Climate Elasticity of Low Flows in the Maritime Western U.S. Mountains

Abstract

The western U.S. climate is characterized by cool, wet winters and warm, dry summers. Streamflow provides a critical water resource during the dry summers here. The minimum streamflow (low flow) usually occurs in September or October, several months after the mountain snowpack has melted. The magnitude of the low flow sets a lower bound on water supply, especially in systems without surface water storage. However, it is not clear whether the magnitude of the low flow is more strongly controlled by how cold and wet the previous winter was versus how warm and dry the summer was. We quantified the percent change in low flows per 1% change in winter precipitation and summer evaporative demand. We found that percent changes in low flows are 4 to 5 times larger per 1% change in summer evaporative demand than winter precipitation. However, year‐to‐year variation in evaporative demand is small so the year‐to‐year variation in low flows is more strongly associated with year‐to‐year variation in winter precipitation. Our results suggest that low flows are highly vulnerable to small changes in evaporative demand, but more work is needed to understand expected changes in evaporation in a warming climate.