The sensitivity of forest water use to the timing of precipitation and snowmelt recharge in the California Sierra: Implications for a warming climate

Forest responses to warming, in the absence of changes in vegetation structure, reflect the balance between the increased atmospheric demand for water and changes in water availability. This study uses a coupled hydroecologic model applied to a snow-dominated mountain watershed to demonstrate how complex topography and interannual variation in climate drivers combine to alter the balance between moisture availability and energy demand. We focus specifically on how often and under what conditions changes in the timing of soil water recharge as precipitation or snowmelt are a significant control on forest actual evapotranspiration (AET) in the Central California Sierra. We show that while interannual variation in precipitation is the dominant control on interannual variation in AET, how much of that recharge accumulates as a seasonal snowpack can act as a second-order control. This sensitivity of AET to snow accumulation and melt occurs across a substantial elevation range (1800-2700 m) and at both aggregate watershed and 90 m patch scales. Model results suggest that the variation in AET due to recharge timing is greatest for patches and years with moderate levels of precipitation or patches that receive substantial lateral moisture inputs. For a 3 degrees C warming scenario, the annual AET increases in some years due to warmer temperatures but decreases by as much as 40% in other years due to an earlier timing of snowmelt. These results help to clarify the conditions under which water availability for forests decreases and highlight scenarios that may lead to increased drought stress under a warming climate in snow-dominated mountain regions.