Freshwater Availability in the Brahmaputra River Basin Under Projected Climate and Land Use Land Cover Change Scenarios

Abstract:

We used the Soil and Water Assessment Tool to evaluate sensitivities and patterns in freshwater availability due to projected climate and land use changes in the Brahmaputra basin. The daily observed discharge at Bahadurabad station in Bangladesh was used to calibrate the model and analyze uncertainties with SUFI-II algorithm for 1988-1997, and to validate the model for 1998-2004. The R², NS, and biases were, respectively, 0.85, 0.85, and -3.2% during calibration, and 0.89, 0.88, and -4.4% during validation for basinwide simulations of monthly streamflow. The sensitivities and impacts of projected climate and land use changes on basin hydrological components were simulated and analyzed relative to a baseline scenario of 1988-2004. Sensitivity analysis identified a doubling of CO₂ concentration to 660 ppm caused average annual evapotranspiration (ET) to decrease by 12%, resulting in increases in water yield by 5%, streamflow by 6%, and groundwater recharge by 8%. With an increase in temperature, annual average ET was predicted to increase, while responses of water yield and streamflow varied by season. An increase in precipitation caused proportional increases in water yield, streamflow, and groundwater recharge, but led to only minor impacts on ET. Annual average water yield, soil water content, ET, streamflow, and groundwater recharge were predicted to increase with higher seasonal variability in response to climate and land use change projections for the A1B and A2 scenarios generated from downscaled CGCM3.1 and IMAGE, respectively. Water yield, soil water content, streamflow, and groundwater recharge were predicted to increase with a strong increasing trend during August to October, indicating exacerbated flood potential, while during May to July, the hydrological components–except soil water content–were predicted to decrease with a strong decreasing trend, indicating enhanced drought potential throughout the 21^st century. Overall, results indicated that the model could provide reasonable predictions of monthly basinwide freshwater availability, therefore, could facilitate strategic decision making through scenario generation that integrates climate change adaptation and hazard mitigation policies to ensure optimized allocation of water resources under a variable and changing climate