Evaluating the influence of meteorological drought on river flow in the transboundary Sunkoshi basin, Nepal

ABSTRACT The transboundary Sun-Koshi River basin, characterized by intricate topography and geo-climatic diversity, has encountered distinct periods of droughts significantly impacting downstream river discharge. This study focuses on assessing temporal and spatial patterns of meteorological drought using SPI and SPEI indices, gauging their influence on river discharge through hydro-meteorological station data and the HEC–HMS model. Severe drought episodes were notably observed in 2010 and 2015. In 2010, prominent drought occurrences extended beyond Nepal's border into China. Conversely, in 2015, Jiri, Okhaldhunga, and Salleri experienced heightened drought conditions. Spatially, over 99% of the basin area experienced drought, varying from moderate to extreme magnitudes during 2010 and 2015. The estimated annual rainfall and basin outlet discharge were 1,800 mm, 1,907 mm, and 1,899 m3/s, 1,086 m3/s in 2010 and 2015, respectively. During drought periods, the stations indicated significantly reduced discharge, indicating a marked departure from normal conditions across the basin. Ultimately, station data performance and the HEC–HMS SCS curve number model showed that discharge in the Sunkoshi River basin is profoundly impacted by drought, notably influencing rainfall intensity on monthly, seasonal, and annual scales. The smaller basins discharge more accurate results compared with the larger outlet basins.