LAUSR

Abstract. Water management throughout the western United States largely relies on the partitioning of cool season mountain precipitation into rain and snow that helps determine water storage in spring snowpack. Recent studies indicate a shift towards increased precipitation falling as rain, consistent with a warming climate. An approach is presented to estimate precipitation partitioning across landscapes from 1948–present by combining fine scale gridded precipitation data with coarse scale freezing-level and precipitation data from an atmospheric reanalysis. A marriage of these datasets allows for a new approach to estimate spatial patterns and trends in precipitation partitioning over elevational and latitudinal gradients in major water supply basins. This product can be used in California as a diagnostic indicator of changing precipitation phase across mountain watersheds. Results show the largest increases in precipitation falling as rain during the past seven decades in lower elevation watersheds located within the climatological rain-snow transition regions of northern California during spring. Further development of the indicator can inform adaptive water management strategy development and implementation in the face of a changing climate.