Researchers: E. Knappe, R. Bendick, H. R. Martens and W. P. Gardner, University of Montana and D. F. Argus, Jet Propulsion Laboratory
Written by Linda Rowan
17 September 2019
Vertical surface motions measured by Global Positioning System (GPS) sites in Idaho, Montana, Nevada and Wyoming can be combined to measure water load over local to regional scales that is useful for water resource management and our understanding of the water cycle. This innovative technique provides information at a scale that satellite data or in situ point measurements cannot. Users can use this technique and the many available geodetic networks to track precipitation, water quantity changes, and the state of aquifers.
GPS sites can measure ground motion in 3-dimensions: the north-south, east-west and vertical directions. For sites that operate over long time periods, the vertical resolution improves as the uncertainties in the measurements are reduced. An array of GPS sites is helpful for measuring surface motion because multiple sites allow researchers to detect and remove artifacts and additional signals. Over time researchers have determined that vertical motions may be due to a variety of processes including tectonic or hydrologic effects. Hydrologic effects include water loading due to precipitation, which causes the GPS site to move downward or water unloading due to runoff or water withdrawal from an aquifer that causes the GPS site to move upward.
Here the authors use 41 GPS sites in Idaho, Montana and Wyoming to measure the hydrologic changes in watersheds. These watersheds are at high elevation, in the mountainous landscape of the Northern Rocky Mountains and provide much of the local water supply. In mountainous regions, the water load is highly variable in space and time and about 50 to 80 percent of the water comes from the annual snowfall. Much of our current knowledge of the changing water quantity in the Northern Rockies comes from snowpack observations from snowpack telemetry stations (part of SNOTEL) and satellites observations from the Gravity Recovery and Climate Experiment (GRACE). However, these observations are limited in their spatial and temporal resolution.
The geodetic measurements, from October 2010 to March 2016, show an average annual vertical response amplitude of 7.5 millimeters. The sites move down as winter comes and the snow accumulates, loading the surface. The sites move up as spring turns to summer and the snow melts and runs off to lower elevations. The sites in the area consistently record this response and are measuring the regional seasonal hydrologic changes. The GPS results are consistent with SNOTEL point measurements and GRACE satellite area measurements (with a resolution of 300 kilometers). To estimate the water load at a local level, the regional signal is removed from the GPS sites. The local vertical amplitude response is on average 2.4 millimeters, accounting for about 30 percent of the total vertical signal. The local response is attributed to localized precipitation and changes in water storage at spatial scales less than 30 kilometers. Thus the GPS is sensitive to local changes in water storage that are more difficult to measure with other techniques.
Knappe, E., Bendick, R., Martens, H. R., Argus, D. F., & Gardner, W. P. (2019). Downscaling vertical GPS observations to derive watershed‐scale hydrologic loading in the northern Rockies. Water Resources Research, 55, 391–401. doi: 10.1029/2018WR023289.
water load, snow, watershed, snowpack
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Last modified: 2020-02-08 01:00:04 America/Denver