Measuring Groundwater Depletion

Scientists are using satellite data to measure changes in the amount of water in underground aquifers. How do they do it? Satellite speed is affected by the pull of gravity, which is partly determined by how much water is underground near Earth's surface. As the first of two satellites approaches a region of the Earth with a large underground aquifer, the pull of gravity increases and the satellite speeds up briefly, increasing the distance between it and a trailing satellite. As the second satellite passes over it too speeds up briefly, closing the gap again. By measuring the changes in distance between the two satellites as they pass over the aquifer and then comparing those distances from year to year, scientists can determine changes in the pull of gravity over time and then estimate how much water has been gained or lost.

Using this technology, scientists have discovered that in just six years, one of the largest aquifers in India has lost a volume of water equal to a lake 30 feet deep and nearly 5,000 square miles in surface area. Most of the groundwater consumed in the region is used for agriculture. Nobody knows how large the aquifer really is or how long it would take to deplete it, but losses of this size just are not sustainable in the long run.

We can expect more of this kind of useful information as the satellite technique becomes more sophisticated. But will we choose to change our water use practices as a result of what we learn?