Satellite-based estimates of groundwater depletion in India

Water resources are at a premium in many parts of the world, and India is one of them. Indirect evidence suggests that groundwater is being consumed faster than it is being naturally replenished in northwest India, but assessments of large-scale rates of depletion are difficult to construct from ground based measurements, which are often scattered and incomplete. Gravity observations from NASA's Gravity Recovery and Climate Experiment (GRACE) satellites, combined with land surface models, have been used to produce monthly time series of groundwater storage variations in India. The analysis reveals a progressively more severe reduction in groundwater in Northwestern India between 2002 and 2008. Groundwater has been depleted at a mean rate of 4.0 cm of water per year averaged over the northern Indian states of Rajasthan, Punjab and Haryana, equivalent to a net loss of 109 km3 of groundwater during that period. The authors consider unsustainable consumption of groundwater for irrigation and other anthropogenic uses as the most likely cause. Indirect evidence suggests that groundwater is being consumed faster than it is naturally being replenished in northwest India, but there has been no regional assessment of the rate of groundwater depletion. Terrestrial water storage-change observations and simulated soil-water variations from a modelling system are now used to show that groundwater is indeed being depleted and that its use for irrigation and other anthropogenic uses is likely to be the cause. Groundwater is a primary source of fresh water in many parts of the world. Some regions are becoming overly dependent on it, consuming groundwater faster than it is naturally replenished and causing water tables to decline unremittingly1. Indirect evidence suggests that this is the case in northwest India2, but there has been no regional assessment of the rate of groundwater depletion. Here we use terrestrial water storage-change observations from the NASA Gravity Recovery and Climate Experiment satellites3 and simulated soil-water variations from a data-integrating hydrological modelling system4 to show that groundwater is being depleted at a mean rate of 4.0 ± 1.0 cm yr-1 equivalent height of water (17.7 ± 4.5 km3 yr-1) over the Indian states of Rajasthan, Punjab and Haryana (including Delhi). During our study period of August 2002 to October 2008, groundwater depletion was equivalent to a net loss of 109 km3 of water, which is double the capacity of India’s largest surface-water reservoir. Annual rainfall was close to normal throughout the period and we demonstrate that the other terrestrial water storage components (soil moisture, surface waters, snow, glaciers and biomass) did not contribute significantly to the observed decline in total water levels. Although our observational record is brief, the available evidence suggests that unsustainable consumption of groundwater for irrigation and other anthropogenic uses is likely to be the cause. If measures are not taken soon to ensure sustainable groundwater usage, the consequences for the 114,000,000 residents of the region may include a reduction of agricultural output and shortages of potable water, leading to extensive socioeconomic stresses.