Toward a process-based understanding of precipitation-phase impacts on streamflow

Empirical evidence suggests that shifting precipitation phase from snow towards rain reduces streamflow from montane catchments, but the physical processes underlying these observations are not well understood. We used a hydro-ecologic model (Regional Hydro-Ecologic Simulation System, RHESSys) to study the impact of changing precipitation phase on streamflow at the rain/snow transition zone in the Southern Sierra Nevada, California. The model was driven by ten years of precipitation and temperature measurements from the Southern Sierra Critical Zone Observatory. We considered two scenarios: one in which we incrementally altered the temperature at which precipitation is partitioned between rain and snow, and one in which we incrementally altered the temperature time series (indirectly affecting the precipitation-phase partitioning). In the phase-change scenario, streamflow was not impacted in most years, and increased with increasing rain fraction in a subset of years, in contrast to the empirical results. In the temperature-change scenario, streamflow generally declined with increasing rain fraction, consistent with empirical findings. Our results suggest that observed streamflow declines with increasing rain fraction may be attributable to factors correlated with precipitation phase (e.g., higher temperatures resulting in greater evapotranspiration), rather than changes in precipitation phase itself.