Dana Ariel Lapides , W Jesse Hahm, Daniella M Rempe, David N Dralle

Water management in snowy mountainous regions hinges on forecasting snowmelt runoff. However, droughts are altering snowpack-runoff relationships with ongoing debate about the driving mechanisms. For example, in 2021 in California, less than half of predicted streamflow arrived. Mechanisms proposed for this `missing' streamflow included changes in evapotranspiration, rainfall, and subsurface moisture conditions. Here, we demonstrate that evapotranspiration in drought years generates dry subsurface conditions that reduce runoff in subsequent years. A model including this legacy of depleted moisture storage reduced median error in 2021 forecasts from 60% to 20% at 15 minimally disturbed basins and from 18% to 2% at 6 water supply basins in the Sierra Nevada (basins range in area from 5-23,051 km2 and mean annual precipitation from 814-1549 mm. Our findings indicate that the relationship between snowpack and runoff will evolve as plant ecosystems respond to climate change and alter subsurface water storage dynamics.