change could increase the frequency, severity, and/or duration of drought⁷³^,¹⁰¹^,¹⁰² and drive aridification, a long-term shift toward a drier climate, which is a concern in already dry parts of the West.⁷⁶

Art × Climate

Meredith Nemirov
Rivers Feed the Trees #467 (Aquifers)
(2022, acrylic on historic topographic map)

Artist’s statement: Rivers Feed the Trees is a series of works on historic maps where blue is painted into the topography to create an abundance of rivers and streams. Since the turn of the 21st century, Colorado has experienced periods of extreme drought. This inspired me to create works where I imagine a CO with no drought. I hope these images will encourage people to learn more about where our water comes from and to look for solutions to the dire situation we are facing regarding the future of our water.

View the full Art × Climate gallery.

Artworks and artists’ statements are not official Assessment products.

Between 1980 and 2022, drought and related heatwaves in the US caused $334.8 billion (in 2023 dollars as of July 2023) in damages; only tropical cyclones and severe storms were more costly (KM 22.1).² Droughts often reduce agricultural productivity and strain water systems,¹⁰³^,¹⁰⁴ driving shortages in water supplies and threatening power generation (KM 5.1).¹⁰⁵ River and lake transportation is also at risk due to drought (KM 24.4).

Drought stresses terrestrial and aquatic ecosystems¹⁰⁶ by leading to increased water temperature and salinity, reduced nutrients, lower oxygen levels, concentrated contaminants (Figure 4.2), loss of surface and groundwater connections, and declining productivity.¹⁰⁷^,¹⁰⁸ In addition, drought can exacerbate other disturbances such as pests and wildfire.¹⁰⁹ Ecosystems can be resilient under normal climate variability, but recovery after drought in a changed climate may not be possible, leading to the loss of ecosystem services and loss or migration of native and invasive species (Figure 8.6).¹¹⁰^,¹¹¹

Groundwater quality is also threatened by heat and drought. Warmer soil and groundwater temperatures can lead to decreased oxygen saturation, lower pH, and enhanced mineral weathering, all of which reduce water quality,¹¹² and coastal and island aquifers are at risk of seawater intrusion, rendering groundwater unpotable and potentially harming infrastructure (Figure 4.2; KMs 9.2, 21.2, 23.3, 28.2, 30.1).

Drought conditions have historically resulted in increased groundwater pumping in some regions of the US, a practice projected to increase with climate change.⁵⁵^,⁵⁶^,¹¹³ Declining groundwater levels due to pumping can reduce streamflow (Figure 4.13)⁴⁸ and result in land subsidence.¹¹⁴

The San Pedro River in Arizona has been depleted by groundwater pumping, drying up wetlands and wildlife habitat.

Figure 4.13. Groundwater pumping can reduce surface water supplies. One example is the San Pedro River in Arizona, where pumping that began in the 1940s has deprived wetlands and wildlife habitat of fresh water.¹¹⁵ Photo credit: CochiseVista/iStock via Getty Images.

Disproportionate Impacts

Climate change creates unequal burdens on people and communities.¹¹⁶^,¹¹⁷^,¹¹⁸ People who live along coasts and rivers or who work in agriculture and fisheries have increased exposure to water-related hazards.¹¹⁹^,¹²⁰^,¹²¹ Older adults, children, and residents of low-income neighborhoods and rural areas are at greatest risk of exposure to pathogens and pollutants from climate change–driven impacts to water quality.¹²²^,¹²³^,¹²⁴

Many Tribal and Indigenous communities reside in areas subject to coastal and riverine flooding and risk displacement from lands with cultural significance.¹²⁵^,¹²⁶^,¹²⁷ Neighborhoods that are home to racial minorities and people with low incomes have the highest inland flood exposures in the South.¹²⁸ Hispanic residents are 50% more likely to live in the 500-year floodplain,¹²⁹ while Black communities are projected to bear a disproportionate share of future flood damages (Figure 4.14; Box 4.2).¹³⁰ Drought can also have unequal impacts depending on economic sector, access to water resources, ability to irrigate, reliance on electricity, and socioeconomic status.¹³¹

A bar chart shows projected increases in annual average losses (abbreviated A A L s) due to floods by 2050. The bars show projections based on percentage of Black residents in US census tracks. The y-axis show percent change in 2050 compared to 2020, with values ranging from 0 to 40 percent. A dashed gray line and text show that the national average increase is projected to be 26.4 percent. In general, the chart shows that the higher the percentage of Black residents, the higher the projected increase in A A L. In Census tracts where less than 1 percent of residents are Black, projected change in A A L is a little less than 20 percent. In tracts where 1 to 2 percent of residents are Black, the figure is slightly more than 20 percent. In tracts where 2 to 7 percent of residents are Black, the figure is slightly above the national average of 26.4 percent. In tracts where 7 to 20 percent of residents are Black, the figure is just above 30 percent. In tracts where more than 20 percent of residents are Black, the figure is about 40 percent.

Losses due to floods are projected to increase disproportionately in US Census tracts with higher percentages of Black residents.

Figure 4.14. Average annual losses—economic damages in a typical year—due to floods in census tracts with a Black population of at least 20% are projected to increase at roughly twice the rate of that in tracts where Black residents make up less than 1% of the population. Black bars represent 95% confidence intervals. Adapted from Wing et al. 2022¹³⁰ [CC BY 4.0].

Art × Climate

Spencer Owen
Catch / Release
(2022, emergency blanket, watercolor, printer ink, magazine collage)

Artist’s statement: This piece shows a worker catching or releasing water droplets, and I use emergency blankets to represent disaster relief. Climate change has increased the intensity of natural disasters, which destroy water infrastructure (for example Hurricane Maria in Puerto Rico). Clean water has also been prioritized for affluent neighborhoods. The residents of Flint, MI, who are mostly low-income and African American, did not have clean water for years. It is a human necessity to have human water, to catch it, yet people are still being forced to release their right to clean water.

View the full Art × Climate gallery.

Artworks and artists’ statements are not official Assessment products.

Box 4.2. Climate Change, Urban Flooding, and Inequality

Hurricane Harvey dropped record-breaking rainfall onto the Houston and Beaumont–Port Arthur metropolitan areas in August 2017 (Figure 4.15). The flooding, exacerbated by extensive urbanization, killed more than 100 people and caused an estimated $147.6 billion in damages (in 2022 dollars).¹³² Harvey’s rainfall was estimated to be about 15% to 20% heavier than it would have been without human-caused warming,¹³³^,¹³⁴^,¹³⁵ which increased the flooded area in the Greater Houston area by 14%,¹³⁶ leading to 32% more homes being flooded.¹³⁷ Many of the flooded properties were located outside FEMA’s designated 100-year floodplains and not covered by federal flood insurance. Such properties were disproportionately inhabited by Black and Hispanic residents.¹³⁸ People with disabilities and residents of subsidized housing were also disproportionately affected.¹³⁹^,¹⁴⁰ Climate change’s impact on flooding is expected to worsen these types of inequalities.

Flooding from Hurricane Harvey inundated residential neighborhoods in Port Arthur, Texas.

Figure 4.15. Photo credit: Staff Sgt. Daniel J. Martinez, US Air National Guard.

Across the Nation, drinking water delivery infrastructure is aging and deteriorating (KM 12.2), increasing the risks of contamination and delivery of unpotable water.¹⁴¹ More than 1,000 community water systems—primarily serving older adults and people who are economically disadvantaged, rural, Indigenous, or with less education¹⁴²—are already providing poor-quality water and are not prepared to cope with climate change-driven flooding, drought, and waterborne diseases (Figure 4.2; KMs 15.1, 15.2). For some Tribal and Indigenous communities, water infrastructure deficiencies threaten their social, physical, and mental well-being and impair their ability to thrive (KM 16.1).¹⁴³^,¹⁴⁴^,¹⁴⁵ Figure 4.16 shows the distribution and severity of sanitation facility deficiencies in American Indian and Alaska Native homes.¹⁴⁶

Maps of the contiguous United States and Alaska show the number of American Indian and Alaska Native homes requiring water and sewer system improvements. Circles are used to indicate locations, with shading indicating deficiency levels ranging from DL2 (pale blue) to DL5 (dark purple) and circle sizes indicating the count of homes at each location. For the contiguous US, circle sizes correspond to the following counts: 0–250, 251–1,000, 1,001–2,500, and 2,501–5,000. The largest concentrations of high home counts and high deficiency ratings occur in northeast Arizona, northwest New Mexico, Oklahoma, and northern Maine, with significant counts also evident in Wisconsin, Minnesota, the Dakotas, Nebraska, Montana, and Washington. Locations are also found in many other states. For Alaska, circle sizes correspond to the following ranges of home counts: 0–250 and 251–1,000. The largest count of homes and highest deficiency levels are evident along Alaska’s western coast, with significant counts and high levels also evident in the central interior and southern coastal regions.

Water infrastructure supporting Tribal and Indigenous Peoples is particularly ill-equipped to handle increases in flooding and drought.

Figure 4.16. The Indian Health Service (IHS) maintains a database of American Indian and Alaska Native (AI/AN) homes requiring sanitation facility improvements within IHS service areas. The figure shows sanitation deficiency levels in AI/AN homes across the country ranging from level 2 (capital improvements are necessary to meet domestic sanitation needs) to level 5 (lacks a safe water supply and a sewage disposal system). The IHS does not collect data for Hawai‘i, the US-Affiliated Pacific Islands, or the US Caribbean, but elevated rates of plumbing deficiencies are documented in those regions.¹⁴² Figure credit: Indian Health Service.

Progress Toward Adaptation Has Been Uneven

The ability of water managers to adapt to changes has improved with better data, advances in decision-making, and steps toward cooperation. However, infrastructure standards and water allocation institutions have been slow to adapt to a changing climate , and efforts are confounded by wet and dry cycles driven by natural climate variability . Frontline, Tribal, and Indigenous communities are heavily impacted but lack resources to adapt effectively, and they are not fully represented in decision-making .

Approaches to Management and Planning

Uncertainty from natural variability has always been part of water resources planning, but as climate change affects different components of the water cycle, uncertainties around extreme events and water availability have increased. Responses to these growing uncertainties include climate adaptation and hazard mitigation through watershed management (KMs 6.1, 6.2);¹⁴⁷ nature-based solutions (KM 8.3); planned relocation;¹⁴⁸^,¹⁴⁹ floodplain management;¹⁵⁰ water conservation and reuse;¹⁵¹^,¹⁵² decision science;¹⁵³^,¹⁵⁴ reservoir optimization and artificial intelligence applications;¹⁵⁵^,¹⁵⁶^,¹⁵⁷ improved weather and streamflow forecasts;¹⁵⁸ municipal planning;¹⁵⁹^,¹⁶⁰^,¹⁶¹ adaptive management systems;¹⁶² stakeholder–scientist partnerships;¹⁶³ and adaptation guidance (KM 31.4).¹⁶⁴^,¹⁶⁵^,¹⁶⁶^,¹⁶⁷

Adaptation Constraints

Climate change is overtaking water resources policymaking,¹⁶⁸^,¹⁶⁹ making risk reduction a continual exercise in catching up. For example, current rates of precipitation change outpace the regulatory changes needed to cope with them. Key rainfall metrics for design and decision-making are widely outdated;¹⁷⁰^,¹⁷¹ updating these metrics is essential to protecting communities. While there have been recent advances in data collection, statistical methods, climate modeling, and weather forecasting, progress is difficult, in part because regulations, codes, and standards involve competing interests and often span multiple jurisdictions.¹⁷²^,¹⁷³^,¹⁷⁴

Conflict, Competition, and Collaboration

Climate change impacts to water supplies can result in competition, collaboration, or conflict. Frequently, water disputes in the western US are resolved through litigation.¹⁷⁵^,¹⁷⁶ However, under current severe drought conditions and in the context of existing legal frameworks, water interests in the Colorado River basin, including Mexico, are struggling to avoid litigation through negotiated settlements and voluntary use reduction (Box 28.1).¹⁷⁷^,¹⁷⁸^,¹⁷⁹ Some of these efforts now include Tribes and other water users who have traditionally been excluded from participation in negotiations, although representation remains uneven.¹⁸⁰

In areas where flood risk is increasing, collaboration on flood hazard management at regional scales has become more urgent, as cooperation can provide solutions that are not available at the local scale (Box 4.3). This is especially true in the Midwest, where flooding is often regional and local solutions can push flood risks downstream.¹⁸¹

Box 4.3. International Cooperation in the Great Lakes

The Great Lakes, which contain the largest quantity of surface fresh water on Earth, are shared by two Canadian provinces, eight US states, and many sovereign Tribes and First Nations. Although ripe for conflict and competition, the waters have been equitably shared since the 1909 Boundary Waters Treaty.¹⁸² In 2017, a management plan regulating Lake Ontario’s levels and outflows was implemented (Figure 4.17).¹⁸³ It was the culmination of more than 16 years of scientific study, public engagement, and governmental review, including a collaboratively built model of the physical, environmental, and economic responses of the system to management and climate alternatives. Performance indicators yielded insights and quantified trade-offs, leading to a plan that balances flooding along the lake’s New York and Ontario shorelines against flooding downstream on the St. Lawrence River at Montreal, Quebec. The plan also aims to restore the health and diversity of coastal wetlands and protect against extreme high and low water levels. An adaptive management committee evaluates the plan’s performance under climate change and recommends adjustments.

