Spatial and temporal variations in water balance components of a Class-II river system in Nepal: a SWAT-based analysis

ABSTRACT The Class-II river systems of Nepal, originating from the Mahabharat range, are crucial yet understudied. This study focuses on the West Rapti Basin, a Class-II river system in western Nepal, examining the spatial and temporal variations in the water balance using the Soil and Water Assessment Tool (SWAT) model. Results indicate that 66% of precipitation becomes surface runoff, 25.7% infiltrates as lateral soil discharge (19.1%) and groundwater flow (6.6%), and 5.8% is lost to evapotranspiration. The basin's potential evapotranspiration is 85.7 mm, with a net water yield of 1,204.5 mm, primarily from the surface flow (75%). The monsoon season is the dominant hydrological driver, with significant increases in rainfall, actual evapotranspiration, and water yield. Spatial variations show higher precipitation in sub-basins 7 and 8 (2,468–2,595 mm) with corresponding higher actual evapotranspiration (AET) rates (105–106 mm), whereas sub-basins 1 and 6 have lower precipitation (665–771 mm) and lower AET. The net water yield (NWY) varies widely (546–2,358 mm), with sub-basin 8 having the highest NWY and sub-basin 6 the lowest. The findings offer critical insights into the hydrological behavior of similar Class-II river systems, aiding in water resource management and planning for the West Rapti Basin.