This reminded me of NASA's GRACE mission [1] that I recently heard about. It can track the depletion of California's aquifers (for example) based on the tiny changes in orbit of the two tandem satellites.

Was mentioned on a recent Still Untitled podcast episode by NASA CTO David Miller [2], the whole thing is worth a listen if you want to hear him eagerly talking about all the cool things NASA is up to.

[1] http://www.nasa.gov/mission_pages/Grace/ [2] http://www.tested.com/science/space/558108-return-rock-still...