Report: Climate change shrinking Western water supplies

WASHINGTON, April 1 2016 -- Farmers in 17 Western states will have to step up their efforts to conserve water in the coming decades, as temperatures rise and droughts and floods become more frequent, the Bureau of Reclamation says in a report.

"More extreme variations in climate will make it difficult for the Bureau to meet competing demands for water,” says the report, required by the SECURE Water Act of 2009. “Warming is expected to continue, causing further impacts on supplies, increasing agricultural water demands, and affecting the seasonal demand for hydropower electricity.”

Droughts and floods are expected to become more intense, raising concerns about infrastructure safety, the ability of fish and wildlife to adapt, and Reclamation’s ability to maintain adequate levels of hydropower production.

During the 21st century, the report forecasts:

• Temperatures will increase 5-7 degrees Fahrenheit;
• Precipitation will increase in the northwestern and north-central parts of the Western U.S. and decrease over the southwest and south-central areas;
• The April 1 snowpack – “a standard benchmark measurement used to project river basin runoff” – will decrease in almost all areas;
• The April-to-July streamflow in several river basins, including the Colorado, the Rio Grande, and the San Joaquin, will decrease by 7 to 27 percent.

Rising temperatures also will result in greater evaporation of water at reservoirs, typically 2 to 6 inches by 2080, the report says.

Demand for irrigation water will be influenced by many factors, including changing cropping patterns driven by market prices and changes in irrigation practices, says the report, which includes chapters on seven different river basins: the Colorado, Columbia, Klamath, Missouri, Rio Grande, Sacramento and San Joaquin River Basins, and the Truckee River Basin. It also discusses the Southern California Coastal and Inland Basins, Great Basin, and the Arkansas-Red-White River Basin.

The report found that some factors will increase agricultural water demand while others would reduce it, but even the good news has some bad aspects to it. For example, the report says that one of the factors that will lessen water demand is “increased crop failure due to increased pests (and) diseases.” Also, there will be “less per-unit crop water use” because there will be more atmospheric CO2 and ozone. Higher ozone levels have been linked to yield losses in many crops, including soybeans and wheat, according to EPA.

Higher temperatures also will mean more acreage will need supplemental irrigation to remain viable, the report said. There also will be an “increase in irrigated lands due to northward warming (and) increased livestock water demands.”

Groundwater, which makes up about a third of total freshwater diversions and withdrawals in the 17 Western states, will also be affected. “Decreased snowpack could result in decreased groundwater infiltration, runoff and ultimately lower base flows in the rivers during the summer,” the report said.

Discussing California’s agriculturally rich Central Valley, the report said: “Projected earlier seasonal runoff will cause reservoirs to fill earlier, thereby reducing overall storage capability, as current flood control constraints limit early season storage in these reservoirs. End-of-September reservoir storage is projected to decrease by 3 percent over the course of the 21st century.”

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