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).