USBR: Study Reveals Climate Change Impacts on Irrigation Demand and Reservoir Evaporation in the West

Hockey Stick based on Mann & Jones 2003
Hockey Stick based on Mann & Jones 2003

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Reflecting current climate projections for the western United States, a new report issued by the Bureau of Reclamation reveals a projected shift in demand for crop irrigation across eight major river basins. The study evaluated irrigation water requirements for the second half of the 20th century and, as compared to projected demand for the second half of the 21st century, found that net irrigation water requirements in the West may be six percent higher. Another area of study revealed in the report – based on a projected temperature increase of approximately 5 degrees Fahrenheit in the region – estimates that annual evaporation at most of the 12 reservoirs modeled by the study could increase 2 to 6 inches by 2080.

The report on irrigation demand and reservoir evaporation projections is the latest in a series of West-Wide Climate Risk Assessments – analyses of overall impacts from climate change on water resources in the West through the Department of the Interior’s WaterSMART Program.

In announcing the report, Reclamation Commissioner Estevan López said the study was an important piece of information about climate change imposing stresses on water resources and will ultimately help inform water planners and stakeholders in confronting future climate-related supply and demand challenges.

“Reclamation and its partners are engaged in critical work to confront a future with increasing disparity between water supply and demand in basins throughout the West,” Commissioner López said. “Understanding how climate change will impact crop irrigation demand and reservoir evaporation provides vital information for the development of alternatives and solutions to meet those challenges and support the nation’s economy.”

Projected future irrigation demands are only estimates and provide a starting point for further analyses and discussions with customers and stakeholders. The results do not account for changing crop patterns and other socioeconomic considerations that are best addressed with stakeholder input within a basin study or other process.

Using climate projections for temperature and precipitation, scientists considered projected irrigation demand in eight major river basins: Colorado, Rio Grande, Sacramento-San Joaquin, Truckee, Columbia, Missouri and Klamath. The water evaporation model was applied to 12 reservoirs in many of those major Reclamation river basins: Lake Powell, Lake Mead, American Falls Reservoir, Lake Roosevelt, Upper Klamath Lake, Canyon Ferry Reservoir, Boysen Reservoir, Elephant Butte Reservoir, Lake Shasta, Millerton Lake, Lake Tahoe and Lahontan Reservoir. This table provides one set of projections of irrigation demand by basin and potential changes in evaporation for the twelve reservoirs when compared to actual figures from 1950 to 1999:


Scientists utilized climate change data to project alterations in precipitation and temperature and to assess evaporation for 12 reservoirs within those river basins, when considering observed and projected climate change impacts. Precipitation projections are highly variable and basin dependent, and they can vary significantly within individual basins as well.

“Through these studies, Reclamation is highlighting climate change impacts and encouraging a collaborative dialogue on the effective management of our water and power resources,” López said. “Facing the challenge in meeting future irrigation demands is one way we are working to underscore our commitment to a strong agricultural economy and national food security.”

Reclamation’s West-Wide Climate Risk Assessment is part of the Department of the Interior’s WaterSMART Program, which focuses on improving water conservation and sustainability, while helping water resource managers make sound decisions about water use. The report may be found at

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