How much water will be available in the upper Colorado River Basin under projected climatic changes?

The Colorado River and its tributaries are the principal source of water for urban and agricultural demands in the arid and semi-arid Southwestern United States. Projected climatic changes in the basin suggest some precipitation increases in the very highest regions, and no changes or declines for the middle and low elevations, while air surface temperatures are expected to increase by about 5 degC by the end of the century. We model the effects of these climatic changes on all hydrologic components (snowmelt, ET, surface runoff, subsurface runoff, groundwater, and streamflow) on the subbasin scale to project future changes in water resources using SWAT. We find that spring season flows in most subbasins are likely to decrease by 20-90%, and summer flows by 50-100%. Many subbasins will turn from semi-arid to arid conditions by the 2080's. The role of the individual hydrologic components and the impact on the availability of water resources for urban and agricultural use in the region are quantified.