Projected Climate Trends and Hydroclimatic Extremes in the West Seti Basin, Nepal

Abstract

Agricultural water management is closely linked to the temporal variations in long-term rainfall and temperature, as well as their associated patterns. However, variations in rainfall and temperature along with extreme rainfall and wet/dry spells, remain highly uncertain in mountainous regions. This study evaluated long-term rainfall and temperature patterns and their associated trends using the latest Coupled Model Intercomparison Project-Phase 6 (CMIP6) Global Climate Models (GCM) projected rainfall and temperature for the West Seti River Basin in Nepal. The investigation showed statistically significant increasing trend in annual rainfall and temperature across future time periods under all Shared Socioeconomic Pathway (SSP) scenarios. Annual rainfall is projected to increase from 1.47–1.54 mm year⁻¹ under SSP126 to 6.50–6.90 mm year⁻¹ under SSP585, while annual maximum and minimum temperatures increased by about 0.01°C–0.05°C year⁻¹ and 0.008°C–0.063°C year⁻¹, respectively. Although there is an increase in annual temperature, a significant decreasing trend in dry spell is projected by most of the climate models, whereas consecutive wet days increased by up to 0.35 days year⁻¹ under SSP585. As the warming continues, the magnitude and frequency of extreme rainfall also increased, with 10–11 out of 13 CMIP6 models projecting significant positive trends in rainfall exceedance at 90th percentile (R90p) and rainfall exceedance at 95th percentile (R95p) under high emissions, reaching up to 5.77 mm year⁻¹. With a greater number of climate models predicting an increase in wet spell and some models suggesting a decrease in dry spell, it is likely that the catchment will experience a wetter hydroclimatic regime in the future. This presents ample opportunities for harvesting, storage, management and allocation of rainwater for irrigation, while emphasizing the need for climate-resilient watershed and water resources management to mitigate the impacts of increasing hydroclimatic extremes.