Abstract
Quantifying the contribution of glacier runoff to water resources is particularly important in regions such High Mountain Asia, where glaciers provide a large percentage of seasonal river discharge and support large populations downstream. In remote areas, direct field measurements of glacier melt rates are difficult to acquire and rarely observed, so hydro-glaciological modeling and remote sensing approaches are needed. Here we present estimates of glacier melt contribution to the Upper Indus watershed over the last 40 years using a suite of seven reanalysis climate datasets that have previously been used in hydrological models for this region, a temperature-index melt model and > 29,000 km2 of ice cover. In particular, we address the uncertainty in estimates of meltwater flux that is introduced by the baseline climate dataset chosen, by comparing the results derived from each. Mean annual glacier melt contribution varies from 8 km3 yr-1 and 169 km3 yr-1, or between 4-78% of the total annual runoff in the Indus, depending on temperature dataset applied. Under projected scenarios of an additional 2-4°C of regional warming by 2100 AD, we find annual meltwater fluxes vary by >200% depending on the baseline climate dataset used and, importantly, span a range of positive and negative trends. Despite significant differences between climate datasets and the resulting spread in meltwater fluxes, the spatial pattern of melt is highly correlated and statistically robust across all datasets. This allows us to conclude with confidence that fewer than 10% of the >20,000 glaciers in the watershed contribute more than 80% of the total glacier runoff to the Indus. These are primarily large, low elevation glaciers in the Karakoram and Hindu Kush. Additional field observations to ground-truth modeled climate data will go far to reduce the uncertainty highlighted here and we suggest that efforts be focused on those glaciers identified to be most significant to water resources.
Highlights
Access to freshwater is becoming increasingly important as world populations grow
We investigate the spatial characteristics of the projected meltwater fluxes, and highlight the key glaciers that contribute most of the meltwater to the Indus River, in order to inform the direction of future research
Positive Degree-day Model for Meltwater Flux In order to calculate the mean annual glacier meltwater flux to the Indus watershed over the past 40 years, we found the melt rate for each glacier with a positive degree-day (PDD) temperature-index melt model (Ambach and Kuhn, 1985; Braithwaite, 1995; Hock, 2003; Bliss et al, 2014; Radic and Hock, 2014)
Summary
In many regions, including High Mountain Asia, glaciers are a significant component of freshwater resources, in the dry summer months. Glacier runoff in the Indus basin in a warming climate, is critical, and especially important in the high mountains of Asia (hereafter HMA) that constitute the “Third Pole,” one of the largest glacierized areas outside the polar icecaps (Dyurgerov and Meier, 2005; Bolch et al, 2012). The necessary financial, political and scientific resources have only sparsely been applied to detailed hydrologic and glaciologic assessments in HMA in order to accurately quantify the impact of climate change over this large and geopolitically sensitive region, and uncertainty remains
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