Runoff response to the uncertainty from key water-budget variables in a seasonally snow-covered mountain basin

Study region: The American River Basin, a seasonally snow-covered mountain basin in the Study focus: Precipitation phase (rain, snow, and mixed rain-snow) partitioning in snowy basins is important for runoff forecasting.This study focuses on how predicted runoff response is affected by uncertainty in rain-snow-transition elevation and other water-budget variables. Using a distributed hydrologic model, we examined predicted runoff differences across 33 extremeprecipitation events and dry seasons during 1982-2020, through a set of modeling experiments with different sources of transition elevation and values of water-budget variables.New hydrological insights for the region: Different transition elevations can result in noticeable differences in predictions of peak snow water equivalent (SWE) and dry-season runoff. Transition elevation modulates runoff response in a nonlinear way, with runoff uncertainty increasing during larger precipitation events. In terms of the relative impacts of water-budget variables on runoff response, precipitation has the greatest impact, followed by antecedent soil moisture, transition elevation, and antecedent SWE. A 2 degrees C warming would push the rain-snow-transition elevation similar to 300-m higher, increasing the number and severity of extreme events. Compared to the historical period, the same transition-elevation uncertainty in climate warming conditions nearly triples its impacts on runoff uncertainty, posing additional challenges to flood control. This study provides implications for addressing uncertainty in flood-control and water-supply decisions confronted with hydrologic extremes and climate warming.