Downscaling Vertical GPS Observations to Derive Watershed‐Scale Hydrologic Loading in the Northern Rockies

• Published in: Water resources research Vol. 55; no. 1; pp. 391 - 401
• Main Authors: Knappe, E., Bendick, R., Martens, H. R., Argus, D. F., Gardner, W. P.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01-01-2019
• Subjects: Climate change; Displacement; Earth; Earth surface; Ecological monitoring; Global positioning systems; GPS; GRACE (experiment); Gravity; Hydrology; Mass transfer; Measurement; Methods; Precipitation; Precipitation patterns; Regional development; Resolution; Satellite navigation systems; Satellite observation; Satellites; Snowpack; Storage; Time series; Topography (geology); Unloading; Water management; Water supply; Watersheds; Wavelength; Weather stations; Weight
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1029/2018WR023289

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 300 km. The local loading, controlled by small‐scale precipitation and storage variability, has much shorter correlation lengths of <30 km. 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.
Key Points
Observations of vertical displacement with GPS can be separated into regional and local surface mass loading contributions
The local loading signal is dominated by watershed scale hydrologic loading
GPS observations are capable of providing an independent measure of total water storage at the watershed scale