Watersheds are determined to be stressed when demand for water from the watershed exceeds the natural supply. Based on a recent study by the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado-Boulder, almost 10 percent of examined watersheds are already stressed.
“By midcentury, we expect to see less reliable surface water supplies in several regions of the United States,” said Kristen Averyt, associate director for science at CIRES and one of the authors of the study. “This is likely to create growing challenges for agriculture, electrical suppliers and municipalities, as there may be more demand for water and less to go around.”
The researchers found that most of the water stress is in the Western United States, where there are fewer surface water resources, compared with the East. According to the report:
On the water supply side, surface and ground water resources have been declining in much of the U.S. Aquifers underlying the Central Valley in California and the Ogallala, which spans the area between Nebraska and Texas, are being drawn down more rapidly than they are being recharged. Approximately 23% of annual freshwater demands rely on groundwater resources, yet the volume of groundwater remaining is unclear.
Average surface water supplies are decreasing, and are expected to continue declining, particularly in the southwestern US.. Also in the southwest, water availability is defined as much by legal regimes as by physical processes. Water rights define how much and when water may be withdrawn from surface water sources irrespective of how much water may or may not be flowing in a given year. Water quality, including temperature and sediment concentration, can also constrain availability for certain users.
Not surprisingly, the water requirement associated with agriculture contributes the most to regional water stress overall with the U.S. West being particularly vulnerable to water stress. In other sections of the country, the water needs of electric power plants represent the biggest demand on water — so much so that a single power plant “has the potential to stress surface supplies in a local area.” In some densely populated regions like Southern California, cities are the greatest stress on the surface water system.
A map created by CIRES shows all of the stressed watersheds in the continental United States, with green signifying minimal stress to red suggesting higher levels of stress:
Existing conservation efforts, more informed use of water by the public and businesses, and greater awareness of the challenges are driving some stability in water demand across the U.S. but climate change will almost certainly drive both increased demand and diminished supply going forward. In the American West, the issue is considerably more top of mind with continuing drought conditions in the Southwest spanning the area from Texas to California, particularly those states that rely heavily on already-stressed watersheds like the Colorado River.
Read the entire CIRES report here.