Energy balance modelling of snow and ice melt for the Naltar catchment (Karakoram, Pakistan) in future climate

Abstract

High Mountain Asia (HMA), including the Hindu Kush-Karakoram Himalayas (HKH) is one of the world's key "water towers", with the resources supporting hundreds of millions of people. Currently, this region is experi encing significant demographic and socio-economic growth. Reliable hydrological projections of the future supply of water resources are essential, given the likelihood that water resources demand will continue to in crease. In this study, CORDEX South Asia (CORDEX-WAS44) regional climate models (RCMs) and the Physically Based Distributed Snow Land and Ice Model, that was calibrated with hourly meteorological data and daily runoff over eight years of monitoring period, are employed in the Naltar catchment located in the Hunza river basin, Upper Indus Basin, Pakistan to project glacio-hydrological regimes in the future climate. For each of the CORDEX-WAS44 simulations, climate change signals for near future (2040-2059) and far future (2080-2099) under three Representative Concentration Pathways (RCPs) namely RCP2.6, RCP4.5, and RCP8.5 are presented with respect to the corresponding present climate (1991-2010). Results show overall significant increases in mean temperature between (+0.9 to + 6.0 ºC, depending upon the scenario) and total precipitation (+6 to + 29 %) from April to September by the end of the century for RCP2.6, RCP4.5, and RCP8.5. The projected simulations of energy and mass balance indicate that snow and ice melt rate will increase consistently in both future periods with an earlier timing of the snowmelt as it appears in June in the near future (2040-2059) and in May in the far future (2080-2099) under the high emission scenario (RCP8.5). The increase in temperature, precipitation and winter snowpack changes are also expected to have a substantial impact on the hydrological regime in the Naltar catchment, with a peak flow occurring one to two months earlier and a total by 2090 and a decrease of total runoff in the monsoon season by -3 to -24 % in the near and far future, respectively, under RCP 8.5 scenario and more neutral changes (-2 to + 3 %) according to RCP 4.5. Based on these results and the discussion above, water availability in the Naltar catchment will be uncertain by the end of the century.