Estimating seasonal snow accumulation and melt in the Sierra Nevada from GPS vertical displacement time series

Streamflow forecasting in the western United States (WUSA) is hindered by large uncertainties in the distribution of mountain snow, as well as the partitioning of the snowpack into streamflow and storage as soil moisture and groundwater. Refined estimates of these quantities may improve water resource management and elucidate mountain hydrologic system dynamics. We use vertical displacement time series from a network of GPS (Global Positioning System) stations located throughout the WUSA to estimate distributed changes in water mass in the Sierra Nevada of California on a season-by-season basis, for the interval 2008-2016. GPS vertical displacements are inverted for distributed snow water equivalent (SWE), as well as water mass retained in the mountains at the end of the melt season. Displacements resulting from lakes, reservoirs, and Central Valley groundwater are removed, leaving a residual combination of snow, soil moisture, and shallow groundwater. We use the Snow Data Assimilation System (SNODAS) to constrain the snow season duration. Relative to snow pillow records and SNODAS, GPS-based peak water mass occurs synchronously, but the melt season rate of water loss is slower. GPS inversions suggest substantial water storage within the mountains at the end of the melt season (70-300 mm equivalent water depth). A modest correlation between the change in storage and the magnitude of peak SWE estimated from SNODAS suggests that storage is controlled in part by the seasonal snowpack, but that other factors such as air temperature or melt timing likely also play important roles.