Climate change will increase evaporation of Colorado River

The Colorado River faces a dual threat from climate change as rising temperatures increase the demand for irrigation water and accelerate evaporation at the river’s two largest reservoirs.

So says a new report from the U.S. Bureau of Reclamation, which predicts an 8 percent increase in irrigation demand on the lower half of the Colorado River Basin and a 10 percent increase in evaporation from Lake Mead by 2080.

The upper half of the basin, above Glen Canyon Dam in Arizona, is expected to see demand for agricultural water jump by almost 23 percent, while Lake Powell loses 7 percent more water to evaporation than it did during the last half of the 20th century.

The estimates are based on a projected temperature increase of about 5 degrees across the region.

In a statement announcing the report Friday, Reclamation Commissioner Estevan Lopez said the findings will help regulators and stakeholders address the challenges that lie ahead.

“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,” Lopez said.

Agriculture accounts for roughly 80 percent of all the water diverted from the Colorado River, so even a small increase in demand can have a significant impact on the system.

The new report is the latest in a series of region-wide risk assessments looking at the impacts of climate change on water resources in the West.

A previous bureau study predicted a 9 percent decline in the Colorado’s flow by 2050 as a result of climate change.

Already, the river has seen its flow reduced significantly amid 15 years of record drought. The surface of Lake Mead has dropped 130 feet since 2000, and current federal projections call for it to fall another 10 feet to a new record low by the end of April.

The Las Vegas Valley draws 90 percent of its water from the reservoir through two intake pipes that will stop working should the lake level drop about 90 feet from where it is now.

To keep water flowing to the community, the Southern Nevada Water Authority is on track to finish an $817 million deep-water intake at Lake Mead by this summer, and plans are now in the works on a $650 million pumping station that will keep that intake working even if the lake drops another 200 feet or more.

Lake Mead is the nation’s largest man-made reservoir by capacity, and each year it loses roughly 800,000 acre-feet of water to evaporation, more than double the amount of Colorado River water used annually in the Las Vegas Valley.

One acre-foot is enough to supply two average valley homes for more than a year.

In all, the Bureau of Reclamation’s new report looks at 12 reservoirs on seven river systems west of the Mississippi. Its outlook is also bleak for the Truckee and Carson Rivers, which supply water to farms and communities in northwestern Nevada, Reno among them.

According to the report, rising temperatures will drive up agricultural demand on the Truckee and the Carson by more than 14 percent over the next 65 years, while evaporation will increase by 14 percent at Lake Tahoe and by 7 percent at Lahontan Reservoir.

The bureau’s projections of future irrigation demand do not account for changing crop patterns or efficiency improvements at farms.

The full report is available for download at usbr.gov/WaterSMART/wcra/.

Contact Henry Brean at hbrean@reviewjournal.com or 702-383-0350. Follow @RefriedBrean on Twitter.