A map shows the drainage basin for Lake Ontario and the St. Lawrence River. A black line indicates the boundary of the drainage basin, and a gray line indicates the international boundary between the United States and Canada, which runs down the center of Lakes Huron, Erie, and Ontario, and the St. Lawrence River as far as Cornwall, before turning eastward. On Lake Ontario, the locations of the cities of Buffalo and Oswego are indicated on the US side, and Toronto and Kingston on the Canada side. The St. Lawrence River flows northeast from Lake Ontario. A red arrow points to the Moses Saunders Dam on the St. Lawrence in the city of Cornwall; the dam marks the divide between the Upper and Lower St. Lawrence River. The Upper River begins at Lake Ontario and ends at the dam (a distance of about 100 miles, according to the map scale), and the Lower begins at the dam and ends at the city of Trois Riviéres (a distance of about 125 miles). The beginning and end of the Upper and Lower segments are indicated by blue arrows. The Ottowa River flows from the northwest and joins the St. Lawrence at Montreal; the city of Ottowa is on its banks, upriver from Montreal. Text at the bottom of the figure reads: Plan 2014 determines outflows to the St. Lawrence River at Cornwall/Massena. Competing regions include Lake Ontario, Upper River, and Lower River. Interests include coastal flooding, erosion, shore protection, commercial navigation, hydropower, recreational boating, municipal and industrial water intakes, and ecological performance.

Plan 2014 was developed to manage Lake Ontario–St Lawrence River water levels, restore ecosystems, and account for climate change.

Figure 4.17. The map shows the geographic setting for an international plan between the US and Canada to cooperatively manage Lake Ontario. The plan balances interests upstream of the Moses-Saunders Dam with downstream interests. The collaborative framework used to develop the plan serves as a model of a successful approach to resolving water conflicts. Adapted from International Joint Commission 2014.¹⁸³

The Effect of Natural Variability on Policy

Historical records and paleoecological evidence, such as tree ring data, show that natural variability in the climate system has resulted in multidecadal wet and dry spells in the past.⁹⁹ Climate projections indicate this pattern will continue, challenging planning and policy formulation for adaptation to climate change, and suggesting that durable and realistic long-term perspectives are necessary for robust policy development. For example, natural variability brought the wettest period in the past 1,200 years to the Colorado River in the early 20th century (Figure 4.18). The Colorado River Compact, negotiated in that period of relative abundance, allocated far more water than the river has since provided.¹⁸⁴ In the last years of the 20th century, sustained high reservoir levels prompted the development of guidelines for surplus allocation, but by the time those guidelines had been finalized, the current 22‑year drought had begun.⁷³ That drought has triggered unprecedented water use restrictions and is leading to more realistic policy discussions (Box 28.1).¹⁷⁷ Similar variability is present in climate and hydrology projections through the end of this century. The amplitude of projected 30-year-average wet and dry spells on the Colorado River may be twice the average projected decrease in streamflow by the end of this century;¹⁸⁵ as a result, multidecadal natural variability almost certainly will again lead to prolonged wet periods,¹⁸⁶ though diminished by higher temperatures.

Two maps of the contiguous United States and two time-series graphs illustrate how natural streamflow variability influences policy. A legend for the maps shows standard deviations from long-term average decadal runoff ranging from minus 2 or less (dark brown) to plus 2 or more (dark green). The left map shows that the Southwest was wetter than average over the period 1915 to 1924, with large areas showing standard deviations of 2 or more wetter than the long-term average. The right map shows that the Southwest was drier than average over the period 2000 to 2009, with standard deviations in many areas of minus 0.5 to minus 2. The top time series graph shows natural flows on the Colorado River at Lees Ferry from 1906 to 2016. The y-axis shows 10-year average natural flow in values ranging from 10 to 20 million acre-feet. Flows were particularly high (about 18 million acre feet) during the 1910s and early 1920s; a wedge indicates that this was when the Colorado River Compact was negotiated. Flows then plunged to below 14 million acre feet in the 1930s, remaining in that range into the 1970s before climbing above 18 million in the 1980s. Flows then declined again, and have been below 14 million since the 1990s. A second wedge indicates that the Colorado River Shortage Guidelines were developed during the current period of low flows. The second time series graph shows reconstructed natural flows on the Colorado River at Lees Ferry over a much longer time period, from the year 1115 through 2015. The y-axis shows 10-year average natural flow in values ranging from 10 to 20 million acre-feet. The graph shows large variability in flows over the millennium, generally ranging between 12 and 17 million acre feet.

Natural hydrologic variability can promote urgency or complacency in long-term planning.

Figure 4.18. The figure shows hydrologic variability in both space and time: (a, b) runoff variability (a surrogate for streamflow variability) across the country between two decades, with the boundary of the Upper Colorado River Basin shown; and streamflow variability across time with (c) estimates of Colorado River flows from historical observations and (d) reconstructed flows from ancient tree rings (blue line), with data from (c) shown in orange. Wedges point to two negotiated policy events. Figure credit: Lynker and University of Colorado Boulder.

Adaptation Challenges Faced by Tribal and Indigenous Communities

To address water-related climate impacts, Tribes have voiced the need for climate impact assessments as a first step to resilience planning and identified information about climate change impacts to water as a top priority.¹⁸⁷ Many Indigenous communities lack data on water quality despite disproportionately experiencing water quality deficiencies.¹⁸⁸ Other data types critical to Tribal water management decisions are streamflow, temperature, precipitation, snowpack, and soil moisture, but these are not always available through federal information sources.¹⁸⁷

Food security, protection of Traditional Knowledge, and Tribal capacity to implement adaptation plans, monitor and collect data, and conduct climate vulnerability assessments are also high priorities. Federally Recognized Tribes are eligible for federal assistance with climate change adaptation, but they face hurdles accessing these limited resources, including agency requirements (e.g., funding matches), lack of Tribal capacity, and navigating interagency processes.

Progress and Gaps in the Quality and Usability of Information

Water resources planning continues to be informed by past hydrologic records that do not reflect the impacts of climate change. Although some federal, state, and larger local agencies do use climate projections in planning, projections of precipitation, streamflow, water use,¹⁸⁹ and extreme events at the scale of local watersheds are rarely available, particularly outside of the contiguous US. Using projections is also costly because tools and techniques are specialized and not standardized. Finally, climate models project a wide range of uncertainty (Figure 4.3), requiring planners to use their best judgment about how to apply the information.

Data are foundational to adaptation. State and federal agencies have been collecting valuable climate, hydrology, and water use data for over a century, but these data are sparse in lightly populated and lower-income areas.¹⁸⁵ Increasingly, modeling and remote-sensing data are filling the gaps. High-resolution elevation and environmental data collected from airborne and spaceborne platforms provide detailed topographical and hydrological information that can be used to map flood hazards and snowpack¹⁹⁰^,¹⁹¹ and refine real-time snow simulation.¹⁹²^,¹⁹³ Evapotranspiration is being estimated using satellite remote sensing combined with vegetation models,¹⁹⁴ providing early warning of emerging droughts,¹⁹⁵ and satellites are now being used to detect groundwater depletion.¹⁹⁶ Nevertheless, expanding direct observational data collection is still key to tracking environmental conditions and supporting development and testing of remotely sensed data and models.

Process Description

With support from the chapter point of contact and the federal coordinating lead author, the chapter lead author selected authors for their expertise in assessing climate impacts to the Nation’s surface and groundwater resources and the consequences of those impacts to human and natural systems, with an emphasis on the authors’ ability to bring diverse perspectives to the team. The team comprises experts drawn from several regions across the country who work under various employment types (i.e., private business, academic institutions, and local, state, and federal governments), come from diverse backgrounds, and represent a range of combinations of age and gender. The team met virtually multiple times to scope the chapter, with each author offering their own priorities about what a chapter about the Nation’s water resources should cover, taking into consideration the goals of this Assessment, the topics covered in previous National Climate Assessments (NCAs), and the topics of the other 31 chapters in the NCA5. The team’s discussions revolved around these questions: How are changes in climate influencing water input volume and movement? How are extremes and the notion of extremes changing? How are changes in climate stressing both natural and human-made systems? What are the environmental justice considerations and the distribution of impacts? Are current climate data and tools adequate for decision-makers? And what are the interconnected climate risks? With these questions in mind, the team iteratively developed a draft outline for the chapter. That outline was made available online for public review and comment. The team presented and participated in a virtual, public, four-hour workshop and discussion, collecting comments and suggestions for the chapter from workshop participants. Workshop comments and formally submitted comments were taken into consideration in development of the chapter text. The Third Order Draft was presented to the public by five of the authors in a webinar hosted by Western Water Assessment at the University of Colorado. The author team met virtually at least twice per month during periods when the draft was not out for review. The team also met in person at the NCA5 All-Author Meeting held in April 2023 in Washington, DC. The meetings were used to set interim deadlines, assess the status of tasks, discuss language choices, find consensus on Key Messages and figures, develop responses to comments on drafts, and support each other with references and text reviews.

KEY MESSAGES

KEY MESSAGE 4.1

Climate Change Will Continue to Cause Profound Changes in the Water Cycle

Read about Confidence and Likelihood

Description of Evidence Base

The hydrologic component maps shown in Figures 4.3, 4.4, 4.5, 4.6, 4.7, and 4.9 constitute part of the evidence base. They show mid-21st-century projections of water cycle components based on an intermediate scenario (RCP4.5). Projections of water cycle components are available for both RCP4.5 and RCP8.5 scenarios, but both scenarios show similar hydrologic responses at midcentury, neither are available as 100-year projections, and space in this chapter is limited; as a result, only RCP4.5 projections are presented here. The central map of the contiguous US (CONUS) in each of these figures represents the average of all 32 Coupled Model Intercomparison Project Phase 5 (CMIP5) projections chosen for this discussion.¹⁹⁷ The Alaska and Hawai‘i maps represent the average of 10 CMIP5 projections. The wettest and driest 20% of projections show the range of outcomes from the 32-projection set for CONUS, illustrating the uncertainty surrounding water cycle responses to climate change. Outside CONUS, downscaled climate projections are limited, especially those needed to map projected changes in hydrologic components for the US Caribbean and US-Affiliated Pacific Islands. The absence of projections for actual evapotranspiration, soil moisture, and runoff contribute to uncertainty when assessing future water security challenges for these regions. Further information about the data used to generate the maps can be found in the figure metadata.

Because the focus of this chapter is terrestrial fresh water, the authors relied heavily on Chapter 2 (Climate Trends) and Chapter 3 (Earth Systems Processes) for their assessments of precipitation trends and projections, particularly extreme precipitation trends and projections.

Regarding evapotranspiration, there is general consensus that warming temperatures will enhance evaporative demand (potential evapotranspiration, PET) across the Nation (Ch. 3);⁴³^,⁴⁴^,⁷⁵ however, uncertainties in vegetation response to warming reduce confidence in evapotranspiration (ET) projections.⁷⁵ In many parts of the country, projected changes in annual evapotranspiration by the end of this century are not robust, and there is disagreement among models across the southern states and parts of the central US.⁴³ The degree and sometimes direction of observed changes in PET and ET are also less certain, particularly east of the Rocky Mountains, due to differences in the trends of the variables that force PET.¹² Nor are these trends well supported by direct observation. There is a lack of information on more recent trends in pan evaporation across the US. Pan evaporation is a useful concept to estimate atmospheric evaporative demand but it is strongly affected by local environmental conditions, which can drive contradictory trends in pan evaporation across a broader region,¹⁹⁸ as is observed across the US.¹⁹⁹ For example, increases in local humidity (e.g., from irrigation) or land-use changes (e.g., changes in tree density near the pans) could affect evaporation from the pans. Therefore, pan evaporation may not provide a reliable indication of regional-scale trends in evaporative demand. The disagreement among observational data and reanalyses limits our confidence in past ET and PET trends. Complexities related to vegetation, as well as the competing effects of multiple evaporation drivers, make assigning nationally consistent likelihood and confidence challenging. However, the balance of evidence suggests with medium confidence that evaporation is expected to increase in places where moisture is not a limiting factor to atmospheric demand.

There is widespread consensus that increases in temperature will decrease the proportion of US precipitation that falls as snow,¹⁴^,¹⁵^,²⁴^,⁴³ decrease snow extents,²⁴^,²⁵ advance the timing of snowmelt rates and pulses,¹⁶^,²⁷ increase the prevalence of rain-on-snow events,⁷⁰^,⁷¹ and influence how snow water resources are partitioned to runoff.¹⁹^,²⁰

Since parts of Alaska and the highest elevations in the contiguous US may be cold enough to sustain snowfall in future climates, some studies have projected increases in snow volume in these locations with future increases in precipitation. However, those increases in snow are expected to be vastly outweighed by the future decreases in snow elsewhere, particularly across the western US and by the late 21st century for all intermediate (RCP4.5 and SSP2-4.5) and higher scenarios.

