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Please see EPA's Climate Change site for current information on climate change and global warming. EPA no longer updates EPA's Global Warming Site, but is maintaining this archive for historical purposes. Thank you for visiting the archive of EPA's Global Warming Site.
Climate Change and the Colorado River Basin
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U.S. Environmental Protection Agency, December 1993
The Colorado River, one of the most important river systems in the western United States, is the principal source of water in a semi-arid basin that covers approximately 243,000 square miles, parts of seven states, and reaches into Mexico. The sensitivity of this river system was assessed by (1) evaluating the effects of changes in temperature and precipitation on runoff and (2) evaluating the effects of runoff changes on water supply, salinity, and hydroelectricity production throughout the Colorado River Basin.
Results suggest that of the hydrology and water supply system of the Colorado River Basin are extremely sensitive to climatic changes that could occur over the next several decades. For a range of plausible changes in climate, streamflow in the basin could either increase or decrease by 30 percent. For climatic changes that would result in significant reductions in runoff, the ability of the existing water supply system to mitigate the worst effects is limited. Under conditions of long-term flow reductions and current operating rules for reservoirs, reservoirs would be drawn almost completely dry, hydroelectricity production would drop dramatically, and salinity in the Colorado River would increase to levels that would frequently exceed legal standards. The results strongly suggest that the current approaches to water management in the basin will have to be modified to balance the many competing demands and priorities under conditions of altered climate, and that current water allocations may well be threatened.
Higher temperatures, with no change in precipitation, would increase evapotranspiration and reduce runoff into the rivers of the basin. A temperature increase of 2°C is estimated to reduce mean annual runoff in the Colorado River Basin 4 to 12 percent. A temperature increase of 4°C is estimated to reduce runoff by 10 to 20 percent. Increases in temperatures decrease winter snowfall and snowpack, increase winter rainfall, and accelerate spring snowmelt. These changes would increase winter runoff and decrease spring runoff, creating a greater potential for spring flooding in some regions, reductions in water storage and deliveries, and increases in average annual salinity. Runoff is even more sensitive to changes in precipitation, which could either increase or decrease in the region and either mitigate or exacerbate the effects of higher temperatures.
If global climate change results in even modest reductions in runoff in the basin of only 5 percent, the Colorado River Compact for allocating water to the upper and lower basins would likely be violated unless changes in water management are made to adapt to the changes in climate. However, the problem of water management planning for climate change is complex because the effects on runoff are uncertain. Increasing flexibility to manage and store potentially more variable runoff may be a desirable response.
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