Subsurface Water Dominates Sierra Nevada Seasonal Hydrologic Storage

Vertical displacements (dz) in permanent Global Positioning System (GPS) station positions enable estimation of water storage changes (ΔS), which historically have been impossible to measure directly. We use dz from 924 GPS stations in the western United States to estimate daily ΔS in California's Sierra Nevada and compare it to seasonal snow accumulation and melt over water years 2008–2017. Seasonal variations in GPS-based ΔS are ~1,000 mm. Typically, only ~30% of ΔS is attributable to snow water equivalent (SWE). ΔS lags the snow cycle, peaking after maximum SWE and remaining positive when all snow has melted (SWE = 0). We conclude that seasonal ΔS fluctuations are not primarily driven by SWE but by rainfall and snowmelt stored in the shallow subsurface (as soil moisture and/or groundwater) and released predominantly through evapotranspiration. Seasonal peak GPS ΔS is larger than accumulated precipitation from the Parameter-elevation Relationships on Independent Slopes Model and North American Land Data Assimilation System, indicating that these standard inputs underestimate mountain precipitation.