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Scientists Explain Mechanisms Affecting Runoff Levels During Drought

January 12, 2018

Scientists at UC Merced’s Sierra Nevada Research Institute (SNRI), UC Irvine, UC Davis and the USDA Forest Service have enumerated the mechanisms that serve as master regulators of streamflow and drought intensity by studying California’s 2012-15 drought. Their findings are detailed in a new paper published in Scientific Reports.

Researchers used measurements from the Southern Sierra Critical Zone Observatory (CZO) in California’s Kings River Basin to pinpoint four distinct mechanisms responsible for regulating runoff levels during the recent drought. Runoff — water from precipitation, snowmelt and natural reservoirs that feeds into mountain streams and rivers — ultimately supplies much of the state’s water.

“Long-term measurements at the CZO allowed us to understand how the drought affected the amount of water supplied by mountains,” study coauthor and UC Irvine Professor Michael Goulden said. “This understanding is needed to more accurately forecast how future droughts will affect water supplies in California and in mountain headwaters around the world, and to help devise strategies to reduce the impact of drought.” 

Two mechanisms — evapotranspiration and warming — decreased runoff levels and intensified the overall effects of the drought.

“Runoff in mountain rivers ultimately reflects the difference between precipitation, which occurs as both rain and snow, and water returned to the atmosphere through evapotranspiration, which is mainly water used by plants plus evaporation from soil,” said Roger Bales, SNRI director and lead author of the new study.

The drought was dominated by evapotranspiration, which varied little from year to year in Sierra Nevada forests, even as precipitation levels declined. As a result, more water was returned to the atmosphere than drained into nearby streams and rivers. This alone led to a 30 percent reduction in springtime runoff. The warming climate only made matters worse.

Temperatures were a full degree Celsius higher than they were the decade prior. This warming resulted in drier, hotter atmospheric conditions favorable to further evapotranspiration. Elevated temperatures ultimately led to an additional 5 percent reduction in runoff.