Partitioning of the water budget in the main river basins in High Mountain Asia with GRACE, model output, and other observations.

Access to freshwater is important as world populations grow, especially in High Mountain Asia, where glaciers are a significant component of the freshwater resources, particularly in summer. Glaciers are sensitive to climate perturbations and affected by climate change. Our understanding of the contribution of glacier runoff to specific watersheds, and projections of glacier runoff in a warming climate, are critical to inform decisions, management and policy development. Here, we quantify changes in glacier mass balance in HMA using GRACE data and determine their contribution to river basin hydrology. We use GRACE data to estimate the HMA glacier mass mas balance and compare the results with changes in total water storage (TWS) for the major watersheds in the HMA regions. We designed ad-hoc mascon configurations to calculate the upstream glacier change in mass balance and contribution to major river basins water supply, determined appropriate corrections and uncertainties for the signal and evaluated the results via comparison with the Water Balance Model (WBM) output and other data (re-analysis data and satellite-derived precipitation and evapotranspiration). Most of the glacier loss is from the Himalaya region (Himalaya, Hengduan Shan S and E Tibet), whereas the western sectors (E and W Tien Shan; and Hindu Kush, Karakoram, W Kunlun, Pamir, Hissar Alay) experienced smaller losses but with larger interannual variability driven by changes in the westerly-driven winter precipitation. For the Indus basin, to evaluate the glacier contribution to the total water budget, we examine the contribution of the upper basin to the lower basin TWS change. Over the Upper Indus basin, we find that the seasonal decline in total water storage between May and September averages 88 Gt during 2002-2012. TRMM cumulative precipitation amounts to 119 Gt, leaving a runoff and evapotranspiration component of 207 Gt. This estimate compares well with an estimate for the WBM modeled runoff of 178 Gt and ET from remote sensing observations. We use these upper basin estimates to close the water budget in the downstream basin using GRACE TWS in conjunction with ancillary data from modeled and observed evapotranspiration, precipitation and runoff. We apply a similar methodology to other major basins. This work was conducted under a NASA contract.