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GPS time series of vertical displacement include the elastic response of the Earth to a combination of regional and local loading signals arising from hydrologic mass transfer. The regional loading, controlled by seasonal, synoptic precipitation patterns, dominates the displacement of individual stations and is highly correlated among stations with separation distances from 10 to 300km. The local loading, controlled by small-scale precipitation and storage variability, has much shorter correlation lengths of <30km. We develop a new method to separate the regional and local contributions using common mode analysis and show that GPS is capable of measuring the local hydrologic load changes at watershed scales of tens of kilometers. Using this methodology, GPS-measured displacement provides an integrated measurement of hydrologic load at a spatial scale between the existing long-wavelength resolution of the Gravity Recovery and Climate Experiment and point measurement resolution of a precipitation station. Thus, GPS time series record critical observations for monitoring integrated hydrologic budgets at scales useful for water management and assessment of the hydro-ecological response to climate change.

Plain Language Summary Even though estimating water quantity is critical for human communities, there are few good methods for doing so, especially in areas with lots of topography. Snow measurements and weather stations are only point observations that do not always represent the surrounding area, and satellite observations do not resolve small mountain watersheds well. We leverage the fact that the surface of the Earth is displaced by the weight of water, and that this deflection can be measured with GPS, to more accurately estimate water loading and unloading. We do this in snowpack-dominated areas of the northern Rockies and show that these methods complement the available tools.