It is well established that groundwater and surface water are connected resources and that groundwater can help stabilize surface water supplies.⁴⁷^,⁴⁸ Similarly, there is agreement that loss of shallow groundwater can exacerbate droughts and decrease streamflow. There is also agreement that warmer temperatures will increase water demand and that this could increase groundwater pumping.⁵²^,⁵³^,⁵⁴

Major Uncertainties and Research Gaps

Uncertainties stem from future projections of climate. This may be particularly true for late-21st-century projections that are dependent on the degree to which societies will respond to climate change. The literature employs different projections and emissions scenarios, as well as metrics and measurements that vary in their degree of climate sensitivity, resulting in studies that are not always directly comparable.

Understanding recent and potential future flood responses to climate change is difficult for several reasons. Floods are the product of complex subseasonal to interannual interactions between rainfall, soil moisture, evapotranspiration, snowpack/melt, and other processes. Isolating climate change impacts on inland flooding is further complicated by the hydrologic “replumbing” wrought by urbanization and dams. For these reasons, the translation of rainfall trends into flood changes is complex and poorly understood. National⁶⁵^,²⁰⁰ and global²⁰¹ examination of historical flood records has concluded that climate influences have been relatively limited, contradicting an earlier study that argued that the largest floods have increased in severity.²⁰² This latter argument is further contradicted by evidence that floods in the central US have become more common but not more intense.²⁰³^,²⁰⁴

However, major floods are by definition rare, making detection and attribution of changes difficult. Thus, a lack of statistically significant trends in observed floods does not necessarily indicate that such events are not changing. Indeed, a relatively limited number of geographically focused case studies have painted complex pictures of climate-related flood changes that are lacking in broader regional and national analyses. Additional place-based case studies—as opposed to regional- or national-scale analyses—could help unravel the complex interactions between climate and non-climate flood drivers.

Given the first-order influence of temperature and precipitation change on snowfall, there is high certainty that future US snow cover, snow volume, and snow persistence will change.²⁴ However, there is some disagreement in the literature about the extent and direction (positive or negative) of change in surface water availability with future changes in climate. Existing studies indicate both increases and decreases in future runoff for different US hydroclimatic regimes.

In particular, there is uncertainty in the degree to which temperature may impact flow in some major river systems in the West.²⁰⁵^,²⁰⁶ Significant disagreements in the direction of observed soil moisture trends remain,³⁸^,³⁹^,⁴⁰ largely because it can be challenging to estimate with remote sensing or models, and the existing in situ soil moisture–monitoring network is insufficient.³⁷ Uncertainties can also be introduced because not all products are directly comparable, capturing trends over slightly different depths, although modest differences are probably not a major source of error. There is also uncertainty in soil moisture projections related to model, season, and soil depth.³⁸^,⁴²^,⁴³

Similarly, there is uncertainty in both the magnitude and direction of groundwater storage changes, primarily due to uncertainty in future groundwater management policy and uncertainty in future recharge. This is due to uncertainty in both the human response to changing climate conditions and research gaps in quantifying natural groundwater recharge. Groundwater pumping is controlled by a myriad of human factors such as population, water policy, crop choices, and irrigation technology. While it is well established that warmer temperatures can increase water demand,⁵²^,⁵³^,⁵⁴ and historical trends demonstrate unsustainable groundwater usage in the past (as discussed in NCA4), future groundwater pumping increases will depend on water management practices and policy. Groundwater recharge is similarly uncertain.⁵⁰^,⁵¹ Projected increases in large precipitation and flooding events are expected to increase recharge (known as episodic recharge events). However, the quantity of this recharge is less certain and highly dependent on the nature and timing of the storms that occur. Also, while increases in recharge may be counteracted by changes in plant water usage and snowpack that can decrease natural recharge, the magnitude of these recharge changes has not been well quantified. Separating the impacts of groundwater pumping from climate trends is particularly challenging due to a lack of long-term groundwater monitoring wells, especially outside of the most heavily groundwater-developed areas.

Description of Confidence and Likelihood

The author team determined that the evidence points to medium confidence that there will continue to be increases in precipitation in Alaska and in the northern and eastern regions of the US and decreases in precipitation in the Caribbean and the Southwest. Despite lingering uncertainties around average precipitation, there is very high confidence from both observations and projections that extreme precipitation events are becoming more frequent nationwide, and that it is very likely this trend will continue in the future. The disagreement among observational data and reanalyses limits our confidence in past ET and PET trends. Complexities related to vegetation, as well as the competing effects of multiple evaporation drivers, make assigning nationally consistent likelihood and confidence challenging. However, the balance of evidence suggests with medium confidence that evaporation will increase in places where moisture is not a limiting factor to atmospheric demand. Based on current trends and climate model projections, there is high confidence and it is very likely that warming temperatures will increase the demand for surface and groundwater for crops and human use. Given the direct influence of rising temperatures on snow, there is high confidence and it is very likely that the extent, volume, and duration of snow cover and melt upon which human and natural systems rely is and will continue to be reduced by warming.

KEY MESSAGE 4.2

Water Cycle Changes Will Affect All Communities, with Disproportionate Impacts for Some

Read about Confidence and Likelihood

Description of Evidence Base

Observational records now span time periods long enough to evaluate changes in the volume, variability, and timing of water availability.¹¹ The magnitude of these changes, and their agreement with model projections, vary with hydroclimate regimes across the US.

While it has been difficult to establish clear linkages between increases in extreme precipitation and trends in “traditional” measures of flood activity such as peak streamflow rate, attribution studies have apportioned some of the historical increases in flood damage to precipitation change.⁶⁸^,⁶⁹ It is probable that many of these increases have been concentrated in urbanized watersheds, which are more sensitive to rainfall than rural and natural settings.⁷⁴ Flood vulnerability, including in urbanized areas, tends to be concentrated in historically marginalized and socioeconomically disadvantaged neighborhoods.¹³⁰ There is increasing consensus that systematically disadvantaged communities have been and will continue to be most impacted by these hazards, due to factors such as inadequate climate/hydrological monitoring, deferred infrastructure maintenance, and insufficient access to recovery resources.²⁰⁷

There is ample literature describing the impacts of floods, fires, and drought events on a wide variety of water quality hazards.⁶ These studies provide insights into impacts to water quality hazards from intensified events due to climate change, and studies specific to climate change impacts on water quality are becoming more prevalent. There are some reports of specific benefits to contaminant concentrations from increased or decreased precipitation, but there is no consensus that water quality will improve with climate change.

There is widespread consensus that increases in air temperature will impact water quality by increasing water temperatures, resulting in less oxygen-rich water, exacerbating harmful algal blooms, increasing pathogens, and creating problems with drinking water taste and odor.⁶^,⁷

Similarly, there is consensus that increased precipitation and intensity will degrade water quality due to urban storm water and combined sewer overflows, increased agricultural runoff, and riverine flooding. There is less certainty in regions of the country where precipitation is not increasing or decreasing. Compounding factors of increasing temperatures and aging stormwater and sewer systems and water reservoirs can exacerbate problems due to too much or too little water.

The literature is rife with observations of segments of the population being negatively affected by climate change, especially water-related hazards. There is consensus that these negative impacts of water-related climate change will be felt disproportionately among marginalized and low-income people.¹²²

Major Uncertainties and Research Gaps

There is moderate uncertainty about the degree to which land-surface changes will drive nonstationary changes to the volume and timing of water resources. There is a lack of research on the linkages between climate change and flooding.⁶⁸ There is uncertainty about the extent to which traditional design storms—that is, storms of particular intensity and duration, used in floodplain and built environment planning—and flooding assumptions based on older observations reflect current and future flood conditions.¹⁷¹ Additional research into the effects of climate change on water quality would improve our understanding of impacts, particularly in the face of compounding factors such as aging infrastructure, wildfires, and increased agricultural runoff.

Description of Confidence and Likelihood

There is strong evidence that climate change imparts a number of important shifts in local and regional hydrologic cycles, and that when combined with land-use changes and other human factors, increases are likely in the frequency, severity, duration, and damages from floods (high confidence) and drought impacts are increasing (medium confidence). There is a more limited body of work on the effects of climate change on water quality; thus there is medium confidence that climate change is degrading water quality. However, there is still uncertainty about how climate drivers may shape harmful algal blooms, a significant factor in water quality. Based on the vast literature documenting current, disparate impacts to frontline communities from floods, droughts, and the exposures they bring, there is high confidence and it is very likely that frontline communities will be at disproportionate risk from water-related hazards exacerbated by climate change.

KEY MESSAGE 4.3

Progress Toward Adaptation Has Been Uneven

Read about Confidence and Likelihood

Description of Evidence Base

There are many examples of climate change overtaking the speed of adaptation,¹⁶⁸^,¹⁶⁹ including communities caught off guard by extreme precipitation and drought events amplified by climate change.⁹⁶^,¹³³ A wide array of literature over the past decade has identified the safety and economic risks posed by aging water systems and changing hydrology.⁸⁷ Since the publication of NCA4, expanded data collection, improved climate projections, and better short- to midterm forecasts support better water resource management and planning. However, local water resource managers are still struggling to find accessible, usable science and data at the appropriate spatial scale, and they continue to rely on historical records that often do not reflect current and future water availability and timing. Disaster management literature contains many examples of public complacency and/or urgency in preparing for extreme events.²⁰⁸

A growing literature focuses on providing scientific information that is more usable for water resource planning and management.¹⁶⁴ There has been less work in assessing success and evaluating how equitable these approaches have been.²⁰⁹

A number of retrospective reviews highlight the omission of frontline, Tribal, and Indigenous voices and benefits from water projects.¹⁸⁰ Long-standing legal entitlements, established before climate change was a consideration, are well documented. The bulk of senior water rights and legal entitlements in the West are held by Tribes and agricultural water users but governed by state and federal decrees, agreements, and compacts that were not written to be flexible or responsive to a changing climate. Current literature is documenting these barriers and assessing emerging approaches to work past them.¹⁷⁷

Major Uncertainties and Research Gaps

Building climate resilience in hydrologic systems is challenging given the high uncertainty of climate variability and change. Gaps in actionable local-scale water data are particularly problematic, especially translating projections from global climate models to the regional and local level. System-level approaches and the use of resilience metrics are also areas ripe for improvement.

There is moderate uncertainty about the degree to which changes to land surface characteristics will drive changes to the volume and timing of water resources, and the degree to which existing infrastructure and historically defined allocations will be able to adapt. A large part of this uncertainty is related to how quickly human actions and policies react to hydrologic hazards.

Description of Confidence and Likelihood

Rising water-related disaster costs, communities ill-prepared for floods and droughts, and basin water users deferring difficult water allocation decisions are just a few of the pieces of evidence leading to high confidence that adaptation efforts are proceeding unevenly relative to the rate of climate change and that this is very likely (with high confidence) due in part to natural climate variability masking long-term changes. The history of water resources decision-making rarely includes participation by frontline, Tribal, or Indigenous individuals or communities. Their exclusion from negotiations, compacts, decrees, and other allocation actions supports an assessment of high confidence that frontline, Tribal, and Indigenous communities have not had full representation in water resources decision-making in the past, despite being affected by those decisions.

