Evaluation and application of a SWAT model to assess the climate change impact on the hydrology of the Himalayan River Basin

The water resources of Tamor River Basin in the eastern Himalayas of Nepal is heavily tapped for agricultural water use and hydropower production. Several studies show that the hydrology of the Himalayan River Basins is vulnerable to climate change. Topographical variations, data scarcity and its complex climate have always been barriers to achieving realistic simulation of the hydrological regime in the Himalayan River Basins, and the Tamor River Basin is no different. In hydrological response unit (HRU) based models, fixing the number of HRUs and sub-basins has a tendency towards confusion. This is especially true in regions with altitudinal and climatic variations. Therefore, this study aims to evaluate the SWAT model performance uncertainty associated with the number of sub-basins, HRUs, and elevation bands and quantify the impact of climate change on streamflows in the Tamor River Basin. Future climate scenarios were developed for three different time frames, the 2030s, 2060s, and 2080s, based on an ensemble of Coupled Model Intercomparison Project Phase 5 (CMIP5) and four Regional Climate Models (RCMs) under the Representative Concentration Pathways (RCP4.5 and RCP8.5) scenarios. The linear scaling method (LSM) of bias correction was used to bias-correct the climate data and then fed into the SWAT model to simulate the future streamflows of the basin. An increase in the annual average maximum temperature ( + 4 degrees C) and minimum temperature ( + 5.5 degrees C), and a decrease in precipitation ( - 4.5%) is projected by the end of the twenty-first century under RCP8.5 scenarios. The study found that the future climate could decrease the streamfiow by over 8.5% during the twenty-first century under RCP8.5 scenarios. The results show elevation bands (EBs) to be more significant compared to HRUs and sub-basins (SBs) in getting a robust hydrological model in the Himalayan region. The estimates prepared in this study would be beneficial for hydropower developers, planners and policymakers, and water resource managers in developing adaptation strategies to offset the negative impact of climate change in the Tamor River Basin.