Beneath northern India’s irrigated fields of wheat, rice, and barley … beneath its densely populated cities of Jaiphur and New Delhi, the groundwater has been disappearing. Halfway around the world, hydrologists, including Matt Rodell of NASA, have been hunting for it.
Where is northern India’s underground water supply going? According to Rodell and colleagues, it is being pumped and consumed by human activities — principally to irrigate cropland — faster than the aquifers can be replenished by natural processes. They based their conclusions — published in the August 20 issue of Nature — on observations from NASA’s Gravity Recovery and Climate Experiment (GRACE).
“Water below the surface can hide from the naked eye, but not from GRACE,” said Rodell. The twin satellites of GRACE can sense tiny changes in Earth’s gravity field and associated mass distribution, including water masses stored above or below Earth’s surface. As the satellites orbit 300 miles above Earth’s surface, their positions change — relative to each other — in response to variations in the pull of gravity. The satellites fly roughly 137 miles apart, and microwave ranging systems measure every microscopic change in the distance between the two.
(First image shows Groundwater resides beneath the soil surface in permeable rock, clay and sand as illustrated in this conceptual image. Many aquifers extend hundreds of feet underground and in some instances have filled with water over the course of thousands of years. Credit: NASA. Second image shows the averaging function (spatial weighting) used to estimate terrestrial water storage changes from GRACE data is mapped. Warmer colors indicate greater sensitivity to terrestrial water storage changes.Credit: NASA/Matt Rodell)
Data provided by India’s Ministry of Water Resources suggested groundwater use was exceeding natural replenishment, but the regional rate of depletion was unknown. Rodell and colleagues had their case study. The team analyzed six years of monthly GRACE gravity data for northern India to produce a time series of water storage changes beneath the region’s land surface.
They found that groundwater levels have been declining by an average of one meter every three years (one foot per year). More than 109 cubic km (26 cubic miles) of groundwater disappeared between 2002 and 2008 — double the capacity of India’s largest surface water reservoir, the Upper Wainganga, and triple that of Lake Mead, the largest man-made reservoir in the United States.
“We don’t know the absolute volume of water in the Northern Indian aquifers, but GRACE provides strong evidence that current rates of water extraction are not sustainable,” said Rodell. “The region has become dependent on irrigation to maximize agricultural productivity, so we could be looking at more than a water crisis.”