IPCC, 2021: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou, Eds. Cambridge University Press, Cambridge, UK and New York, NY, USA, 2391 pp. https://doi.org/10.1017/9781009157896
NCEI, 2023: U.S. Billion-Dollar Weather and Climate Disasters. National Oceanic and Atmospheric Administration, National Environmental Satellite, Data, and Information Service, National Centers for Environmental Information. https://www.ncei.noaa.gov/access/billions/
Lall, U., T. Johnson, P. Colohan, A. Aghakouchak, C. Brown, G. McCabe, R. Pulwarty, and A. Sankarasubramanian, 2018: Ch. 3. Water. In: Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II. Reidmiller, D.R., C.W. Avery, D. Easterling, K. Kunkel, K.L.M. Lewis, T.K. Maycock, and B.C. Stewart, Eds. U.S. Global Change Research Program, Washington, DC, USA, 145–173. https://doi.org/10.7930/nca4.2018.ch3
Delpla, I., A.-V. Jung, E. Baures, M. Clement, and O. Thomas, 2009: Impacts of climate change on surface water quality in relation to drinking water production. Environment International, 35 (8), 1225–1233. https://doi.org/10.1016/j.envint.2009.07.001
NCEH, 2022: Climate Effects on Health. Centers for Disease Control and Prevention, National Center for Environmental Health, accessed May 3, 2022. https://www.cdc.gov/climateandhealth/effects/default.htm
Nijhawan, A. and G. Howard, 2022: Associations between climate variables and water quality in low- and middle-income countries: A scoping review. Water Research, 210, 117996. https://doi.org/10.1016/j.watres.2021.117996
Shukla, N., S. Gupta, and S. Rai, 2023: Ch. 7. Potential impacts of climatic changes and human activity on water quality. In: Environmental Processes and Management: Tools and Practices for Groundwater. Shukla, P., P. Singh, and R.M. Singh, Eds. Springer, Cham, Switzerland, 103–111. https://doi.org/10.1007/978-3-031-20208-7_7
Chow, A.T., K.-P. Tsai, T.S. Fegel, D.N. Pierson, and C.C. Rhoades, 2019: Lasting effects of wildfire on disinfection by-product formation in forest catchments. Journal of Environmental Quality, 48 (6), 1826–1834. https://doi.org/10.2134/jeq2019.04.0172
Robinne, F.-N., D.W. Hallema, K.D. Bladon, M.D. Flannigan, G. Boisramé, C.M. Bréthaut, S.H. Doerr, G. Di Baldassarre, L.A. Gallagher, A.K. Hohner, S.J. Khan, A.M. Kinoshita, R. Mordecai, J.P. Nunes, P. Nyman, C. Santín, G. Sheridan, C.R. Stoof, M.P. Thompson, J.M. Waddington, and Y. Wei, 2021: Scientists' warning on extreme wildfire risks to water supply. Hydrological Processes, 35 (5), e14086. https://doi.org/10.1002/hyp.14086
Corson-Dosch, H.R., C.S. Nell, R.E. Volentine, A.A. Archer, E. Bechtel, J.L. Bruce, N. Felts, T.A. Gross, D. Lopez-Trujillo, C.E. Riggs, and E.K. Read, 2023: The Water Cycle. USGS General Information Product 221, 1 sheet. U.S. Geological Survey, Reston, VA. https://doi.org/10.3133/gip221
Jasinski, M.F., J.S. Borak, S.V. Kumar, D.M. Mocko, C.D. Peters-Lidard, M. Rodell, H. Rui, H.K. Beaudoing, B.E. Vollmer, K.R. Arsenault, B. Li, J.D. Bolten, and N. Tangdamrongsub, 2019: NCA-LDAs: Overview and analysis of hydrologic trends for the National Climate Assessment. Journal of Hydrometeorology, 20 (8), 1595–1617. https://doi.org/10.1175/jhm-d-17-0234.1
Albano, C.M., J.T. Abatzoglou, D.J. McEvoy, J.L. Huntington, C.G. Morton, M.D. Dettinger, and T.J. Ott, 2022: A Multidataset assessment of climatic drivers and uncertainties of recent trends in evaporative demand across the continental United States. Journal of Hydrometeorology, 23 (4), 505–519. https://doi.org/10.1175/jhm-d-21-0163.1
Tercek, M.T., D. Thoma, J.E. Gross, K. Sherrill, S. Kagone, and G. Senay, 2021: Historical changes in plant water use and need in the continental United States. PLoS ONE, 16 (9), e0256586. https://doi.org/10.1371/journal.pone.0256586
Hale, K.E., A.N. Wlostowski, A.M. Badger, K.N. Musselman, B. Livneh, and N.P. Molotch, 2022: Modeling streamflow sensitivity to climate warming and surface water inputs in a montane catchment. Journal of Hydrology: Regional Studies, 39, 100976. https://doi.org/10.1016/j.ejrh.2021.100976
Harpold, A.A., M.L. Kaplan, P.Z. Klos, T. Link, J.P. McNamara, S. Rajagopal, R. Schumer, and C.M. Steele, 2017: Rain or snow: Hydrologic processes, observations, prediction, and research needs. Hydrology and Earth System Sciences, 21 (1), 1–22. https://doi.org/10.5194/hess-21-1-2017
Musselman, K.N., N. Addor, J.A. Vano, and N.P. Molotch, 2021: Winter melt trends portend widespread declines in snow water resources. Nature Climate Change, 11 (5), 418–424. https://doi.org/10.1038/s41558-021-01014-9
Barnhart, T.B., N.P. Molotch, B. Livneh, A.A. Harpold, J.F. Knowles, and D. Schneider, 2016: Snowmelt rate dictates streamflow. Geophysical Research Letters, 43 (15), 8006–8016. https://doi.org/10.1002/2016gl069690
Evan, A. and I. Eisenman, 2021: A mechanism for regional variations in snowpack melt under rising temperature. Nature Climate Change, 11 (4), 326–330. https://doi.org/10.1038/s41558-021-00996-w
Harpold, A.A. and P.D. Brooks, 2018: Humidity determines snowpack ablation under a warming climate. Proceedings of the National Academy of Sciences of the United States of America, 115 (6), 1215–1220. https://doi.org/10.1073/pnas.1716789115
Musselman, K.N., M.P. Clark, C. Liu, K. Ikeda, and R. Rasmussen, 2017: Slower snowmelt in a warmer world. Nature Climate Change, 7 (3), 214–219. https://doi.org/10.1038/nclimate3225
Breckheimer, I.K., E.J. Theobald, N.C. Cristea, A.K. Wilson, J.D. Lundquist, R.M. Rochefort, and J. HilleRisLambers, 2020: Crowd-sourced data reveal social–ecological mismatches in phenology driven by climate. Frontiers in Ecology and the Environment, 18 (2), 76–82. https://doi.org/10.1002/fee.2142
Gergel, D.R., B. Nijssen, J.T. Abatzoglou, D.P. Lettenmaier, and M.R. Stumbaugh, 2017: Effects of climate change on snowpack and fire potential in the western USA. Climatic Change, 141 (2), 287–299. https://doi.org/10.1007/s10584-017-1899-y
Nolin, A.W. and C. Daly, 2006: Mapping “at risk” snow in the Pacific Northwest. Journal of Hydrometeorology, 7 (5), 1164–1171. https://doi.org/10.1175/jhm543.1
Siirila-Woodburn, E.R., A.M. Rhoades, B.J. Hatchett, L.S. Huning, J. Szinai, C. Tague, P.S. Nico, D.R. Feldman, A.D. Jones, W.D. Collins, and L. Kaatz, 2021: A low-to-no snow future and its impacts on water resources in the western United States. Nature Reviews Earth & Environment, 2 (11), 800–819. https://doi.org/10.1038/s43017-021-00219-y
Vano, J.A., B. Nijssen, and D.P. Lettenmaier, 2015: Seasonal hydrologic responses to climate change in the Pacific Northwest. Water Resources Research, 51 (4), 1959–1976. https://doi.org/10.1002/2014wr015909
Rauscher, S.A., J.S. Pal, N.S. Diffenbaugh, and M.M. Benedetti, 2008: Future changes in snowmelt-driven runoff timing over the western US. Geophysical Research Letters, 35 (16). https://doi.org/10.1029/2008gl034424
Li, D., M.L. Wrzesien, M. Durand, J. Adam, and D.P. Lettenmaier, 2017: How much runoff originates as snow in the western United States, and how will that change in the future? Geophysical Research Letters, 44 (12), 6163–6172. https://doi.org/10.1002/2017gl073551
Qin, Y., J.T. Abatzoglou, S. Siebert, L.S. Huning, A. AghaKouchak, J.S. Mankin, C. Hong, D. Tong, S.J. Davis, and N.D. Mueller, 2020: Agricultural risks from changing snowmelt. Nature Climate Change, 10 (5), 459–465. https://doi.org/10.1038/s41558-020-0746-8
Sturm, M., M.A. Goldstein, and C. Parr, 2017: Water and life from snow: A trillion dollar science question. Water Resources Research, 53 (5), 3534–3544. https://doi.org/10.1002/2017wr020840
Fountain, A.G., C. Gray, B. Glenn, B. Menounos, J. Pflug, and J.L. Riedel, 2022: Glaciers of the Olympic Mountains, Washington—The past and future 100 years. Journal of Geophysical Research: Earth Surface, 127 (4), e2022JF006670. https://doi.org/10.1029/2022jf006670
Hugonnet, R., R. McNabb, E. Berthier, B. Menounos, C. Nuth, L. Girod, D. Farinotti, M. Huss, I. Dussaillant, F. Brun, and A. Kääb, 2021: Accelerated global glacier mass loss in the early twenty-first century. Nature, 592 (7856), 726–731. https://doi.org/10.1038/s41586-021-03436-z
Menounos, B., R. Hugonnet, D. Shean, A. Gardner, I. Howat, E. Berthier, B. Pelto, C. Tennant, J. Shea, M.-J. Noh, F. Brun, and A. Dehecq, 2019: Heterogeneous changes in western North American glaciers linked to decadal variability in zonal wind strength. Geophysical Research Letters, 46 (1), 200–209. https://doi.org/10.1029/2018gl080942
Mizukami, N., A.J. Newman, J.S. Littell, T.W. Giambelluca, A.W. Wood, E.D. Gutmann, J.J. Hamman, D.R. Gergel, B. Nijssen, M.P. Clark, and J.R. Arnold, 2022: New projections of 21st century climate and hydrology for Alaska and Hawaiʻi. Climate Services, 27, 100312. https://doi.org/10.1016/j.cliser.2022.100312
Frans, C., E. Istanbulluoglu, D.P. Lettenmaier, A.G. Fountain, and J. Riedel, 2018: Glacier recession and the response of summer streamflow in the Pacific Northwest United States, 1960–2099. Water Resources Research, 54 (9), 6202–6225. https://doi.org/10.1029/2017wr021764
Yang, L., G. Sun, L. Zhi, and J. Zhao, 2018: Negative soil moisture-precipitation feedback in dry and wet regions. Scientific Reports, 8 (1), 4026. https://doi.org/10.1038/s41598-018-22394-7
Zhou, S., A.P. Williams, B.R. Lintner, A.M. Berg, Y. Zhang, T.F. Keenan, B.I. Cook, S. Hagemann, S.I. Seneviratne, and P. Gentine, 2021: Soil moisture–atmosphere feedbacks mitigate declining water availability in drylands. Nature Climate Change, 11 (1), 38–44. https://doi.org/10.1038/s41558-020-00945-z
Ford, T.W. and S.M. Quiring, 2019: Comparison of contemporary in situ, model, and satellite remote sensing soil moisture with a focus on drought monitoring. Water Resources Research, 55 (2), 1565–1582. https://doi.org/10.1029/2018wr024039
Cheng, S., J. Huang, F. Ji, and L. Lin, 2017: Uncertainties of soil moisture in historical simulations and future projections. Journal of Geophysical Research: Atmospheres, 122 (4), 2239–2253. https://doi.org/10.1002/2016jd025871
Deng, Y., S. Wang, X. Bai, G. Luo, L. Wu, Y. Cao, H. Li, C. Li, Y. Yang, Z. Hu, and S. Tian, 2020: Variation trend of global soil moisture and its cause analysis. Ecological Indicators, 110, 105939. https://doi.org/10.1016/j.ecolind.2019.105939
Gu, X., J. Li, Y.D. Chen, D. Kong, and J. Liu, 2019: Consistency and discrepancy of global surface soil moisture changes from multiple model-based data sets against satellite observations. Journal of Geophysical Research: Atmospheres, 124 (3), 1474–1495. https://doi.org/10.1029/2018jd029304
Su, L., Q. Cao, M. Xiao, D.M. Mocko, M. Barlage, D. Li, C.D. Peters-Lidard, and D.P. Lettenmaier, 2021: Drought variability over the conterminous United States for the past century. Journal of Hydrometeorology, 22 (5), 1153–1168. https://doi.org/10.1175/jhm-d-20-0158.1
Berg, A., J. Sheffield, and P.C.D. Milly, 2017: Divergent surface and total soil moisture projections under global warming. Geophysical Research Letters, 44 (1), 236–244. https://doi.org/10.1002/2016gl071921
Marvel, K., B.I. Cook, C. Bonfils, J.E. Smerdon, A.P. Williams, and H. Liu, 2021: Projected changes to hydroclimate seasonality in the continental United States. Earth's Future, 9 (9), e2021EF002019. https://doi.org/10.1029/2021ef002019
Condon, L.E., A.L. Atchley, and R.M. Maxwell, 2020: Evapotranspiration depletes groundwater under warming over the contiguous United States. Nature Communications, 11 (1), 873. https://doi.org/10.1038/s41467-020-14688-0
Maxwell, R.M. and S.J. Kollet, 2008: Interdependence of groundwater dynamics and land-energy feedbacks under climate change. Nature Geoscience, 1 (10), 665–669. https://doi.org/10.1038/ngeo315
Russo, T.A. and U. Lall, 2017: Depletion and response of deep groundwater to climate-induced pumping variability. Nature Geoscience, 10 (2), 105–108. https://doi.org/10.1038/ngeo2883
Condon, L.E. and R.M. Maxwell, 2019: Simulating the sensitivity of evapotranspiration and streamflow to large-scale groundwater depletion. Science Advances, 5 (6), 4574. https://doi.org/10.1126/sciadv.aav4574
Jasechko, S., H. Seybold, D. Perrone, Y. Fan, and J.W. Kirchner, 2021: Widespread potential loss of streamflow into underlying aquifers across the USA. Nature, 591 (7850), 391–395. https://doi.org/10.1038/s41586-021-03311-x
Lovelace, J.K., M.G. Nielsen, A.L. Read, C.J. Murphy, and M.A. Maupin, 2020: Estimated Groundwater Withdrawals from Principal Aquifers in the United States, 2015. Circular 1464. U.S. Geological Survey, Reston, VA, 70 pp. https://doi.org/10.3133/cir1464
Meixner, T., A.H. Manning, D.A. Stonestrom, D.M. Allen, H. Ajami, K.W. Blasch, A.E. Brookfield, C.L. Castro, J.F. Clark, D.J. Gochis, A.L. Flint, K.L. Neff, R. Niraula, M. Rodell, B.R. Scanlon, K. Singha, and M.A. Walvoord, 2016: Implications of projected climate change for groundwater recharge in the western United States. Journal of Hydrology, 534, 124–138. https://doi.org/10.1016/j.jhydrol.2015.12.027
Niraula, R., T. Meixner, F. Dominguez, N. Bhattarai, M. Rodell, H. Ajami, D. Gochis, and C. Castro, 2017: How might recharge change under projected climate change in the western U.S.? Geophysical Research Letters, 44 (20), 10407–10418. https://doi.org/10.1002/2017gl075421
Alam, S., M. Gebremichael, R. Li, J. Dozier, and D.P. Lettenmaier, 2019: Climate change impacts on groundwater storage in the Central Valley, California. Climatic Change, 157 (3), 387–406. https://doi.org/10.1007/s10584-019-02585-5
Taylor, R., 2014: When wells run dry. Nature, 516 (7530), 179–180. https://doi.org/10.1038/516179a
Wada, Y. and M.F.P. Bierkens, 2014: Sustainability of global water use: Past reconstruction and future projections. Environmental Research Letters, 9 (10), 104003. https://doi.org/10.1088/1748-9326/9/10/104003
