How important are glaciers for Indus water resources?

How important are glaciers for Indus water resources? Looking into the literature reveals a wide range of estimates to this question. They partly diverge because of different definitions of a 'water resource' and partly due to the different estimates of the contribution from the glaciers. Defining the Indus water resources as precipitation (rain and snowfall) minus the loss of water due to evapotranspiration and sublimation for non-glaciated regions and the total runoff from the glaciated regions (sum of seasonal snowmelt on top of the glacier, percolating rainfall and loss of glacier ice), we have attempted to estimate the fraction of the total water resource that is coming from the glaciated regions and the fraction due to loss of glacier ice alone (which is the part that may be lost if a glacier disappears). Here we present estimates of water resources from non-glaciated regions using a wide range of estimates from observationally based, reanalysis and land data assimilation systems. Our results indicate large differences between the different estimates even for the annual values averaged over the whole basin. The implication of this finding is that the large uncertainties in the water resources of the non-glaciated regions will prevent us from making narrow estimates of the importance of the glaciers. For estimating the contribution from the glaciers we use an energy balance model with snow metamorphosis forced with 3 hourly reanalysis data (including perturbed precipitation and temperature runs based on the uncertainties in temperature and precipitation found from 10 different datasets). As there is over 10 000 glaciers in Indus it is not possible to model individual glaciers. Instead we model ';representative' glaciers where we get the average elevation of terminus and top, avg. glacier depth etc. from the Extended World Glacier Inventory in a given sub basin (using the FAO HydroSHEDS dataset to divide the Indus basin into subbasins). The mass balance and glacier runoff is then calculated for every 50 m elevation of the ';representative' glacier and the results scale up with total glacier area of that subbasin to get the total mass balance and glacier runoff for the subbasin. This is done for all subbasins that have glaciers. Our results using the above approach indicate that runoff from glaciated areas contributes to 8-24% of the Indus water resources, but ice melt alone only accounts for 2-10% the rest is seasonal snowmelt on top of the glacier and percolation of rain through the glacier. Thus if all glaciers are lost the effect on the water resources of the Indus basin will probably be less than 10%. We found the key uncertainties in estimating Indus water resources to be the lack of observations of precipitation. While the rate of sublimation from the glacier is a key factor in estimating how much of the glacier loss that actually becomes runoff. A factor often ignored or just prescribed as a constant factor when simple degree day models are used.