South Asia is a region with a large and rising population, a high dependenceon water intense industries, such as agriculture and a highly variableclimate. In recent years, fears over the changing Asian summer monsoon (ASM)and rapidly retreating glaciers together with increasing demands for waterresources have caused concern over the reliability of water resources and thepotential impact on intensely irrigated crops in this region. Despite theseconcerns, there is a lack of climate simulations with a high enoughresolution to capture the complex orography, and water resource analysis islimited by a lack of observations of the water cycle for the region. In thispaper we present the first 25 km resolution regional climate projections ofriver flow for the South Asia region. Two global climate models (GCMs), whichrepresent the ASM reasonably well are downscaled (1960–2100) using aregional climate model (RCM). In the absence of robust observations,ERA-Interim reanalysis is also downscaled providing a constrained estimate ofthe water balance for the region for comparison against the GCMs(1990–2006). The RCM river flow is routed using a river-routing model toallow analysis of present-day and future river flows through comparison withavailable river gauge observations. We examine how useful these simulationsare for understanding potential changes in water resources for the South Asiaregion. In general the downscaled GCMs capture the seasonality of the riverflows but overestimate the maximum river flows compared to the observationsprobably due to a positive rainfall bias and a lack of abstraction in themodel. The simulations suggest an increasing trend in annual mean river flowsfor some of the river gauges in this analysis, in some cases almost doublingby the end of the century. The future maximum river-flow rates still occurduring the ASM period, with a magnitude in some cases, greater than thepresent-day natural variability. Increases in river flow could meanadditional water resources for irrigation, the largest usage of water in thisregion, but has implications in terms of inundation risk. These projectedincreases could be more than countered by changes in demand due to depletedgroundwater, increases in domestic use or expansion of water intenseindustries. Including missing hydrological processes in the model would makethese projections more robust but could also change the sign of theprojections.
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