CONSTRAINTS ON REGIONAL HYDROLOGY OF THE INDUS BASIN FROM GRACE DATA

Analysis of GRACE mascon data from NASA Goddard Space Flight Center provides some useful constraints on Indus Basin hydrology. With a 2o by 2o spatial and ten-day temporal resolution, individual mascon cells can be used to address issues of recharge and discharge on a sub-basin scale over six seasons from 2003-2009. Some 35 mascon cells cover the entire Indus Basin from the Tibetan Plateau to the Arabian Sea. All these cells show a pronounced annual cycle of gravity due to seasonal precipitation and snow/ice melt. Individual mascon cell data have been analyzed with a time series calculator provided at the GSFC portal, varying annual and semi-annual contributions and lag to find a minimum RMS solution. With this solution, monsoon precipitation can be distinguished in time and space from westerlies precipitation, and matched quantitatively to ground measurements. Spatial analysis of the entire basin was pursued using ANOVA of annual GRACE totals for individual mascon cells, which resulted in statistically significant differentiation of hydrologically sensible sub-basins. Most importantly the main agricultural area centered on the Punjab of both India and Pakistan was found to have distinctively large negative change of gravity values consistent with ground water mining. This is in agreement with previous regional studies, but with greater resolution provided by individual mascon cells. The negative magnitude of the GRACE gravity anomaly in cm of water matches the modeled drawdown of ground water based on subsurface measurements. The linear trend of 2003-2009 GRACE data showed the negative anomaly increasing in time. GRACE data were also found to be useful in consideration of hydrological issues in the mountains. Spectral analysis of individual mascon cells supported the major distinction between orographic recharge in the mountain ranges and relatively minor precipitation on the Tibetan Plateau. Calculation of the 2003-2009 linear trends for individual cells in the mountains shows generally decreasing trends in the Himalaya, suggesting shrinking glacier storage. However, in the Karakoram, the GRACE trend is increasing, consistent with an increase in water storage through glacier growth, as also suggested by field workers.