Divergent runoff regime revealed by hydrological simulations with corrected precipitation in the upper Indus

The upper Indus basin (UIB) supplies water for the largest irrigation system in Asia and for the livelihoods of millions of people downstream. Yet a large gap still remains in the current knowledge of the hydrological pro-cesses in this region due to data scarcity, especially at sub-basin scales. In this work, we evaluated eight pre-cipitation datasets and screened out an optimized (MERRA-2) to be inversely corrected through the Variable Infiltration Capacity (VIC)-glacier hydrological model across the UIB. Divergent runoff regimes, changes, and their attributions were quantified at sub-basin scales based on corrected MERRA-2 precipitation and the well-constrained VIC-glacier model against observed streamflow and glacier/snow changes. Spatial variations exist in the corrected precipitation over the UIB with the lowest mean annual average of 414 mm in the Kharmong and the highest of 1318 mm in the Astore, and a mean of 646 mm over the entire basin in 1980-2013. Three runoff regimes were identified across the UIB basins, including the glacial regime of Shigar, Hunza, and Shyok with glacier runoff dominating (41.6%-53.4%), nival-pluvial regime of Astore and Gilgit with rainfall (46.9%-51.9%) and snowmelt (33.1%-40.7%) runoff dominating, and pluvial regime of UIB-M and Kharmong with rainfall runoff (56.7%-60.9%) dominating. Total runoff generally exhibited stable or insignificant positive tendencies over the UIB basins during 1980-2013. Runoff increases in the glacial regime basins were mainly controlled by the increased glacier runoff, while runoff changes in the non-glacial regime basins were mostly attributed to the changes of precipitation-induced runoff from non-glacierized areas. The increased glacier runoff served as a buffer against hydrological drought by offsetting the non-glacierized runoff decrease caused by the overall decreased precipitation. This study is expected to further the understanding of divergent hydrological regimes in the UIB, and provide essential information for policy decisions on sustainable water resource management in this water-stressed area.