Runoff simulation of ungauged catchments : importance in the Nepalese context

Nepal is a landlocked country in the foothills of the Himalayan region in South Asia and a country endowed with rich water resources. However, the country is unable to utilize and manage the full potential of available water resources. One of the reasons for this is the lack of an adequate network of river gauging stations necessary to collect hydrologic data. Installation of hydrological stations is an expensive proposition and not financially viable for small water resources projects (water supply, irrigation, mini and micro-hydro projects). This research aims to address the challenges via an alternative strategy - i.e. the use of a hydrological model which can reliably simulate runoff in ungauged catchments even in the absence of adequate hydrologic data. SWAT (Soil & Water Assessment Tool), a popular simulation model with ArcGIS and QGIS interface, was chosen to simulate flow in an ungauged catchment in the mid-western region of Nepal. The model was applied to the West Rapti River basin using five years (1981-1985) of data from the Global Runoff Data Centre (GRDC). The GRDC was the only source, and the dataset was incomplete, limiting the model calibration and validation process. This limitation was addressed by using another simulation model, HEC-HMS (Hydrologic Engineering Centre’s Hydrologic Modeling System), for comparison. The results of the SWAT model were compared with those from HEC-HMS, one of the most widely used rainfall-runoff simulation models. Comparative analysis showed that both models generated comparable results. Historical rainfall data (1979-2009) were extracted from the Global Weather Data for SWAT to predict the rainfall trend in the West Rapti Watershed. This trend in rainfall pattern was used to extract rainfall and simulate runoff for 2023 to 2026, considering rainfall data of 2013 as a baseline. The simulated results showed a minor shift in time to peak and increased peak discharge. Similarly, the simulated runoff trends matched perfectly with the observed rainfall trend in SWAT. Thus, the results proved the reliability of SWAT to simulate runoff in the West Rapti Basin. The conclusion was drawn that the SWAT model can be used reliably to predict runoff in ungauged catchments that assist with managing water resources and contribute to the development of Nepal’s economy