Groundwater levels in critical agricultural areas worldwide such as India and California’s Central Valley are dropping rapidly. For example, groundwater levels are dropping up to six feet a year in the San Joaquin Valley.
Two technologies that use remote sensing—one airplane-based and one satellite-based—are being used to track groundwater levels worldwide either at the large-scale or at the small-scale. GRACE (Gravity Recovery and Climate Experiment) performs large-scale, long-term analysis using changes in gravity at the earth’s surface to examine the removal of groundwater from large aquifers worldwide.
According to satellite data from GRACE, groundwater levels in the San Joaquin Valley dropped 2 to 6 feet per year from October 2003 – March 2009 while groundwater levels dropped from 0.3 to 0.5 feet per year over that same time period in the northern Sacramento Valley.
GRACE, through a partnership with NASA and the German Aerospace Center, tracks the monthly changes in the Earth's gravity field caused by the movement of water. Variations in the Earth's gravity field are mapped using two identical satellites flying about 220 kilometers apart in a polar orbit 500 kilometers above the Earth. Gravity fluctuations measured by the satellites correlate with variations in the density of the land surface below. When groundwater is extracted, the land surface density changes slightly and GRACE data can detect those changes.
GRACE measurements show that India’s breadbasket region may be running out of water. According to NASA satellite data, groundwater levels in aquifers in northwest India have declined one foot per year over the past decade. Researchers conclude the loss is due almost entirely to groundwater pumping and consumption by human activities, such as irrigating cropland. As a result, aquifers are being drained much faster than they can be replenished by rainfall or river runoff.
In December 2009, scientists from NASA and the University of California, Irvine, detailed California's groundwater changes and described GRACE-based research on other global aquifers at the American Geophysical Union meeting in San Francisco. Since late 2003, the Sacramento and San Joaquin drainage basins combined have lost more than 30 cubic kilometers of water, said Jay Famiglietti, a hydrologist from the University of California, Irvine. (One cubic kilometer is about 264.2 billion gallons or 811 thousand acre feet.) The bulk of the loss occurred in California's agricultural Central Valley.
“GRACE data reveal groundwater in these basins is being pumped for irrigation at rates that are not sustainable if current trends continue,” Famiglietti said. “This is leading to declining water tables, water shortages, decreasing crop sizes and continued land subsidence. The findings have major implications for the U.S. economy, as California's Central Valley is home to one sixth of all U.S. irrigated farmland, and the state leads the nation in agricultural production and exports.”
Groundwater resides beneath the soil surface in permeable rock, clay and sand. Many aquifers extend hundreds of feet underground and in some instances have filled with water over the course of thousands of years. Removal of groundwater at rates that exceed the rate of natural recharge is mining an aquifer. One byproduct of aquifer mining is that the ground surface drops as the emptied aquifer compresses. This process of compaction results in subsidence of the Earth’s surface.
Land subsidence is a gradual settling or sudden sinking of the Earth’s surface owing to subsurface movement of earth materials. According to a fact sheet from U.S. Geological Survey (USGS), the principal causes of subsidence are aquifer-system compaction, drainage of organic soils, underground mining, hydrocompaction, natural compaction, sinkholes and thawing permafrost.
In addition to GRACE, an aircraft-based technology looks at ground subsidence at the scale of millimeters to look at smaller scale and shorter-term depletion of groundwater. This powerful new mapping tool, InSAR—Interferometric Synthetic Aperture Radar—is capable of remotely sensing small changes in land surface elevation with excellent detail through collection of airborne or spaceborne radar data. The subsidence maps allow monitoring and managing of subsidence caused by the compaction of aquifer systems.
Scott Hensley, a scientist at NASA’s Jet Propulsion Laboratory (JPL) working on the use of synthetic aperture radar for measuring deformation, explained how the measurements work. “To make deformation measurements the radar makes two images of the region of interest, one prior to the deformation and one after. Since the radar is measuring the distance to each pixel in both the before and after images and after compensating for changes in the platform trajectory, it is possible to measure the change in distance and hence deformation (see cartoon below) due to surface deformation.”
“To measure subsidence in the Central Valley of California,” Hensley said, “two radar passes are made by either an airborne or spaceborne radar and then by measuring precise range changes using radar interferometric techniques, the subsidence due to groundwater discharge can then be measured at the sub-centimeter level.” This high degree of accuracy allows precise measurement of subsidence.
“The technique of InSAR has been used in several studies of subsidence elsewhere, including Los Angeles, Las Vegas, and Phoenix,” said Tom Farr, a geologist with JPL. “However, the Central Valley is proving difficult because of the agricultural vegetation and plowing that change the ground surface too much for the InSAR technique to work well. So far, we have some preliminary data that indicates we can make the measurements sometimes for some locations, but what we’d like to do is create a complete time series, or ‘movie’ of the ground deformation caused by water withdrawal and recharge,” he added.
"Groundwater resources are being rapidly depleted in many regions of the world," Famiglietti said in an article last August in Scientific American. "These signals of groundwater loss, particularly in the Central Valley, are very strong. Measurements of subsidence and groundwater depletion now can be measured accurately by remote sensing from satellites and airplanes.
The more difficult task is coming up with management actions that will reverse or slow subsidence and groundwater mining.