Subsurface water dominates seasonal hydrologic storage

Vertical displacements (dz) in permanent GPS station positions enable estimation of water storage changes (DS), which historically have been impossible to measure directly. We use dz from 924 GPS stations in the western United States to estimate daily DS in California’s Sierra Nevada, and compare it to seasonal snow accumulation and melt over water years 2008-2017. Seasonal variations in GPS-based DS are ~1000 mm. Typically, only ~30% of DS is attributable to snow water equivalent (SWE). DS lags the snow cycle, peaking after maximum SWE and remaining positive when all snow has melted (SWE=0). We conclude that seasonal DS 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 DS is larger than accumulated precipitation from PRISM and NLDAS, indicating that these standard inputs underestimate mountain precipitation.