Hanson, R.T., L.E. Flint, A.L. Flint, M.D. Dettinger, C.C. Faunt, D. Cayan, and W. Schmid, 2012: A method for physically based model analysis of conjunctive use in response to potential climate changes. Water Resources Research, 48 (6). https://doi.org/10.1029/2011wr010774
Scanlon, B.R., C.C. Faunt, L. Longuevergne, R.C. Reedy, W.M. Alley, V.L. McGuire, and P.B. McMahon, 2012: Groundwater depletion and sustainability of irrigation in the US High Plains and Central Valley. Proceedings of the National Academy of Sciences of the United States of America, 109 (24), 9320–9325. https://doi.org/10.1073/pnas.1200311109
Dudley, R.W., G.A. Hodgkins, M.R. McHale, M.J. Kolian, and B. Renard, 2017: Trends in snowmelt-related streamflow timing in the conterminous United States. Journal of Hydrology, 547, 208–221. https://doi.org/10.1016/j.jhydrol.2017.01.051
Das, T., D.W. Pierce, D.R. Cayan, J.A. Vano, and D.P. Lettenmaier, 2011: The importance of warm season warming to western U.S. streamflow changes. Geophysical Research Letters, 38 (23). https://doi.org/10.1029/2011gl049660
Chegwidden, O.S., D.E. Rupp, and B. Nijssen, 2020: Climate change alters flood magnitudes and mechanisms in climatically-diverse headwaters across the northwestern United States. Environmental Research Letters, 15 (9), 094048. https://doi.org/10.1088/1748-9326/ab986f
Naz, B.S., S.-C. Kao, M. Ashfaq, H. Gao, D. Rastogi, and S. Gangrade, 2018: Effects of climate change on streamflow extremes and implications for reservoir inflow in the United States. Journal of Hydrology, 556, 359–370. https://doi.org/10.1016/j.jhydrol.2017.11.027
England Jr., J.F., T.A. Cohn, B.A. Faber, J.R. Stedinger, W.O. Thomas Jr., A.G. Veilleux, J.E. Kiang, and R.R. Mason Jr., 2019: Guidelines for Determining Flood Flow Frequency—Bulletin 17C. Techniques and Methods, book 4, chap. B5. U.S. Geological Survey, Reston, VA, 148 pp. https://doi.org/10.3133/tm4b5
Serinaldi, F. and C.G. Kilsby, 2015: Stationarity is undead: Uncertainty dominates the distribution of extremes. Advances in Water Resources, 77, 17–36. https://doi.org/10.1016/j.advwatres.2014.12.013
Sivapalan, M. and J.M. Samuel, 2009: Transcending limitations of stationarity and the return period: Process-based approach to flood estimation and risk assessment. Hydrological Processes, 23 (11), 1671–1675. https://doi.org/10.1002/hyp.7292
Yu, G., D.B. Wright, Z. Zhu, C. Smith, and K.D. Holman, 2019: Process-based flood frequency analysis in an agricultural watershed exhibiting nonstationary flood seasonality. Hydrology and Earth System Sciences, 23 (5), 2225–2243. https://doi.org/10.5194/hess-23-2225-2019
Hodgkins, G.A., R.W. Dudley, S.A. Archfield, and B. Renard, 2019: Effects of climate, regulation, and urbanization on historical flood trends in the United States. Journal of Hydrology, 573, 697–709. https://doi.org/10.1016/j.jhydrol.2019.03.102
Yu, G., D.B. Wright, and Z. Li, 2020: The upper tail of precipitation in convection-permitting regional climate models and their utility in nonstationary rainfall and flood frequency analysis. Earth's Future, 8 (10), e2020EF001613. https://doi.org/10.1029/2020ef001613
Grillakis, M.G., A.G. Koutroulis, J. Komma, I.K. Tsanis, W. Wagner, and G. Blöschl, 2016: Initial soil moisture effects on flash flood generation–A comparison between basins of contrasting hydro-climatic conditions. Journal of Hydrology, 541, 206–217. https://doi.org/10.1016/j.jhydrol.2016.03.007
Sharma, A., C. Wasko, and D.P. Lettenmaier, 2018: If precipitation extremes are increasing, why aren’t floods? Water Resources Research, 54 (11), 8545–8551. https://doi.org/10.1029/2018wr023749
Wasko, C. and R. Nathan, 2019: Influence of changes in rainfall and soil moisture on trends in flooding. Journal of Hydrology, 575, 432–441. https://doi.org/10.1016/j.jhydrol.2019.05.054
Il Jeong, D. and L. Sushama, 2018: Rain-on-snow events over North America based on two Canadian regional climate models. Climate Dynamics, 50 (1), 303–316. https://doi.org/10.1007/s00382-017-3609-x
Musselman, K.N., F. Lehner, K. Ikeda, M.P. Clark, A.F. Prein, C. Liu, M. Barlage, and R. Rasmussen, 2018: Projected increases and shifts in rain-on-snow flood risk over western North America. Nature Climate Change, 8 (9), 808–812. https://doi.org/10.1038/s41558-018-0236-4
Ebel, B.A., 2020: Temporal evolution of measured and simulated infiltration following wildfire in the Colorado Front Range, USA: Shifting thresholds of runoff generation and hydrologic hazards. Journal of Hydrology, 585, 124765. https://doi.org/10.1016/j.jhydrol.2020.124765
Williams, A.P., B.I. Cook, and J.E. Smerdon, 2022: Rapid intensification of the emerging southwestern North American megadrought in 2020–2021. Nature Climate Change, 12 (3), 232–234. https://doi.org/10.1038/s41558-022-01290-z
Hettiarachchi, S., C. Wasko, and A. Sharma, 2018: Increase in flood risk resulting from climate change in a developed urban watershed—The role of storm temporal patterns. Hydrology and Earth System Sciences, 22 (3), 2041–2056. https://doi.org/10.5194/hess-22-2041-2018
Hobbins, M., I. Rangwala, J. Barsugli, and C. Dewes, 2019: Ch. 25. Extremes in evaporative demand and their implications for droughts and drought monitoring in the 21st century. In: Extreme Hydrology and Climate Variability. Melesse, A.M., W. Abtew, and G. Senay, Eds. Elsevier, 325–341. https://doi.org/10.1016/b978-0-12-815998-9.00025-7
Overpeck, J.T. and B. Udall, 2020: Climate change and the aridification of North America. Proceedings of the National Academy of Sciences of the United States of America, 117 (22), 11856–11858. https://doi.org/10.1073/pnas.2006323117
Fosu, B.O., S.-Y. Simon Wang, and J.-H. Yoon, 2016: The 2014/15 snowpack drought in Washington State and its climate forcing. Bulletin of the American Meteorological Society, 97 (12), S19–S24. https://doi.org/10.1175/bams-d-16-0154.1
Belmecheri, S., F. Babst, E.R. Wahl, D.W. Stahle, and V. Trouet, 2016: Multi-century evaluation of Sierra Nevada snowpack. Nature Climate Change, 6 (1), 2–3. https://doi.org/10.1038/nclimate2809
Margulis, S.A., G. Cortés, M. Girotto, L.S. Huning, D. Li, and M. Durand, 2016: Characterizing the extreme 2015 snowpack deficit in the Sierra Nevada (USA) and the implications for drought recovery. Geophysical Research Letters, 43 (12), 6341–6349. https://doi.org/10.1002/2016gl068520
Carlton, J., 2015: Snow drought saps Washington state’s economy. Wall Street Journal, July 1, 2015. https://www.wsj.com/articles/snow-drought-saps-washington-states-economy-1435801630
Sugg, Z.P., 2018: An equity autopsy: Exploring the role of water rights in water allocations and impacts for the Central Valley Project during the 2012–2016 California drought. Resources, 7 (1), 12. https://doi.org/10.3390/resources7010012
Davenport, F.V., M. Burke, and N.S. Diffenbaugh, 2021: Contribution of historical precipitation change to US flood damages. Proceedings of the National Academy of Sciences of the United States of America, 118 (4), e2017524118. https://doi.org/10.1073/pnas.2017524118
Alaoui, A., M. Rogger, S. Peth, and G. Blöschl, 2018: Does soil compaction increase floods? A review. Journal of Hydrology, 557, 631–642. https://doi.org/10.1016/j.jhydrol.2017.12.052
Mississippi River Commission, 2012: 2011 Mississippi River and Tributaries Flood Report. Mississippi River Commission, 45 pp. https://www.mvd.usace.army.mil/portals/52/docs/mrc/mrc_2011_flood_report.pdf
Harris, A.R., E.N. Fidan, N.G. Nelson, R.E. Emanuel, T. Jass, S. Kathariou, J. Niedermeyer, M. Sharara, F.L. de los Reyes, D.A. Riveros-Iregui, and J.R. Stewart, 2021: Microbial contamination in environmental waters of rural and agriculturally-dominated landscapes following Hurricane Florence. ACS ES&T Water, 1 (9), 2012–2019. https://doi.org/10.1021/acsestwater.1c00103
Schaffer-Smith, D., S.W. Myint, R.L. Muenich, D. Tong, and J.E. DeMeester, 2020: Repeated hurricanes reveal risks and opportunities for social-ecological resilience to flooding and water quality problems. Environmental Science & Technology, 54 (12), 7194–7204. https://doi.org/10.1021/acs.est.9b07815
ASCE, 2021: A Comprehensive Assessment of America’s Infrastructure: 2021 Report Card for America’s Infrastructure. American Society of Civil Engineers. https://infrastructurereportcard.org/
Roseboro, A., M.N. Torres, Z. Zhu, and A.J. Rabideau, 2021: The impacts of climate change and porous pavements on combined sewer overflows: A case study of the city of Buffalo, New York, USA. Frontiers in Water, 3, 725174. https://doi.org/10.3389/frwa.2021.725174
National Academies of Sciences, Engineering, and Medicine, 2019: Framing the Challenge of Urban Flooding in the United States. The National Academies Press, Washington, DC, 100 pp. https://doi.org/10.17226/25381
Pieper, K.J., C.N. Jones, W.J. Rhoads, M. Rome, D.M. Gholson, A. Katner, D.E. Boellstorff, and R.E. Beighley, 2021: Microbial contamination of drinking water supplied by private wells after Hurricane Harvey. Environmental Science & Technology, 55 (12), 8382–8392. https://doi.org/10.1021/acs.est.0c07869
Wells, M.L., B. Karlson, A. Wulff, R. Kudela, C. Trick, V. Asnaghi, E. Berdalet, W. Cochlan, K. Davidson, M. De Rijcke, S. Dutkiewicz, G. Hallegraeff, K.J. Flynn, C. Legrand, H. Paerl, J. Silke, S. Suikkanen, P. Thompson, and V.L. Trainer, 2020: Future HAB science: Directions and challenges in a changing climate. Harmful Algae, 91, 101632. https://doi.org/10.1016/j.hal.2019.101632
AghaKouchak, A., A. Mirchi, K. Madani, G. Di Baldassarre, A. Nazemi, A. Alborzi, H. Anjileli, M. Azarderakhsh, F. Chiang, E. Hassanzadeh, L.S. Huning, I. Mallakpour, A. Martinez, O. Mazdiyasni, H. Moftakhari, H. Norouzi, M. Sadegh, D. Sadeqi, A.F. Van Loon, and N. Wanders, 2021: Anthropogenic drought: Definition, challenges, and opportunities. Reviews of Geophysics, 59 (2), e2019RG000683. https://doi.org/10.1029/2019rg000683
Crausbay, S.D., J. Betancourt, J. Bradford, J. Cartwright, W.C. Dennison, J. Dunham, C.A.F. Enquist, A.G. Frazier, K.R. Hall, J.S. Littell, C.H. Luce, R. Palmer, A.R. Ramirez, I. Rangwala, L. Thompson, B.M. Walsh, and S. Carter, 2020: Unfamiliar territory: Emerging themes for ecological drought research and management. One Earth, 3 (3), 337–353. https://doi.org/10.1016/j.oneear.2020.08.019
Mankin, J.S., I. Simpson, A. Hoell, R. Fu, J. Lisonbee, A. Sheffield, and D. Barrie, 2021: NOAA Drought Task Force Report on the 2020–2021 Southwestern U.S. Drought. National Oceanic and Atmospheric Administration Drought Task Force, Modeling, Analysis, Predictions, and Projections Program, and National Integrated Drought Information System. https://www.drought.gov/documents/noaa-drought-task-force-report-2020-2021-southwestern-us-drought
Overpeck, J.T., 2013: The challenge of hot drought. Nature, 503 (7476), 350–351. https://doi.org/10.1038/503350a
Otkin, J.A., M. Woloszyn, H. Wang, M. Svoboda, M. Skumanich, R. Pulwarty, J. Lisonbee, A. Hoell, M. Hobbins, T. Haigh, and A.E. Cravens, 2022: Getting ahead of flash drought: From early warning to early action. Bulletin of the American Meteorological Society, 103 (10), E2188–E2202. https://doi.org/10.1175/bams-d-21-0288.1
Svoboda, M., D. LeComte, M. Hayes, R. Heim, K. Gleason, J. Angel, B. Rippey, R. Tinker, M. Palecki, D. Stooksbury, D. Miskus, and S. Stephens, 2002: The Drought Monitor. Bulletin of the American Meteorological Society, 83 (8), 1181–1190. https://doi.org/10.1175/1520-0477-83.8.1181
Cook, B.I., J.E. Smerdon, E.R. Cook, A.P. Williams, K.J. Anchukaitis, J.S. Mankin, K. Allen, L. Andreu-Hayles, T.R. Ault, S. Belmecheri, S. Coats, B. Coulthard, B. Fosu, P. Grierson, D. Griffin, D.A. Herrera, M. Ionita, F. Lehner, C. Leland, K. Marvel, M.S. Morales, V. Mishra, J. Ngoma, H.T.T. Nguyen, A. O’Donnell, J. Palmer, M.P. Rao, M. Rodriguez-Caton, R. Seager, D.W. Stahle, S. Stevenson, U.K. Thapa, A.M. Varuolo-Clarke, and E.K. Wise, 2022: Megadroughts in the Common Era and the Anthropocene. Nature Reviews Earth & Environment, 3, 735–745. https://doi.org/10.1038/s43017-022-00329-1
Gangopadhyay, S., C.A. Woodhouse, G.J. McCabe, C.C. Routson, and D.M. Meko, 2022: Tree rings reveal unmatched 2nd century drought in the Colorado River Basin. Geophysical Research Letters, 49 (11), e2022GL098781. https://doi.org/10.1029/2022gl098781
Lachniet, M.S., 2020: Illuminating the meaning of Asian monsoon cave speleothem records. Paleoceanography and Paleoclimatology, 35 (1), e2019PA003841. https://doi.org/10.1029/2019pa003841
Martin, J.T., G.T. Pederson, C.A. Woodhouse, E.R. Cook, G.J. McCabe, K.J. Anchukaitis, E.K. Wise, P.J. Erger, L. Dolan, M. McGuire, S. Gangopadhyay, K.J. Chase, J.S. Littell, S.T. Gray, S. St. George, J.M. Friedman, D.J. Sauchyn, J.-M. St-Jacques, and J. King, 2020: Increased drought severity tracks warming in the United States’ largest river basin. Proceedings of the National Academy of Sciences of the United States of America, 117 (21), 11328–11336. https://doi.org/10.1073/pnas.1916208117
Williams, A.P., E.R. Cook, J.E. Smerdon, B.I. Cook, J.T. Abatzoglou, K. Bolles, S.H. Baek, A.M. Badger, and B. Livneh, 2020: Large contribution from anthropogenic warming to an emerging North American megadrought. Science, 368 (6488), 314–318. https://doi.org/10.1126/science.aaz9600
Boyer, J.S., P. Byrne, K.G. Cassman, M. Cooper, D. Delmer, T. Greene, F. Gruis, J. Habben, N. Hausmann, N. Kenny, R. Lafitte, S. Paszkiewicz, D. Porter, A. Schlegel, J. Schussler, T. Setter, J. Shanahan, R.E. Sharp, T.J. Vyn, D. Warner, and J. Gaffney, 2013: The U.S. drought of 2012 in perspective: A call to action. Global Food Security, 2 (3), 139–143. https://doi.org/10.1016/j.gfs.2013.08.002
Van Loon, A.F., 2015: Hydrological drought explained. WIREs Water, 2 (4), 359–392. https://doi.org/10.1002/wat2.1085
