Evapotranspiration feedbacks shift annual precipitation-runoff relationships during multi-year droughts in a Mediterranean mixed rain-snow climate

Abstract

Abstract. Focusing on the headwaters of the California's Feather River, we investigated how multi-year droughts affect the water balance of Mediterranean mixed rain-snow catchments. Droughts in these catchments saw a lower fraction of precipitation allocated to runoff compared to non-drought years. This shift in precipitation-runoff relationship was larger in a surface-runoff-dominated than in a subsurface-flow-dominated catchment – 39 % and 18 % less runoff, respectively, for a representative precipitation amount. The performance of the PRMS hydrologic model in these catchments decreased during droughts, particularly those causing larger shifts in the annual precipitation-runoff relationship. Evapotranspiration (ET) was the only water-balance component for which predictive accuracy during drought vs. non-drought years was consistently different. Besides a systematic bias during all years, the model tended to relatively overestimate drought ET and to underestimate non-drought ET. Modeling errors for ET during droughts were somewhat correlated with maximum and minimum annual temperature as well as changes in sub-surface storage (r = −0.45, −0.57, and 0.23, respectively). These correlations point to the interannual response of ET to climate, or climate elasticity of ET, as the likely driver of the observed shifts in precipitation-runoff relationship during droughts in Mediterranean mixed rain-snow regions; underestimation of this response caused increased modeling inaccuracy during droughts. Improved predictions of interannual variability of ET are necessary to support water-supply management in a warming climate and could be achieved by explicitly parametrizing feedback mechanisms across atmospheric demand for moisture, ET, and multi-year carryover of subsurface storage.

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