Yes, it rained and snowed a lot this winter and left us with an epic Sierra snowpack. Now forget all that. The long-range outlook: still dry.
A new report from the federal Bureau of Reclamation may offer the most comprehensive forecast yet for western water in the 21st century — but few surprises.
The report, Managing Water in the West, breaks down the outlook for eight key river systems, including three vital to California. The overall message is predictably sobering.
The risks that California faces from climate change are pretty well known, says Peter Gleick of the Oakland-based Pacific Institute. He says the 200-plus-page report “doesn’t offer any new surprises about those risks — but it does reaffirm those risks in an increasingly compelling way.”
Some media coverage of the report seems to conclude that California gets off lightly in the study. But the section of the report covering the critical Sacramento and San Joaquin basins seems sobering at best. While it does predict a small (0.6%) increase in annual precipitation on the Sacramento, the report also foresees a drop in San Joaquin precipitation of somewhere between 4.2% and 5.3% by 2050.
Any decrease in precipitation would be significant, especially for a river system like the San Joaquin, which Gleick calls “grossly oversubscribed.” He says that given the other stresses on water supply, such as rising temperatures, evaporation and demand, “Precipitation has to go up, just to break even.”
The report projects a 5-6-degree rise in temperatures during this century, in California’s Central Valley. And whether total precipitation rises or falls, how and when it comes down is just as critical to water supplies. The DOI report predicts that as more arrives as rain, rather than snow, we’ll see more runoff occurring in the winter and less in summer, when it’s most needed. And the infrastructure of dams, reservoirs and aqueducts in place in California were designed for “yesterday’s climate, not tomorrow’s climate,” according to Gleick, who has studied the state’s mountain hydrology for decades.
“It’s a very comprehensive report. It really integrates the state of the science,” says Gleick. “They’re telling us we really need to pay attention to these problems.”