Quantifying Uncertainties in Climate Change Projection and Its Impact on Water Availability in the Thuli Bheri River Basin, Nepal

Increase in global mean surface temperature due to greenhouse gases and rapid urbanization has resulted in climate change in both regional and global scale. The Intergovernmental Panel on Climate Change (IPCC) estimated the change in global mean surface temperature in the range of 0.3–0.7 °C for a period of 2016–2035 relative to 1986–2005 under four Representative Concentration Pathways (RCPs). In this study, we aim to quantify the uncertainties associated with projecting the future climate and their impacts on the water availability in the Thuli Bheri River Basin of Nepal. We used Soil and Water Assessment Tool (SWAT) as a hydrological model to simulate the runoff from the basin. Five COordinated Regional Climate Downscaling EXperiment-South Asia (CORDEX-SA) regional climate model (RCM) experiments have been used to analyze the impact of different climate models (CMs) on the future river discharge of the basin. The CMs were bias-corrected using quantile mapping (QM) method. Change in river discharge is evaluated for three future time windows, namely near future (2021–2040), mid-future (2041–2070), and far future (2071–2099). Further, we aim to outline the range of uncertainty arising from different projections of the CMs under the two RCPs 4.5 and 8.5 using the probability density function (PDF). The climate projection analysis indicated a significant increase in temperature in the future. Annual precipitation was projected to change from −4% to 16% under five CMs and two RCPs. The ensemble of the five CMs for both RCPs predicted the change of 9–13% in the future period. This uncertainty in climate projection has impacted water availability in different time periods. It also revealed that the uncertainties due to CMs are significantly higher during the high flow season. The results of this research would be helpful to practitioners, researchers, and decision/policymakers to regulate the issues of water availability in the future.