Amazing GRACE – Groundwater from Space and the Future of Hydrometry

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Gravity Recovery and Climate Experiment Mission (GRACE) data is being streamed to Time Square in New York City until April 22nd in honor of World Water Day.

If you haven’t heard about GRACE, you should check it out http://www.csr.utexas.edu/grace/. The earth is a closed system so when water leaves one region it will appear somewhere else.  The twin GRACE satellites, which have been in a paired orbit for 10 years, measure their distance relative to each other using laser beams. This distance changes as the satellites pass over anomalies in the mass of the earth, allowing for the mapping of the strength to the earth’s gravitational field. The mass of water in, on, and above the earth’s surface changes the total mass between the satellite and the centre of the earth between successive orbits.  A complete global map of these anomalies is processed every 30 days.

The imagery allows you to ‘see’:

  • the seasonal water balance in the major basins
  • effects of groundwater extraction in northern India and California
  • changes in ice mass from glaciers and ice sheets around the world
  • as well as non-hydrologic changes such as occurred after the Fukushima earthquake

When I was at the University of Texas recently (for the OGC Hydro Domain Working Group meetings) I was able to talk to a researcher at the Center for Space Research. The mission is beyond its life expectancy and measures are being taken to extend the life of the satellites as long as possible. There is almost no chance that GRACE will still be functional by the time a replacement mission is launched in 2017. I was worried that without a suitable period of overlap there would be no comparability between the GRACE anomalies and those produced by the Follow On mission (GRACE-FO). Apparently there has been a lively discussion about this at UT-CSR but the consensus is that the data will be inter-comparable because what is being measured is straight up physics. The only parameter is the speed of light, which they are fairly confident in.

Whereas it is very difficult to measure glacier mass balance using field observation techniques, GRACE has been able to measure a loss of 250 gigatons of mass from the Greenland Ice sheet over the past 10 years.

This, I think, is reason enough to get excited about the role of remote sensing in the future of hydrometry.

Nobody will be giving up their snow courses, ablation stakes, groundwater wells or precipitation gauges any time soon to be replaced by remotely sensed data at monthly time step and a 300 km spatial resolution. However, it is clear that remote sensing does provide valuable information that cannot be easily measured on the ground.

We need to start thinking about our methods of measurement and data management to figure how to combine what remote sensing does well (extensive coverage) with what field observations do well (high resolution).


Stu Hamilton

Stu Hamilton

Senior Hydrologist

Hydrology field work done today, if managed well, becomes part of a legacy of information that will serve for generations to come. As an avid canoeist and whitewater kayaker I was easily drawn into a career in hydrometry in spite of an undergraduate education in biology. Shortly after graduating from the University of Alaska I started work with the Water Survey of Canada in Whitehorse, Yukon. The initial appeal was the freedom to travel extensively to some of the most beautiful landscapes on the planet to measure streamflow. The highlight of my career was measuring 7040 m3s-1 of flow on the Porcupine River using a small, under-powered, aluminum skiff, kevlar tagline and a 150 pound sounding weight. It took 4 tries to string the line, while uprooted trees and large ice floes came down the river. I am guilty of being a data philosopher. I think we have to first be able to clearly articulate what an ideal data set should look like and then we can influence the direction of technological development to make that ideal achievable.

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