Simulation of the projected climate change impacts on the river flow regimes under CMIP5 RCP scenarios in the westerlies dominated belt, northern Pakistan

Burgeoning hydropower and irrigation demands will wield substantial stresses on available freshwater resources in the Hindukush-Karakoram-Himalayan (HKH) Range. Probable changes in the river flow regimes as a consequence of changes in precipitation, temperature, and snowpack shall further exacerbate this critical situation. However, probable climate change impacts over the Hindukush range's river flow regimes are unexplored. Here, we assessed the potential impacts of projected climate changes on the outflows of a humid subtropical basin which is nested in the westerlies dominated part of the Hindukush Mountains. Downscaled precipitation and temperature projections from six different Global Climate Models (GCMs), under two Representative Concentration Pathways (RCPs 4.5 and 8.5), were forced in a calibrated Soil and Water Assessment Tool (SWAT) model to investigate the climate change impacts on the Swat River flows. Compared to the baseline period (1981-2010), outputs of all selected GCMs indicated a significant increase in the annual average temperature, ranging from 2.22 to 4.18 degrees C and 4.63 to 8.49 degrees C under RCP 4.5 and 8.5, respectively, by the end of the 21st century. Outputs of four GCMs suggested a concurrent relative increase in the annual average precipitation, ranging from 1.51-22.52% and 1.05-35.98% under RCP 4.5 and 8.5, respectively. However, two GCMs showed decrease in the precipitation, ranging from -9.74 to -2.99 and - 13.72 to -3.24 under RCP 4.5 and 8.5, respectively. Simulated results of the SWAT model showed a significant impact of projected climate change on the Swat River flows by the end of the century that includes (a) increase in the annual average flow under both RCPs (ranging from 0.3 to 44.4% compared to the annual average flow in the baseline period), (b) an increase in the low and medium flows, and (c) an advancement in the peak flow month (from July to June). We conclude that the projected climate changes could substantially influence the seasonality of the Swat River flows. Findings of this study imply that it is important to consider the climate-induced impacts on freshwater resources for sustainable regional planning and management of hydropower projects as well as for irrigation scheduling in the future.