There was a time when the world’s greatest cities were located on the banks of rivers. Now, in many dry places across the planet, water stored beneath the earth’s surface is a critical resource for human society. Groundwater consumption has become a critical element of urban development and the expansion of human populations into regions that would be otherwise uninhabitable.
Much of the world’s groundwater comes from “fossil” aquifers. That means that water last replenished the aquifer a very long time ago: on the order of 10,000-20,000 years, when our planet was in its last glacial period. In many cases, if we use this water today, it won’t be recharged during our lifetimes. So there is a finite amount of water for use presently, and excessive withdrawal becomes unsustainable.
Unfortunately, groundwater use is difficult to monitor globally. Even in the U.S., wells that are drilled on private property can be exempt from official monitoring. This is a somewhat illogical concept, as aquifers themselves (the underground basins that hold groundwater reserves) often exceed the boundaries of private property, and the water in them flows freely between the land of several property owners. As water is a shared resource, it should be managed with common benefits and conservation in mind.
NASA’s Gravity Recovery and Climate Experiment (GRACE) satellite mission, launched in 2002, provides the first opportunity to directly measure groundwater changes from space (Figure 1). 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.
Figure 1. NASA's GRACE satellite mission is providing new, space-based insights into the global nature of groundwater depletion. The ongoing California drought is evident in these maps of dry season (September-November) total water storage anomalies (in mm equivalent water height; anomalies with respect to 2005-2010) in the western United States. The maps were constructed using GRACE Mascons solutions from NASA's Jet Propulsion Laboratory [Watkins et al. 2015]. California's Sacramento and San Joaquin river basins have lost roughly 15km3 of total water per year since 2011 - more water than all 38 million Californians use for domestic and municipal supplies annually - over half of which is due to groundwater pumping in the Central Valley. Image from Famiglietti , created by Felix Landerer, NASA Jet Propulsion Laboratory, California Institute of Technology, USA.
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 (40km3) during the last 10 years. This is a large amount of water. It’s estimated that about 30 km3 of this was groundwater.
These numbers are interesting because one of them describes the natural impact on water resources (i.e. changes in snow, rain and surface runoff). The other number is attributable to human behavior. When it is dry and there is less water in rivers, we tend to substitute more groundwater for all of our agricultural, domestic and industrial needs. So human activity can directly cause an increase in the local drying initiated by climate.
Big aquifers in Australia (the Canning basin), the middle east, the north china plain, northern India, the southern High Plains aquifer in the US, and of course California’s Central Valley, are all being hit hard during the GRACE record. GRACE is able to estimate how much water is “missing” from these aquifers, and therefore how much water would have to be replaced in order to arrive again at “normal” levels (Figure 2). In the most recent IPCC report (AR5), groundwater pumping is estimated to contribute about 15-25% of the current rate of global sea level rise.