aerial of the 230 mile long California Aqueduct near Palmdale CA that delivers water to the Los Angeles basin
Over a third of American vegetables are grown in California, largely in the state’s Central Valley. The region also produces two-thirds of the nation’s fruits and nuts. These crops—and the many Americans who produce and consume them—are heavily reliant on California’s water supply. But, given recurrent and severe droughts, the state’s groundwater supply has been strained.
When surface water supplies run low, most arid regions worldwide turn instead to their groundwater. But past mismanagement of the groundwater in California has caused parts of the state to sink as much as 30 feet and has also increased the frequency of earthquakes along the San Andreas fault.
Just as importantly, the state’s groundwater storage may have been depleted to a point where recovery may take many decades. But, given that this supply is—as its name suggests—in the ground, changes to groundwater aren’t the easiest to measure; the available approaches each have advantages and disadvantages. A new study uses a combination of four of the leading methods to show that California’s aquifers haven’t been recovering from overdrafts during the droughts over the last two decades—and they’re unlikely to do so unless policymakers put more limits in place soon.
Measuring a hidden resource
Past groundwater depletion estimates have typically relied on one, or maybe two, approaches to estimating water supplies. But, in this latest study, the authors suggest that using a combination of multiple methods can give a better picture of how much ground water has been used and how quickly it’s being replenished. The four methods the researchers relied on included gravitational measures; the actual water-level measurements of wells; a newly developed water balance approach; and a hydrologic simulation model.
Through the data collected by NASA’s GRACE satellites (Gravity Recovery and Climate Experiment), it’s possible to detect groundwater loss via the small change in gravity that results from the decreased mass. These measurements have been continuous since the start of the program in 2002 and encompass the entire valley. However, the weakness of this approach is the resolution of the measurement footprint, which precludes more detailed information on a smaller scale. Well measurements are at the other end of the spectrum—they are very localized, but continuous datasets are rare and coverage is scattered. Inconsistent use of aquifer storage coefficients has also caused some uncertainties in well-based calculations.
Just as importantly, the state’s groundwater storage may have been depleted to a point where recovery may take many decades. But, given that this supply is—as its name suggests—in the ground, changes to groundwater aren’t the easiest to measure; the available approaches each have advantages and disadvantages. A new study uses a combination of four of the leading methods to show that California’s aquifers haven’t been recovering from overdrafts during the droughts over the last two decades—and they’re unlikely to do so unless policymakers put more limits in place soon.
Measuring a hidden resource
Past groundwater depletion estimates have typically relied on one, or maybe two, approaches to estimating water supplies. But, in this latest study, the authors suggest that using a combination of multiple methods can give a better picture of how much ground water has been used and how quickly it’s being replenished. The four methods the researchers relied on included gravitational measures; the actual water-level measurements of wells; a newly developed water balance approach; and a hydrologic simulation model.
Through the data collected by NASA’s GRACE satellites (Gravity Recovery and Climate Experiment), it’s possible to detect groundwater loss via the small change in gravity that results from the decreased mass. These measurements have been continuous since the start of the program in 2002 and encompass the entire valley. However, the weakness of this approach is the resolution of the measurement footprint, which precludes more detailed information on a smaller scale. Well measurements are at the other end of the spectrum—they are very localized, but continuous datasets are rare and coverage is scattered. Inconsistent use of aquifer storage coefficients has also caused some uncertainties in well-based calculations.