Evaluation of long-term changes in water balances in the Nepal Himalayas

The Himalayan region is one of the world's youngest and greatest mountain ranges, serving as the source of several major rivers in Asia. Nepal, situated in the middle of the Himalayas, has a diverse climate that varies from tropical to polar, with highly heterogeneous precipitation patterns and hydrological regimes. This study analyzes long-term changes in water balance parameters from 1961 to 2020 across Nepal using TerraClimate (a global gridded dataset), splitting the period into two 30-year phases, reflecting pre-development (1961-1990) and development (1991-2020). The water balance parameters include precipitation, actual and potential evapotranspiration, runoff, climate water deficit, soil moisture, minimum temperature, and maximum temperature. These parameters illustrate the various in- and out-fluxes of a hydrologic system and the net change in water storage. This study also examines the temporal variation and monotonic trends in water balance in different physiographical and spatial regions. We divide the country into nine regions, and additionally, look at seasonal and annual scales. Results show that maximum and minimum temperatures have an increasing trend and, therefore, indicate a positive deviation during the later period (1991-2020) compared to the earlier period (1961-1990) in all regions. Furthermore, we find that precipitation and soil moisture are slightly decreasing in the later period, while climate water deficit is increasing, indicating increased water stress, particularly in the western mountains. In addition, several springs in the mountains of Nepal are disappearing, which indicates an increase in water scarcity and vulnerability. Seasonally, post-monsoon and winter temperatures are increasing faster than pre-monsoon and monsoon temperatures. Looking at the precipitation pattern, monsoon and post-monsoon precipitation are decreasing overall, and pre-monsoon precipitation is mostly increasing. These findings can help to better comprehend water availability in a region and guide water management strategies.