Projections of declining surface-water availability for the southwestern United States

Abstract

Global warming driven by rising greenhouse-gas concentrations is expected to cause wet regions of the tropics and mid to high latitudes to get wetter and subtropical dry regions to get drier and expand polewards 1-4 . Over southwest North America, models project a steady drop in precipitation minus evapotranspiration, P − E, the net flux of water at the land surface 5-7 , leading to, for example, a decline in Colorado River flow [8][9][10][11] . This would cause widespread and important social and ecological consequences 12-14 . Here, using new simulations from the Coupled Model Intercomparison Project Five, to be assessed in Intergovernmental Panel on Climate Change Assessment Report Five, we extend previous work by examining changes in P, E, runoff and soil moisture by season and for three different water resource regions. Focusing on the near future, 2021-2040, the new simulations project declines in surface-water availability across the southwest that translate into reduced soil moisture and runoff in California and Nevada, the Colorado River headwaters and Texas.The global climate models used in this study include all simulations for all models that were continuous from 1950 to 2040 and that provided all of the data required. Historical simulations to December 2005 and future projections to 2040 using the Representative Concentration Pathway (RCP)85 scenario whereby anthropogenic radiative forcing equals 8.5 W m −2 by 2100 (refs. 15, 16) were analysed (see Methods). The RCP85 scenario involves stronger anthropogenic radiative forcing than the Special Report on Emissions Scenario A1B for Coupled Model Intercomparison Project (CMIP) 3/Intergovernmental Panel on Climate Change Assessment Report Four analysed in ref. 5, and was chosen to reflect the present lack of any international action to limit CO 2 emissions. The models are being evaluated as part of CMIP5 and have been shown to model important aspects of North American hydroclimate with fidelity but with biases, for example, to excess P in the interior of western North America (see Supplementary Information).In both the models and the observations, California gets almost all of its annual P in the winter, whereas inland regions such as the Colorado headwaters and Texas have a more even distribution of P throughout the year (see Supplementary Information). However, in all cases, winter P has a disproportionate importance from the water-resource perspective because E is lowest during winter and P is effective at increasing soil moisture, streamflows and reservoir storage, often with a delay until snowmelt 8 . Summer rains are however very important for dry farming, rangelands and ecosystems and for influencing fire risk 17 . Figure 1 shows the changes in the multimodel ensemble mean P and P − E for North America for 2021-2040 minus 1951-2000 To the north, in central and northern California, Nevada, Utah and Colorado the models project increased winter P. In spring (April-June, AMJ) the region of reduced P extends to include all of California and most of...

Riparian trees and aridland streams of the southwestern United States: An assessment of the past, present, and future

“…There is high model agreement for a continued increase in temperature and evaporation in this region , which will reduce the amount of precipitation that falls as snow and decrease the amount of rainfall that enters streams (Das et al, 2011;Wi et al, 2012;Vano et al, 2014). This warming will reduce discharge volume in several southwestern streams, even if amount of precipitation is unaffected (Christensen et al, 2004;Seager et al, 2013). Similar reductions in total discharge have been modeled at the Verde River and Salt River by Ellis et al (2008).…”

“…Climatologists predict that, under current and projected levels of CO 2 emissions, higher temperatures and smaller snowpacks will reduce the amount of runoff that enters streams (Pierce et al, 2008;Seager and Vecchi, 2010;Seager et al, 2013). Droughts, which have been a regular feature in recent years, will continue to occur, but with increasing severity, resulting in further reductions in discharge volume (Cayan et al, 2010;Woodhouse et al, 2010;Gutzler and Robbins, 2011).…”

Combined impacts of current and future dust deposition and regional warming on Colorado River Basin snow dynamics and hydrology

“…Anthropogenic warming and enhanced evapotranspiration over the coming decades put this supply at risk. Coupled climate-hydrology model simulations show a 5-20 % decrease in flow depending on the study, indicative of a greater risk of a dry future (Christensen et al, 2004;Christensen and Lettenmaier, 2007;McCabe and Wolock, 2007;Reclamation, 2011a, b;Colorado Water Conservation Board, 2012;Vano et al, 2012;Seager et al, 2012).…”

Predicting the river’s blue line for fish conservation

“…1). Exceptional droughts and climate change are one cause of drying-surface water is scarcer and that trend is not changing (1)(2)(3)(4). Increasing human appropriation of groundwaterespecially during drought-is another cause (5,6).…”