Upstream hydrology and the importance of snowmelt in buffering droughts in the Karnali basin in Nepal

Understanding the hydrology in the upstream mountainous part of the Karnali basin in Nepal is vital, considering the importance of streamflow for downstream nature conservation and water supply. We use a fully distributed hydrological model to understand the current hydrology, the associated vulnerability of the basin, and the importance of the different hydrological components in regulating flow. Downscaled ERA5 meteorological data is used to force the model for the period 1991–2022 at a high spatial resolution (500 meters). We calibrate our model using observed discharges, and the model performance is considered good with a reported Kling-Gupta efficiency of 0.84 and a bias of −3.33%. Our results show that 40% of the overall discharge generated in the Karnali basin originates from rain runoff, 35% from baseflow, 24% from snowmelt, and a negligible 0.8% from glaciers. The water balance components vary spatially in magnitude, but the overall monthly patterns are comparable. On average, the basin receives 1,485 mm/year of precipitation, peaking in July, and is a pronounced southwest region. The annual average evapotranspiration in the basin is 574 mm/year, and discharge is 914 mm/year. Analysis of anomalies reveals that the discharge has become increasingly more variable over the last decades and, therefore, less predictable. Our results also reveal that the basin is frequently experiencing meteorological droughts, often translating into a hydrological drought with a lag time of a month. The average duration of a hydrological drought period in the basin was about 6 months. Snow storage plays an important role in modulating these droughts, and variability in initial snow storage impacts basin streamflow for up to 6 months. A climate change-induced shift from snow to rain may therefore impact the climate resilience of the Karnali considerably.