Agriculture is depleting world aquifers, new satellite measurements show

Agriculture’s heavy demand on the world’s freshwater resources is well understood from the output end — of all water consumption for all uses, the United Nations estimates, 70 percent goes to produce food. But the problem has been more difficult at the sourcing end, which requires distinguishing between perpetually replenished surface water from lakes and streams on the one hand, and essentially nonrenewable underground reserves on the other.Quantifying the impact of withdrawals from aquifers has become a little easier since the introduction about 15 years ago of the satellite program known as GRACE, for Gravity Recovery and Climate Experiment, developed in a collaboration of the U.S. and German space programs. But where is the water going?To answer that question, the team made what appears to be the first effort to overlay depletion data with country-by-country statistics on agricultural output, to see how much of the loss could be attributed to food production.They called the resulting measurement GWD — groundwater depletion for irrigation — and the numbers were rather grim in terms of the acceleration rate. Of course, agricultural depletion is not uniform across the globe. About two-thirds of the GWD calculated for 2010 was in just four countries: India (7.35 km3), Iran (3.33 km3), Pakistan (2.75 km3) and China (2.40 km3). Almost 85 percent occurred in 10 nations — the top four plus the United States (1.62 km3), Mexico (1.11 km3), Libya (.25 km3), Turkey (.20 km3), and Italy (.20 km3).During the decade that ended in 2010, the acceleration of GWD was most rapid in India (23 percent), China (102 percent) and the United States (31 percent).