Turner, S.W., N. Voisin, K.D. Nelson, and V.C. Tidwell, 2022: Drought Impacts on Hydroelectric Power Generation in the Western United States: A Multiregional Analysis of 21st Century Hydropower Generation. PNNL-33212. U.S. Department of Energy, Pacific Northwest National Laboratory, Richland, WA. https://doi.org/10.2172/1887470
Crausbay, S.D., A.R. Ramirez, S.L. Carter, M.S. Cross, K.R. Hall, D.J. Bathke, J.L. Betancourt, S. Colt, A.E. Cravens, M.S. Dalton, J.B. Dunham, L.E. Hay, M.J. Hayes, J. McEvoy, C.A. McNutt, M.A. Moritz, K.H. Nislow, N. Raheem, and T. Sanford, 2017: Defining ecological drought for the twenty-first century. Bulletin of the American Meteorological Society, 98 (12), 2543–2550. https://doi.org/10.1175/bams-d-16-0292.1
EcoAdapt, 2021: Freshwater Marshes, Wetlands, and Ponds: Climate Change Vulnerability Assessment Summary for the Santa Cruz Mountains Climate Adaptation Project. EcoAdapt, Bainbridge Island, WA, 7 pp. https://ecoadapt.org/data/documents/EcoAdapt_SantaCruzMtnsVASummary_Freshwatermarsheswetlandsandponds_FINAL_Mar2021.pdf
Poff, B., K.A. Koestner, D.G. Neary, and D. Merritt, 2012: Threats to Western United States Riparian Ecosystems: A Bibliography. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fort Collins, CO, 78 pp. https://doi.org/10.2737/rmrs-gtr-269
Vose, J.M., J.S. Clark, C.H. Luce, and T. Patel-Weynand, 2016: Effects of Drought on Forests and Rangelands in the United States: A Comprehensive Science Synthesis. Gen. Tech. Rep. WO-93b. U.S. Department of Agriculture, Forest Service, Washington Office, Washington, DC, 289 pp. https://doi.org/10.2737/wo-gtr-93b
Höök, T.O., C.J. Foley, P. Collingsworth, L. Dorworth, B. Fisher, J.T. Hoverman, E. LaRue, M. Pyron, and J. Tank, 2020: An assessment of the potential impacts of climate change on freshwater habitats and biota of Indiana, USA. Climatic Change, 163 (4), 1897–1916. https://doi.org/10.1007/s10584-019-02502-w
Johnson, W.C., B. Werner, and G.R. Guntenspergen, 2016: Non-linear responses of glaciated prairie wetlands to climate warming. Climatic Change, 134 (1), 209–223. https://doi.org/10.1007/s10584-015-1534-8
Riedel, T., 2019: Temperature-associated changes in groundwater quality. Journal of Hydrology, 572, 206–212. https://doi.org/10.1016/j.jhydrol.2019.02.059
Bloomfield, J.P., B.P. Marchant, and A.A. McKenzie, 2019: Changes in groundwater drought associated with anthropogenic warming. Hydrology and Earth System Sciences, 23 (3), 1393–1408. https://doi.org/10.5194/hess-23-1393-2019
Galloway, D.L., D. R. Jones, and S.E. Ingebritsen, 1999: Land Subsidence in the United States. USGS Numbered Series 1182. U.S. Geological Survey, 177 pp. https://doi.org/10.3133/cir1182
Borunda, A., 2019: We pump too much water out of the ground—And that’s killing our rivers. National Geographic. https://www.nationalgeographic.com/science/article/groundwater-pumping-killing-rivers-streams
CJWG, 2017: Advancing Climate Justice in California: Guiding Principles and Recommendations for Policy and Funding Decisions. California Climate Justice Working Group. https://www.healthyworldforall.org/en/express-img/17081516-3570-img1.pdf
Goldsmith, L., V. Raditz, and M. Méndez, 2022: Queer and present danger: Understanding the disparate impacts of disasters on LGBTQ+ communities. Disasters, 46 (4), 946–973. https://doi.org/10.1111/disa.12509
Nelson, M., R. Ehrenfeucht, T. Birch, and A. Brand, 2022: Getting by and getting out: How residents of Louisiana’s frontline communities are adapting to environmental change. Housing Policy Debate, 32 (1), 84–101. https://doi.org/10.1080/10511482.2021.1925944
Edmonds, D.A., R.L. Caldwell, E.S. Brondizio, and S.M.O. Siani, 2020: Coastal flooding will disproportionately impact people on river deltas. Nature Communications, 11 (1), 4741. https://doi.org/10.1038/s41467-020-18531-4
Reckien, D., F. Creutzig, B. Fernandez, S. Lwasa, M. Tovar-Restrepo, D. Mcevoy, and D. Satterthwaite, 2017: Climate change, equity and the Sustainable Development Goals: An urban perspective. Environment and Urbanization, 29 (1), 159–182. https://doi.org/10.1177/0956247816677778
Thiault, L., C. Mora, J.E. Cinner, W.W.L. Cheung, N.A.J. Graham, F.A. Januchowski-Hartley, D. Mouillot, U.R. Sumaila, and J. Claudet, 2019: Escaping the perfect storm of simultaneous climate change impacts on agriculture and marine fisheries. Science Advances, 5 (11), 9976. https://doi.org/10.1126/sciadv.aaw9976
Ebi, K.L., J. Vanos, J.W. Baldwin, J.E. Bell, D.M. Hondula, N.A. Errett, K. Hayes, C.E. Reid, S. Saha, J. Spector, and P. Berry, 2021: Extreme weather and climate change: Population health and health system implications. Annual Review of Public Health, 42 (1), 293–315. https://doi.org/10.1146/annurev-publhealth-012420-105026
EPA, 2022: Climate Change and Human Health: Who’s Most at Risk? U.S. Environmental Protection Agency, accessed March 20, 2022. https://www.epa.gov/climate-change/climate-change-and-human-health-whos-most-risk
Leffers, J.M., 2022: Climate change and health of children: Our borrowed future. Journal of Pediatric Health Care, 36 (1), 12–19. https://doi.org/10.1016/j.pedhc.2021.09.002
Bronen, R., J. Maldonado, E. Marino, and P. Hardison, 2018: Ch. 12. Climate change and displacement: Challenges and needs to address an imminent reality. In: Challenging the Prevailing Paradigm of Displacement and Resettlement: Risks, Impoverishment, Legacies, Solutions. Cernea, M.M. and J.K. Maldonado, Eds. Routledge, London, UK, 252–272. https://doi.org/10.4324/9781315163062
MacDonald, J.P., A.C. Willox, J.D. Ford, I. Shiwak, and M. Wood, 2015: Protective factors for mental health and well-being in a changing climate: Perspectives from Inuit youth in Nunatsiavut, Labrador. Social Science & Medicine, 141, 133–141. https://doi.org/10.1016/j.socscimed.2015.07.017
Marino, E., 2015: Fierce Climate, Sacred Ground: An Ethnography of Climate Change in Shishmaref, Alaska. University of Alaska Press, Fairbanks, AK, 122 pp. https://upcolorado.com/university-of-alaska-press/item/5674-fierce-climate-sacred-ground
Tate, E., M.A. Rahman, C.T. Emrich, and C.C. Sampson, 2021: Flood exposure and social vulnerability in the United States. Natural Hazards, 106 (1), 435–457. https://doi.org/10.1007/s11069-020-04470-2
Titus, J.G., 2023: Population in floodplains or close to sea level increased in US but declined in some counties—Especially among black residents. Environmental Research Letters, 18 (3), 034001. https://doi.org/10.1088/1748-9326/acadf5
Wing, O.E.J., W. Lehman, P.D. Bates, C.C. Sampson, N. Quinn, A.M. Smith, J.C. Neal, J.R. Porter, and C. Kousky, 2022: Inequitable patterns of US flood risk in the Anthropocene. Nature Climate Change, 12 (2), 156–162. https://doi.org/10.1038/s41558-021-01265-6
Engström, J., K. Jafarzadegan, and H. Moradkhani, 2020: Drought vulnerability in the United States: An integrated assessment. Water, 12 (7), 2033. https://doi.org/10.3390/w12072033
NHC, 2018: Costliest U.S. Tropical Cyclones Tables Updated. National Oceanic and Atmospheric Administration, National Weather Service, National Hurricane Center, Miami, FL, 3 pp. https://www.nhc.noaa.gov/news/UpdatedCostliest.pdf
Risser, M.D. and M.F. Wehner, 2017: Attributable human-induced changes in the likelihood and magnitude of the observed extreme precipitation during Hurricane Harvey. Geophysical Research Letters, 44 (24), 12457–12464. https://doi.org/10.1002/2017gl075888
van Oldenborgh, G.J., K. van der Wiel, A. Sebastian, R. Singh, J. Arrighi, F. Otto, K. Haustein, S. Li, G. Vecchi, and H. Cullen, 2017: Attribution of extreme rainfall from Hurricane Harvey, August 2017. Environmental Research Letters, 12 (12), 124009. https://doi.org/10.1088/1748-9326/aa9ef2
Wang, S.Y.S., L. Zhao, J.-H. Yoon, P. Klotzbach, and R.R. Gillies, 2018: Quantitative attribution of climate effects on Hurricane Harvey’s extreme rainfall in Texas. Environmental Research Letters, 13 (5), 054014. https://doi.org/10.1088/1748-9326/aabb85
Wehner, M. and C. Sampson, 2021: Attributable human-induced changes in the magnitude of flooding in the Houston, Texas region during Hurricane Harvey. Climatic Change, 166 (1), 1–13. https://doi.org/10.1007/s10584-021-03114-z
Smiley, K.T., I. Noy, M.F. Wehner, D. Frame, C.C. Sampson, and O.E.J. Wing, 2022: Social inequalities in climate change-attributed impacts of Hurricane Harvey. Nature Communications, 13 (1), 3418. https://doi.org/10.1038/s41467-022-31056-2
Smiley, K.T., 2020: Social inequalities in flooding inside and outside of floodplains during Hurricane Harvey. Environmental Research Letters, 15 (9), 0940b3. https://doi.org/10.1088/1748-9326/aba0fe
Chakraborty, J., S.E. Grineski, and T.W. Collins, 2019: Hurricane Harvey and people with disabilities: Disproportionate exposure to flooding in Houston, Texas. Social Science & Medicine, 226, 176–181. https://doi.org/10.1016/j.socscimed.2019.02.039
Chakraborty, J., A.A. McAfee, T.W. Collins, and S.E. Grineski, 2021: Exposure to Hurricane Harvey flooding for subsidized housing residents of Harris County, Texas. Natural Hazards, 106 (3), 2185–2205. https://doi.org/10.1007/s11069-021-04536-9
Vacs Renwick, D., A. Heinrich, R. Weisman, H. Arvanaghi, and K. Rotert, 2019: Potential public health impacts of deteriorating distribution system infrastructure. Journal-American Water Works Association, 111 (2), 42–53. https://doi.org/10.1002/awwa.1235
Mueller, J.T. and S. Gasteyer, 2021: The widespread and unjust drinking water and clean water crisis in the United States. Nature Communications, 12 (1), 3544. https://doi.org/10.1038/s41467-021-23898-z
Cozzetto, K., K. Chief, K. Dittmer, M. Brubaker, R. Gough, K. Souza, F. Ettawageshik, S. Wotkyns, S. Opitz-Stapleton, S. Duren, and P. Chavan, 2014: Ch. 6. Climate change impacts on the water resources of American Indians and Alaska Natives in the U.S. In: Climate Change and Indigenous Peoples in the United States: Impacts, Experiences and Actions. Maldonado, J.K., R.E. Pandya, and B.J. Colombi, Eds. Springer, Cham, Switzerland, 61–76. https://doi.org/10.1007/978-3-319-05266-3_6
NCAI, 2017: Tribal Infrastructure: Investing in Indian Country for a Stronger America. National Congress of American Indians, Washington, DC. http://www.ncai.org/ncai-infrastructurereport-final.pdf
Tanana, H., J. Combs, and A. Hoss, 2021: Water is life: Law, systemic racism, and water security in Indian Country. Health Security, 19 (1), 78–82. https://doi.org/10.1089/hs.2021.0034
IHS, 2019: Annual Report to the Congress of the United States on Sanitation Deficiency Levels for Indian Homes and Communities. Indian Health Service, Office of Environmental Health and Engineering. https://www.ihs.gov/sites/newsroom/themes/responsive2017/display_objects/documents/FY_2019_RTC_Sanitation_Deficiencies_Report.pdf
Anderson, E.P., S. Jackson, R.E. Tharme, M. Douglas, J.E. Flotemersch, M. Zwarteveen, C. Lokgariwar, M. Montoya, A. Wali, G.T. Tipa, T.D. Jardine, J.D. Olden, L. Cheng, J. Conallin, B. Cosens, C. Dickens, D. Garrick, D. Groenfeldt, J. Kabogo, D.J. Roux, A. Ruhi, and A.H. Arthington, 2019: Understanding rivers and their social relations: A critical step to advance environmental water management. WIREs Water, 6 (6), e1381. https://doi.org/10.1002/wat2.1381
Carey, J., 2020: Managed retreat increasingly seen as necessary in response to climate change’s fury. Proceedings of the National Academy of Sciences of the United States of America, 117 (24), 13182–13185. https://doi.org/10.1073/pnas.2008198117
Li, J. and K. Spidalieri, 2021: Home is where the safer ground is: The need to promote affordable housing laws and policies in receiving communities. Journal of Environmental Studies and Sciences, 11 (4), 682–695. https://doi.org/10.1007/s13412-021-00702-4
ASFPM, 2014: No Adverse Impact Planning How-To Guide. Association of State Floodplain Managers. https://asfpm-library.s3-us-west-2.amazonaws.com/FSC/NAI/ASFPM_NAI_Planning_2014.pdf
Chang, S., W. Graham, J. Geurink, N. Wanakule, and T. Asefa, 2018: Evaluation of impacts of future climate change and water use scenarios on regional hydrology. Hydrology and Earth System Sciences, 22 (9), 4793–4813. https://doi.org/10.5194/hess-22-4793-2018
SNWA, 2019: Joint Water Conservation Plan. Southern Nevada Water Authority. https://www.snwa.com/assets/pdf/reports-conservation-plan-2019.pdf
Smith, R., E. Zagona, J. Kasprzyk, N. Bonham, E. Alexander, A. Butler, J. Prairie, and C. Jerla, 2022: Decision science can help address the challenges of long-term planning in the Colorado River Basin. Journal of the American Water Resources Association, 58 (5), 735–745. https://doi.org/10.1111/1752-1688.12985
Woodhouse, C.A., R.M. Smith, S.A. McAfee, G.T. Pederson, G.J. McCabe, W.P. Miller, and A. Csank, 2021: Upper Colorado River Basin 20th century droughts under 21st century warming: Plausible scenarios for the future. Climate Services, 21, 100206. https://doi.org/10.1016/j.cliser.2020.100206
Fleming, S.W., D.C. Garen, A.G. Goodbody, C.S. McCarthy, and L.C. Landers, 2021: Assessing the new Natural Resources Conservation Service water supply forecast model for the American West: A challenging test of explainable, automated, ensemble artificial intelligence. Journal of Hydrology, 602, 126782. https://doi.org/10.1016/j.jhydrol.2021.126782
Goharian, E., S.J. Burian, and M. Karamouz, 2018: Using joint probability distribution of reliability and vulnerability to develop a water system performance index. Journal of Water Resources Planning and Management, 144 (2), 04017081. https://doi.org/10.1061/(asce)wr.1943-5452.0000869
