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

Abstract

Under global warming, arid subtropical regions are expected to get drier and expand polewards. This study uses model simulations to examine changes in hydrological parameters for the southwestern United States. The predictions for 2021–2040 show declines in surface-water availability, resulting in reduced soil moisture and runoff. 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 polewards1,2,3,4. Over southwest North America, models project a steady drop in precipitation minus evapotranspiration, P−E, the net flux of water at the land surface5,6,7, leading to, for example, a decline in Colorado River flow8,9,10,11. This would cause widespread and important social and ecological consequences12,13,14. 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.