Spatio-temporal dynamics of hydrologic changes in the Himalayan river basins of Nepal using high-resolution hydrological-hydrodynamic modeling

Abstract The hydrology of the Himalayan region, known as the water tower of Asia, is undergoing rapid transformations due to climate change and growing human influences, and it is known that this region is one of those most vulnerable to climate change. Numerous studies have examined the changes in the hydrology of Nepal, which includes a significant upstream portion of the Himalayas. However, there is a lack of holistic studies on the spatial-temporal evolution of hydrologic dynamics over the entire nation and over long periods. In this study, we present a comprehensive assessment of the changes in river discharge, flood occurrence, and terrestrial water storage (TWS) across all river basins in Nepal using hydrological-hydrodynamic simulations spanning for 40 years (1979-2018) at ∼5km spatial resolution, and downscaled flood attributes at ∼90m resolution. The spatio-temporal variations in river discharge and inundation extent are examined through mapping of decadal trends using a quantile analysis method. The results indicate that the dynamics of river discharge has evolved varyingly across different river basins. The evolution pattern within a basin generally agrees with that at the basin outlet, but notable exceptions are found, indicating high hydro-climatic heterogeneity within the basins. The decadal evaluation of flood dynamics over major flooded areas suggests that inundation dynamics is strongly influenced by various flow characteristics, including the timing, duration, and magnitude, and that the evolution of flood dynamics is more complex than that of river discharge. Results indicate that the TWS dynamics over entire Nepal is strongly modulated by the variations in subsurface water storage, and groundwater storage has been in continuous decline (-1.74 cm/year) in the recent decades (2002-2016). This study provides a basis to advance the understanding of long-term hydrologic changes in the Himalayan region with important implications for improved water resources management.