Groundwater consumption has become a critical element of urban development and the expansion of human populations into regions that would be otherwise uninhabitable. Unfortunately, groundwater use is difficult to monitor globally.
NASA’s Gravity Recovery and Climate Experiment (GRACE) satellite mission, launched in 2002, provides the first opportunity to directly measure groundwater changes from space (image from Famiglietti [2014]). By observing changes in the Earth’s gravity field, scientists can estimate changes in the amount of water stored in a region, which cause changes in gravity. GRACE provides a more than 10 year-long data record (and counting) for scientific analysis. Plans are underway to continue this record with the GRACE Follow-On (GRACE-FO) satellite mission, with a tentative launch date in 2017. This makes a huge difference for scientists and water managers who want to understand trends in how our resources are being consumed over the long term.
GRACE has returned data on some of the world’s biggest aquifers and how their water storage is changing. If we use estimates of changes in snow and surface soil moisture, scientists can calculate an exact change in groundwater in volume. For instance, it is estimated that the Central Valley aquifer in California has lost roughly 1.5 times the full volume of Lake Mead (40 km3) during the last 10 years. This is a large amount of water. It’s estimated that about 30 km3 of this was groundwater.