Effects of land use and land cover change on streamflow in the Tamor Basin, Nepal: A distributed hydrological modeling approach for hydropower assessment

The alteration of land use and land cover (LULC) is a key driver of hydrologic change that is often overlooked in high Himalayan basins, where studies typically emphasize climate and glaciers controls over the poorly quantified role of LULC as an independent variable. This study quantifies the LULC-induced component of the streamflow in the Tamor River basin, Nepal, using an experimental approach by imposing a constant meteorological forcing from 2009 and varying the LULC data for 2009, 2014, and 2019 obtained from ICIMOD. The Spatial Processes in Hydrology (SPHY) model was first calibrated (NSE = 0.78, PBIAS = 2.11%) and then validated (NSE = 0.61, PBIAS = -13.32%) against observed discharge at Majitar station. Contraction of cropland and bare-rock cover between 2009 and 2019 lowered the area-weighted crop coefficient, reducing evapotranspiration and increasing both baseflow and rainfall runoff by more than one third. The annual energy generation potential of a representative run-of-river hydropower project with a head of 100 m increased by 17%, and the firm energy projection increased by 51%. These findings demonstrate that LULC change produced a hydrological signal of sufficient magnitude to materially affect both the dry-season energy reliability and monsoon-season operational risks of run-of-river hydropower projects.