Monitoring Snow Water Equivalent in Mountain Watersheds with GPS Vertical Displacement Time Series

Snow water equivalent (SWE) from mountain headwater regions is a critical component of the annual water budget in the majority of western United States watersheds. Accurate estimation of mountain SWE is challenging due to high spatial heterogeneity in snow deposition and snowpack density, yet crucial to water resource management decision-making. Vertical displacements from networks of permanently anchored GPS (Global Positioning System) receivers have shown promise as a means of estimating changes in SWE at the mountain range scale; however, their utility at finer spatial scales is uncertain. Here, we evaluate the accuracy of GPS-based SWE estimates via comparison to in situ measurements from the California Department of Water Resources (CDWR) and Natural Resources Conservation Service (NRCS) snow telemetry (SNOTEL) snow pillow networks in the California Sierra Nevada and Colorado Rocky Mountains. We further investigate the agreement between snow pillow SWE measurements and GPS-based SWE estimates as it relates to station density and distribution. We use the National Weather Service (NWS) River Forecast Centers (RFC) basin groupings to compare GPS- and snow-pillow-based SWE estimates at the basin scale. Snow pillow data is often physiographically biased, as many stations are situated at middle elevations in flat clearings. The accuracy of GPS-based estimates, meanwhile, depends on the number and distribution of GPS stations in and around a basin of interest. We therefore consider the possibility of incorporating snow pillow and GPS data into a composite solution in order to better constrain spatial patterns of SWE accumulation and melt.