Steinschneider, S., R. McCrary, S. Wi, K. Mulligan, L.O. Mearns, and C. Brown, 2015: Expanded decision-scaling framework to select robust long-term water-system plans under hydroclimatic uncertainties. Journal of Water Resources Planning and Management, 141 (11), 04015023. https://doi.org/10.1061/(asce)wr.1943-5452.0000536
Towler, E., D. Woodson, S. Baker, M. Ge, J. Prairie, B. Rajagopalan, S. Shanahan, and R. Smith, 2022: Incorporating mid-term temperature predictions into streamflow forecasts and operational reservoir projections in the Colorado River Basin. Journal of Water Resources Planning and Management, 148 (4), 04022007. https://doi.org/10.1061/(asce)wr.1943-5452.0001534
Kim, Y., T. Carvalhaes, A. Helmrich, S. Markolf, R. Hoff, M. Chester, R. Li, and N. Ahmad, 2022: Leveraging SETS resilience capabilities for safe-to-fail infrastructure under climate change. Current Opinion in Environmental Sustainability, 54, 101153. https://doi.org/10.1016/j.cosust.2022.101153
Kim, Y., D.A. Eisenberg, E.N. Bondank, M.V. Chester, G. Mascaro, and B.S. Underwood, 2017: Fail-safe and safe-to-fail adaptation: Decision-making for urban flooding under climate change. Climatic Change, 145 (3), 397–412. https://doi.org/10.1007/s10584-017-2090-1
Salinas Rodriguez, C.N.A., R. Ashley, B. Gersonius, J. Rijke, A. Pathirana, and C. Zevenbergen, 2014: Incorporation and application of resilience in the context of water-sensitive urban design: Linking European and Australian perspectives. WIREs Water, 1 (2), 173–186. https://doi.org/10.1002/wat2.1017
Gorelick, D.E., L. Lin, H.B. Zeff, Y. Kim, J.M. Vose, J.W. Coulston, D.N. Wear, L.E. Band, P.M. Reed, and G.W. Characklis, 2020: Accounting for adaptive water supply management when quantifying climate and land cover change vulnerability. Water Resources Research, 56 (1), e2019WR025614. https://doi.org/10.1029/2019wr025614
Misra, V., T. Irani, L. Staal, K. Morris, T. Asefa, C. Martinez, and W. Graham, 2021: The Florida Water and Climate Alliance (FloridaWCA): Developing a stakeholder–scientist partnership to create actionable science in climate adaptation and water resource management. Bulletin of the American Meteorological Society, 102 (2), 367–382. https://doi.org/10.1175/bams-d-19-0302.1
Kruk, M.C., B. Parker, J.J. Marra, K. Werner, R. Heim, R. Vose, and P. Malsale, 2017: Engaging with users of climate information and the coproduction of knowledge. Weather, Climate, and Society, 9 (4), 839–849. https://doi.org/10.1175/wcas-d-16-0127.1
Sullivan, A., 2022: Climate-Resilient Planning and Design Guidance: Building Our Future Today. Philadelphia Water Department, Philadelphia, PA. https://water.phila.gov/pool/files/climate-resilient-guidance.pdf
Vincent, K., M. Daly, C. Scannell, and B. Leathes, 2018: What can climate services learn from theory and practice of co-production? Climate Services, 12, 48–58. https://doi.org/10.1016/j.cliser.2018.11.001
WUCA, 2021: An Enhanced Climate-Related Risks and Opportunities Framework and Guidebook for Water Utilities Preparing for a Changing Climate. Project No. 5056. Water Utility Climate Alliance. https://www.wucaonline.org/assets/pdf/project-5056-guidebook.pdf
Hoylman, Z.H., R.K. Bocinsky, and K.G. Jencso, 2022: Drought assessment has been outpaced by climate change: Empirical arguments for a paradigm shift. Nature Communications, 13 (1), 2715. https://doi.org/10.1038/s41467-022-30316-5
Xu, Y., V. Ramanathan, and D.G. Victor, 2018: Global warming will happen faster than we think. Nature, 564 (7734), 30–32. https://doi.org/10.1038/d41586-018-07586-5
Lopez-Cantu, T. and C. Samaras, 2018: Temporal and spatial evaluation of stormwater engineering standards reveals risks and priorities across the United States. Environmental Research Letters, 13 (7), 074006. https://doi.org/10.1088/1748-9326/aac696
Wright, D.B., C.D. Bosma, and T. Lopez-Cantu, 2019: U.S. hydrologic design standards insufficient due to large increases in frequency of rainfall extremes. Geophysical Research Letters, 46 (14), 8144–8153. https://doi.org/10.1029/2019gl083235
Brekke, L.D., J.E. Kiang, J.R. Olsen, R.S. Pulwarty, D.A. Raff, D.P. Turnipseed, R.S. Webb, and K.D. White, 2009: Climate Change and Water Resources Management: A Federal Perspective. USGS Circular 1331. U.S. Geological Survey, Reston, VA, 65 pp. http://pubs.usgs.gov/circ/1331/
Mulroy, P., 2017: Water Problem: Climate Change and Water Policy in the United States. The Brookings Institution, Washington, DC, 208 pp. https://www.brookings.edu/book/the-water-problem/
Olmstead, S.M., 2014: Climate change adaptation and water resource management: A review of the literature. Energy Economics, 46, 500–509. https://doi.org/10.1016/j.eneco.2013.09.005
MacDonnell, L., 2020: Colorado River Governance: A Model? Social Science Research Network, 4 pp. https://papers.ssrn.com/abstract=3743051
MacDonnell, L., 2021: The Law of the Colorado River: Coping with Sever Sustained Drought, Part II. Social Science Research Network, 14 pp. https://ssrn.com/abstract=3811024
Fleck, J. and A. Castle, 2022: Green light for adaptive policies on the Colorado River. Water, 14 (1), 2. https://doi.org/10.3390/w14010002
Juricich, R., 2020: Colorado River Basin governance, decision making, and alternative approaches. In: World Environmental and Water Resources Congress 2020: Water Resources Planning and Management and Irrigation and Drainage. American Society of Civil Engineers, 121–130. https://doi.org/10.1061/9780784482957.013
Sullivan, A., D.D. White, and M. Hanemann, 2019: Designing collaborative governance: Insights from the drought contingency planning process for the lower Colorado River Basin. Environmental Science and Policy, 91, 39–49. https://doi.org/10.1016/j.envsci.2018.10.011
Karambelkar, S. and A.K. Gerlak, 2020: Collaborative governance and stakeholder participation in the Colorado River Basin: An examination of patterns of inclusion and exclusion. Natural Resources Journal, 60 (1), 1–46. https://www.jstor.org/stable/26912770
Reed, T., L.R. Mason, and C.C. Ekenga, 2020: Adapting to climate change in the Upper Mississippi River Basin: Exploring stakeholder perspectives on river system management and flood risk reduction. Environmental Health Insights, 14, 1178630220984153. https://doi.org/10.1177/1178630220984153
International Joint Commission, 2016: The Boundary Waters Treaty of 1909. International Joint Commission. https://www.ijc.org/en/boundary-waters-treaty-1909
International Joint Commission, 2014: Lake Ontario–St. Lawrence River Plan 2014: Protecting Against Extreme Water Levels, Restoring Wetlands and Preparing for Climate Change. International Joint Commission, Ottawa, Ontario, 98 pp. https://ijc.org/sites/default/files/IJC_LOSR_EN_Web.pdf
Kuhn, E. and J. Fleck, 2019: Science Be Dammed: How Ignoring Inconvenient Science Drained the Colorado River. University of Arizona Press, Tucson, AZ. https://uapress.arizona.edu/book/science-be-dammed
Lukas, J.J. and E.A. Payton, 2020: Colorado River Basin Climate and Hydrology: State of the Science. Western Water Assessment. University of Colorado Boulder, Cooperative Institute for Research in Environmental Sciences, Boulder, CO. https://doi.org/10.25810/3hcv-w477
Salehabadi, H., D.G. Tarboton, B. Udall, K.G. Wheeler, and J.C. Schmidt, 2022: An assessment of potential severe droughts in the Colorado River Basin. JAWRA Journal of the American Water Resources Association, 58 (6), 1053–1075. https://doi.org/10.1111/1752-1688.13061
Fillmore, H. and L. Singletary, 2021: Climate data and information needs of Indigenous communities on reservation lands: Insights from stakeholders in the Southwestern United States. Climatic Change, 169 (3), 37. https://doi.org/10.1007/s10584-021-03285-9
Conroy-Ben, O. and R. Richard, 2018: Disparities in water quality in Indian Country. Journal of Contemporary Water Research & Education, 163 (1), 31–44. https://doi.org/10.1111/j.1936-704x.2018.03268.x
Dahm, K., T. Hawbaker, R. Frus, A. Monroe, J. Bradford, W. Andrews, A. Torregrosa, E. Anderson, D. Dean, and S. Qi, 2023: Colorado River Basin Actionable and Strategic Integrated Science and Technology Project—Science Strategy. USGS Circular 1502. U.S. Geological Survey, 57 pp. https://doi.org/10.3133/cir1502
Margulis, S.A., Y. Fang, D. Li, D.P. Lettenmaier, and K. Andreadis, 2019: The utility of infrequent snow depth images for deriving continuous space-time estimates of seasonal snow water equivalent. Geophysical Research Letters, 46 (10), 5331–5340. https://doi.org/10.1029/2019gl082507
Painter, T.H., D.F. Berisford, J.W. Boardman, K.J. Bormann, J.S. Deems, F. Gehrke, A. Hedrick, M. Joyce, R. Laidlaw, D. Marks, C. Mattmann, B. McGurk, P. Ramirez, M. Richardson, S.M. Skiles, F.C. Seidel, and A. Winstral, 2016: The Airborne Snow Observatory: Fusion of scanning lidar, imaging spectrometer, and physically-based modeling for mapping snow water equivalent and snow albedo. Remote Sensing of Environment, 184, 139–152. https://doi.org/10.1016/j.rse.2016.06.018
Pflug, J.M., S.A. Margulis, and J.D. Lundquist, 2022: Inferring watershed-scale mean snowfall magnitude and distribution using multidecadal snow reanalysis patterns and snow pillow observations. Hydrological Processes, 36 (6), e14581. https://doi.org/10.1002/hyp.14581
Yang, K., K.N. Musselman, K. Rittger, S.A. Margulis, T.H. Painter, and N.P. Molotch, 2022: Combining ground-based and remotely sensed snow data in a linear regression model for real-time estimation of snow water equivalent. Advances in Water Resources, 160, 104075. https://doi.org/10.1016/j.advwatres.2021.104075
Melton, F.S., J. Huntington, R. Grimm, J. Herring, M. Hall, D. Rollison, T. Erickson, R. Allen, M. Anderson, J.B. Fisher, A. Kilic, G.B. Senay, J. Volk, C. Hain, L. Johnson, A. Ruhoff, P. Blankenau, M. Bromley, W. Carrara, B. Daudert, C. Doherty, C. Dunkerly, M. Friedrichs, A. Guzman, G. Halverson, J. Hansen, J. Harding, Y. Kang, D. Ketchum, B. Minor, C. Morton, S. Ortega-Salazar, T. Ott, M. Ozdogan, P.M. ReVelle, M. Schull, C. Wang, Y. Yang, and R.G. Anderson, 2021: OpenET: Filling a critical data gap in water management for the western United States. JAWRA Journal of the American Water Resources Association, 58 (6), 971–994. https://doi.org/10.1111/1752-1688.12956
Senay, G.B., S. Kagone, and N.M. Velpuri, 2020: Operational global actual evapotranspiration: Development, evaluation, and dissemination. Sensors, 20 (7), 1915. https://doi.org/10.3390/s20071915
Tapley, B.D., M.M. Watkins, F. Flechtner, C. Reigber, S. Bettadpur, M. Rodell, I. Sasgen, J.S. Famiglietti, F.W. Landerer, D.P. Chambers, J.T. Reager, A.S. Gardner, H. Save, E.R. Ivins, S.C. Swenson, C. Boening, C. Dahle, D.N. Wiese, H. Dobslaw, M.E. Tamisiea, and I. Velicogna, 2019: Contributions of GRACE to understanding climate change. Nature Climate Change, 9 (5), 358–369. https://doi.org/10.1038/s41558-019-0456-2
Vano, J., J. Hamman, E. Gutmann, A. Wood, N. Mizukami, M. Clark, D.W. Pierce, D.R. Cayan, C. Wobus, K. Nowak, and J. Arnold, 2020: Comparing Downscaled LOCA and BCSD CMIP5 Climate and Hydrology Projections: Release of Downscaled LOCA CMIP5 Hydrology. Bureau of Reclamation, Livermore, CA, 96 pp. https://gdo-dcp.ucllnl.org/downscaled_cmip_projections/techmemo/LOCA_BCSD_hydrology_tech_memo.pdf
Chapman, R.A., G.F. Midgley, and K. Smart, 2021: Diverse trends in observed pan evaporation in South Africa suggest multiple interacting drivers. South African Journal of Science, 117 (7/8). https://doi.org/10.17159/sajs.2021/7900
Hobbins, M.T., J.A. Ramírez, and T.C. Brown, 2004: Trends in pan evaporation and actual evapotranspiration across the conterminous U.S.: Paradoxical or complementary? Geophysical Research Letters, 31 (13). https://doi.org/10.1029/2004gl019846
Hodgkins, G.A., P.H. Whitfield, D.H. Burn, J. Hannaford, B. Renard, K. Stahl, A.K. Fleig, H. Madsen, L. Mediero, J. Korhonen, C. Murphy, and D. Wilson, 2017: Climate-driven variability in the occurrence of major floods across North America and Europe. Journal of Hydrology, 552, 704–717. https://doi.org/10.1016/j.jhydrol.2017.07.027
Do, H.X., S. Westra, and M. Leonard, 2017: A global-scale investigation of trends in annual maximum streamflow. Journal of Hydrology, 552, 28–43. https://doi.org/10.1016/j.jhydrol.2017.06.015
Milly, P.C.D., R.T. Wetherald, K.A. Dunne, and T.L. Delworth, 2002: Increasing risk of great floods in a changing climate. Nature, 415 (6871), 514–517. https://doi.org/10.1038/415514a
Hirsch, R.M. and S.A. Archfield, 2015: Not higher but more often. Nature Climate Change, 5 (3), 198–199. https://doi.org/10.1038/nclimate2551
Mallakpour, I. and G. Villarini, 2015: The changing nature of flooding across the central United States. Nature Climate Change, 5 (3), 250–254. https://doi.org/10.1038/nclimate2516
Hoerling, M., J. Eischeid, X. Quan, and A. Badger, 2019: Causes for the century-long decline in Colorado River flow. Journal of Climate, 32 (23), 8181–8203. https://doi.org/10.1175/jcli-d-19-0207.1
Milly, P.C.D. and K.A. Dunne, 2020: Colorado River flow dwindles as warming-driven loss of reflective snow energizes evaporation. Science, 367 (6483), 1252–1255. https://doi.org/10.1126/science.aay9187
Tanana, H., J. Garcia, A. Olaya, C. Colwyn, H. Larsen, R. Williams, and J. King, 2021: Universal Access to Clean Water for Tribes in the Colorado River Basin. Water and Tribes Initiative, Colorado River Basin. http://www.naturalresourcespolicy.org/docs/water-tribes/wti-full-report-4.21.pdf
Cutter, S.L., 2021: The changing nature of hazard and disaster risk in the Anthropocene. Annals of the American Association of Geographers, 111 (3), 819–827. https://doi.org/10.1080/24694452.2020.1744423
Dilling, L., A. Prakash, Z. Zommers, F. Ahmad, N. Singh, S. de Wit, J. Nalau, M. Daly, and K. Bowman, 2019: Is adaptation success a flawed concept? Nature Climate Change, 9 (8), 572–574. https://doi.org/10.1038/s41558-019-0539-0

