Grace of Water

Despite its great influence, all the freshwater that makes up the lakes, rivers, streams, creeks, marshes, potholes, bogs, fens, mires, swamps, ponds, billabongs, lagoons, mud holes and groundwater of Earth has only recently been accounted for. Monitoring where water goes is a big job and can only really be done affordably for the entire planet from space.

NASA's Gravity Recovery and Climate Experiment (GRACE) satellites do this task by measuring local changes in gravity over time. All matter ??? including water ??? has mass and gravity. So when there's less water in a particular area, its gravity is slightly less. More water ??? whether in lakes, streams or underground ??? means the gravity is greater. GRACE has now managed to watch as the continents swell and shrink with water on a seasonal basis ??? showing Earth's water cycle actually at work on a global scale.

Link: For more on this story see: http://dsc.discovery.com/convergence/planet-earth/guide/fresh-water-02.html.