The Bureau of Reclamation released its West-Wide Climate Risk Assessment on February 6, 2015 showing the impact of projected climate change for the western United States on irrigation demand and reservoir evaporation. In announcing the report, Reclamation Commissioner Estevan Lopez 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.”
The study examined eight major river basins: Colorado, (Upper) Rio Grande, Sacramento-San Joaquin, Truckee, Columbia, Missouri and Klamath. Projected future irrigation demands do not account for changing crop patterns and other socioeconomic considerations. Climate projections for temperature and precipitation were used in projecting irrigation demand and increased reservoir evaporation. The projections are compared to irrigation demands and reservoir evaporation from 1950 to 1999.
Projected climate change has its greatest impact on irrigation water demand in the Upper Colorado River Basin (22.9%) followed by the (Upper) Rio Grande (18.7%), the Truckee and Carson River Basins (14.6%) and Klamath (14.0%)—see table. The projected increase in irrigation water demand due to projected climate change was relatively smaller in the Sacramento-San Joaquin River Basins (6.8%), Columbia River Basin (6.3%) and smallest in the Imperial Valley (1.4%). The massive Missouri River Basin had a range of estimated impacts, ranging from an large increase in irrigation water demand in the Western Missouri (13.6%), to a moderate increase in the Northern Missouri (3.6%) and a small decline in the Southeastern Missouri (-1%).
| Basin | % Increase in Irrigation Water Demand | & Increase in Reservoir Evaporation |
| Upper Colorado River Basin | 22.9 | 7.1 (Lake Powell) |
| Lower Colorado River Basin | 1.4 (Imperial Valley) | 10.1 (Lake Mead) |
| Columbia River Basin | 6.3 | 6.0 (American Falls)
5.4 (Lake Roosevelt) |
| Klamath River Basin | 14.0 | 8.2 (Upper Klamath Lake) |
| Missouri River Basin | ||
| Western | 13.6 | 4.3 (Boysen Reservoir) |
| Northern | 3.6 | 6.9 (Canyon Ferry) |
| Southeastern | -1.0 | |
| (Upper Rio Grande) | 18.7 | 9.5 (Elephant Butte) |
| Sacramento/San Joaquin River | 6.8 | 14.7 (Lake Shasta)
12.3 (Millerton Lake) |
| Truckee/Carson River Basins | 14.6 | 14.4 (Lake Tahoe)
7.1 (Lahontan Reservoir) |
The study cautions that “precipitation projections are highly variable and basin dependent, and they can vary significantly within individual basins as well.”
Reclamation’s West-Wide Climate Risk Assessment is part of the Department of Interior’s WaterSMART Program that focuses on water conservation and sustainability.
The full report can be found at:
http://www.usbr.gov/WaterSMART/wcra/docs/irrigationdemand/WWCRAdemands.pdf.
Written by Rodney T. Smith, Ph.D.