Have GRACE satellites overestimated groundwater depletion in the Northwest India Aquifer?

The Northwest India Aquifer (NWIA) has been shown to have the highest groundwater depletion (GWD) rate globally, threatening crop production and sustainability of groundwater resources. Gravity Recovery and Climate Experiment (GRACE) satellites have been emerging as a powerful tool to evaluate GWD with ancillary data. Accurate GWD estimation is, however, challenging because of uncertainties in GRACE data processing. We evaluated GWD rates over the NWIA using a variety of approaches, including newly developed constrained forward modeling resulting in a GWD rate of 3.1 +/- 0.1 cm/a (or 14 +/- 0.4 km(3)/a) for Jan 2005-Dec 2010, consistent with the GWD rate (2.8 cm/a or 12.3 km(3)/a) from groundwater-level monitoring data. Published studies (e.g., 4 +/- 1 cm/a or 18 +/- 4.4 km(3)/a) may overestimate GWD over this region. This study highlights uncertainties in GWD estimates and the importance of incorporating a priori information to refine spatial patterns of GRACE signals that could be more useful in groundwater resource management and need to be paid more attention in future studies.