Co-authors (from left) Matthew Rodell, Isabella Velicogna and James Famiglietti. 
In huge stretches of arid northern India, depleting groundwater has long been an issue but the menace was not rigorously quantified. A team of hydrologists led by Matthew Rodell of NASA's Goddard Space Flight Center have found that northern India's underground water supply is being pumped and consumed by human activities, primarily to irrigate cropland, faster than the aquifers can be replenished by natural processes.
Rodell and colleagues Isabella Velicogna of NASA's Jet Propulsion Laboratory and the University of California-Irvine, and James Famiglietti of UC Irvine show that more than 109 cubic km (26 cubic miles) of groundwater disappeared from the region's aquifers between 2002 and 2008. This is double the capacity of India's largest surface water reservoir, the Upper Wainganga.
In the hotbed are Rajasthan, Punjab, and Haryana — states with staggering population growth, rapid economic development, and water-hungry farms — accounting for about 95 percent of groundwater use in the region.
The finding is based on data from NASA's Gravity Recovery and Climate Experiment (GRACE), a pair of satellites that can sense changes in Earth's gravity field and associated mass distribution, including water masses stored above or below Earth's surface. As the twin satellites orbit 300 miles above Earth's surface, their positions change relative to each other in response to variations in the pull of gravity.
"We were among the first scientists to apply GRACE gravity observations for hydrology. We began to study the feasibility of using GRACE for hydrology years before the satellites launched in 2002," Rodell told Nature India. "Water below the surface can hide from the naked eye, but not from GRACE," he said.
India's Ministry of Water Resources has data to suggest that groundwater use across the country exceeds natural replenishment, but the regional rate of depletion was still unknown. Rodell and colleagues analysed six years of monthly GRACE data for northern India to produce a time series of water storage changes beneath the land surface.
The situation in India got the NASA team interested when they read newspaper articles and reports that parts of the country had issues with wells running dry. Also noticing a trend in the GRACE data in northern India, they decided to take a closer look. "The data processing was not much different than we had performed for other hydrological studies, except that we had to be extra careful to account for all possible sources of water mass variability in the region, including glacier melt in the Himalayas, lake and reservoir storage, and soil moisture, so that we could be sure that the apparent trend we saw in groundwater was real," Rodell said.
The study has confirmed what India's Central Ground Water Board (CGWB) knew all along. CGWB Chairman Braj Mohan Jha said while the method used by Rodell and colleagues is good for detecting groundwater trends in large areas, it might not be as effective for small areas.
"We estimate groundwater depletion at the block level through the water balance estimation method. So, while the GRACE study is great in terms of getting further authentication for our data, we need to do more micro-scale analysis," he told Nature India. CGWB estimates groundwater depletion in northern India at the rate of 13.2 km cube per year, very close to the GRACE data of 17.7±4.5 km cube per year.
Co-authors (from left) Matthew Rodell, Isabella Velicogna and James Famiglietti.
GRACE has been used to measure rates of ice melt in Greenland, Antarctica and certain mountain glaciers caused by climate change. "When you look at a trend map based on seven years of GRACE data, with positive water storage trends in blue and negative trends in red, those are the biggest, reddest features on the map. Unfortunately for India, one of the next things that catch your eye is a dark red blob centered near the intersection of Haryana, Punjab, and Rajasthan," Rodell said.
The major advantage of GRACE over other remote sensing systems is that it can monitor changes in water storage at all levels, even groundwater, so it is a huge advance for global water cycle research. "We had previously proven that GRACE is useful for monitoring seasonal variations in groundwater, and we wanted to demonstrate its value for monitoring inter-annual variations and identifying trends," Rodell added.
However, one of the frustrating things about studying regional scale groundwater variability is the general unavailability of groundwater well level data in much of the world, he said.
As groundwater declines in affected areas, farmers respond by digging deeper wells, which is an unsustainable means to procure water. Jha said monsoon rains are the only way to recharge groundwater in the region. "This year the situation is going to be very grim, what with the monsoons failing us," he said. In this context, the study results are disturbing since the depletion occurred during 2002-2008, when rainfall trends were slightly above normal, pointing to enhanced use of groundwater for irrigation purposes.
Rodell said if measures are not taken to ensure sustainable groundwater usage, the consequences for the 114 million residents of the region may include a collapse of agricultural output and severe shortages of potable water.