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Likelihood

Virtually Certain	Very Likely	Likely	As Likely as Not	Unlikely	Very Unikely	Exceptionally Unlikely
99%–100%	90%–100%	66%–100%	33%–66%	0%–33%	0%–10%	0%–1%

Confidence Level

Very High	High	Medium	Low
Strong evidence (established theory, multiple sources, well-documented and accepted methods, etc.) High consensus	Moderate evidence (several sources, some consistency, methods vary and/or documentation limited, etc.) Medium consensus	Suggestive evidence (a few sources, limited consistency, methods emerging, etc.) Competing schools of thought	Inconclusive evidence (limited sources, extrapolations, inconsistent findings, poor documentation and/or methods not tested, etc.) Disagreement or lack of opinions among experts

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INTRODUCTION

Climate Change Will Continue to Cause Profound Changes in the Water Cycle

Precipitation Changes

Evapotranspiration Changes

Snow and Glacier Changes

Soil Moisture Changes

Groundwater Changes

Runoff Changes

Extreme Events: Floods and Droughts

Water Cycle Changes Will Affect All Communities, with Disproportionate Impacts for Some

Flood Impacts

Drought Impacts

Disproportionate Impacts

Progress Toward Adaptation Has Been Uneven

Approaches to Management and Planning

Adaptation Constraints

Conflict, Competition, and Collaboration

The Effect of Natural Variability on Policy

Adaptation Challenges Faced by Tribal and Indigenous Communities

Progress and Gaps in the Quality and Usability of Information

TRACEABLE ACCOUNTS

Process Description

KEY MESSAGES

KM 4.1

Climate Change Will Continue to Cause Profound Changes in the Water Cycle

Description of Evidence Base

Major Uncertainties and Research Gaps

Description of Confidence and Likelihood

KM 4.2

Water Cycle Changes Will Affect All Communities, with Disproportionate Impacts for Some

Description of Evidence Base

Major Uncertainties and Research Gaps

Description of Confidence and Likelihood

KM 4.3

Progress Toward Adaptation Has Been Uneven

Description of Evidence Base

Major Uncertainties and Research Gaps

Description of Confidence and Likelihood

REFERENCES

Likelihood

Confidence Level