Use of CORDEX Ensemble to Simulate Climate Change Impacts on Flow Regime of the Upper Chamkhar Catchment, Bhutan

This study uses glacio-hydrologic analyses and semi-distributed glacio-hydrological modelling to examine the flow regime of the upper Chamkhar catchment for various climate scenarios. The basin encompasses an area of 1446 sq.km with 5% glacierized area. Climate and spatial data are processed to calibrate the modified positive degree day(MPPD) model for runoff simulations. The MPPD model is a simple yet robust melt modelling tool which requires limited forcing data and is suitable for river runoff simulations in a glacierized basin. The 21st century runoff simulations are obtained by driving the MPPD model with climate data from the six regional climate models (RCMs) of CORDEX South Asia experiments for representative concentration pathways RCP 4.5 and RCP 8.5. The model simulates runoff very well for the observed period (1996-2008) with the Nash-Sutcliffe Efficiency (NSE) 0.74 and 2.65 % volume bias. The contribution of snow and ice-melt in the total runoff is 12 % for the observed period. The bias corrected precipitation and temperature data obtained from all six regional climate models along with their ensemble show increasing temperature trends and decreasing precipitation trends. These data are forced into MPPD to generate future river runoff in the basin. Between 2009 and 2099, the average river runoff in the basin is 49.96 m3/s and 53.7 m3/s for RCP4.5 ad RCP 8.5 scenario respectively which is slightly less than the present river runoff ( 55.01 m3/s) in the catchment. Snow ad ice melt contribution in both scenarios seem to increase slightly in the catchment from 12 % to 14% in RCP4.5 and 16% in RCP8.5. The results from this study can be applied for operational and strategic decision support by water planners and policy makers to improve water resources management in this region for present and future uses.