Amrith Gunasekara, California Department of Food and Agriculture
Nancy Huntly, Utah State University
Julie K. Maldonado, Livelihoods Knowledge Exchange Network
Nathan J. Mantua, National Oceanic and Atmospheric Administration
Helene G. Margolis, University of California, Davis
Skyli McAfee, The Nature Conservancy (through 2017)
Beth Rose Middleton, University of California, Davis
Bradley H. Udall, Colorado State University
Cristina Bradatan, Texas Tech University
Mary E. Black, University of Arizona
Shallin Busch, National Oceanic and Atmospheric Administration
Brandon Goshi, Metropolitan Water District of Southern California
Fredric Lipschultz, Senior Scientist and Regional Coordinator
Christopher W. Avery, Senior Manager
<b>Gonzalez</b>, P., G.M. Garfin, D.D. Breshears, K.M. Brooks, H.E. Brown, E.H. Elias, A. Gunasekara, N. Huntly, J.K. Maldonado, N.J. Mantua, H.G. Margolis, S. McAfee, B.R. Middleton, and B.H. Udall, 2018: Southwest. In <i>Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II</i> [Reidmiller, D.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, K.L.M. Lewis, T.K. Maycock, and B.C. Stewart (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 1101–1184. doi: 10.7930/NCA4.2018.CH25
Water for people and nature in the Southwest has declined during droughts, due in part to human-caused climate change. Intensifying droughts and occasional large floods, combined with critical water demands from a growing population, deteriorating infrastructure, and groundwater depletion, suggest the need for flexible water management techniques that address changing risks over time, balancing declining supplies with greater demands.
Water for people and nature in the Southwest has declined during droughts, due in part to human-caused climate change. Intensifying droughts and occasional large floods, combined with critical water demands from a growing population, deteriorating infrastructure, and groundwater depletion suggest the need for flexible water management techniques that address changing risks over time, balancing declining supplies with greater demands.
Higher temperatures intensified the recent severe drought in California and are amplifying drought in the Colorado River Basin. In California, the higher temperatures intensified the 2011–2016 drought,14,56,97,98,99 which had been initiated by years of low precipitation,57,58 causing water shortages to ecosystems, cities, farms, and energy generators. In addition, above-freezing temperatures through the winter of 2014–2015 led to the lowest snowpack in California (referred to as a warm snow drought) on record.47,55,98,100 Through increased temperature, climate change may have accounted for one-tenth to one-fifth of the reduced soil moisture from 2012 to 2014 during the recent California drought.14 In the ongoing Colorado River Basin drought, high temperatures due mainly to climate change have contributed to lower runoff12,59 and to 17%–50% of the record-setting streamflow reductions between 2000 and 2014 (Figure 25.3).13 In the Rio Grande, higher temperatures have been linked to declining runoff efficiency60 and reductions in snowpack.45
Increased temperatures, especially the earlier occurrence of spring warmth,101 have significantly altered the water cycle in the Southwest region. These changes include decreases in snowpack and its water content,46,47,48,49,102 earlier peak of snow-fed streamflow,103 and increases in the proportion of rain to snow.49,103 These changes, attributed mainly to climate change,49,103 exacerbate hydrological drought.
With continued greenhouse gas emissions, higher temperatures would cause more frequent and severe droughts in the Southwest.11,56,62,65,80 This would also lead to drier future conditions for the region.70,74 Higher temperatures sharply increase the risk of megadroughts—dry periods lasting 10 years or more.61,62,65 Under the higher scenario (RCP8.5), models project annual declines of river flow in southern basins (the Rio Grande and the lower Colorado River) and either no change or modest increases in northern basins (northern California and the upper Colorado River).78,104,105,106,107 Snowpack supplies a major portion of water in the Southwest, but with continued emissions, models project substantial reductions in snowpack, less snow and more rain, shorter snowfall seasons, earlier runoff,55,71,78,79,108,109 and warmer late-season stream temperatures.110 Fewer days with precipitation would lead to increased year-to-year variability.111,112,113 Substantial increases in precipitation would be needed to overcome temperature-induced decreases in river flow.13 The combination of reduced river flows in California and the Colorado River Basin and increasing population in southern California, which imports most of its water, would increase the probability of future water shortages.114
In response to the recent California drought, the state government implemented a water conservation plan in 2014 that set allocations for water utilities and major users and banned wasteful practices such as watering during or after a rainfall, hosing off sidewalks, and irrigating ornamental turf on public street medians.115 As a result, the people of the state reduced water use 25% from 2014 to 2017, when abundant rains allowed the state to lift many restrictions while continuing to promote water conservation as a way of life.116
The Southern Nevada Water Authority used similar measures to reduce water use per person 38% from 2002 to 2016.117 Water utilities in the Colorado Front Range also used similar conservation practices to reduce water use more than 20% in the early 2000s.118 While many southwestern cities have reduced total and per-person water use since the 1990s despite growing populations,119 ongoing drought has increased competition for reliable water supplies in many locations. In parts of Colorado, Nevada, and Utah, population growth has prompted proposals for new water diversions and transfers from agriculture. While desalination of seawater and brackish water has been proposed as a partial solution to water scarcity, its high energy requirement creates greenhouse gas emissions and its capital costs are high.15
Atmospheric rivers, which have caused many large floods in California,120 may increase in severity and frequency under climate change.82,83,107,121,122,123,124 In the winter of 2016–2017, a series of strong atmospheric rivers generated high runoff in northern California and filled reservoirs. At Oroville Dam, high flows eroded the structurally flawed emergency spillway, caused costly damage, and led to the preventive evacuation of people living downstream. In addition to the immediate threat to human life and property, this incident revealed two water supply risks. First, summer water supplies are reduced when protective flood control releases of water from reservoirs are necessary in the spring.108 Second, several studies have concluded that deteriorating dams, spillways, and other infrastructure require substantial maintenance and repair.125,126 In U.S.–Mexico border cities with chronic urban storm water and pollutant runoff problems127 and populations vulnerable to flooding,127,128 projected increases in heavy precipitation88 would increase risks of floods.
Wet periods present a water resource opportunity because increased infiltration from the surface into the ground recharges groundwater aquifers. Groundwater was critical for farmers during the California drought, especially for fruit and nut trees and grapevines.129,130,131 Overdraft of groundwater, however, caused land subsidence (sinking), which can permanently reduce groundwater storage capacity and damage infrastructure as the ground deforms.132
In light of projected future changes in the hydrologic cycle, water resource planners and scientists are testing new techniques to combine results from multiple climate and hydrology models, downscale climate model output to finer geographic scales, calculate changing water demands, and use forecasts for flood control.133,134,135,136 Integrating data from satellites, climate and hydrology models, and field observations remains difficult with existing water management tools, methods, and legal requirements.