Understanding the seasonal and spatial variation of water balance in the Karnali basin in Nepal

The hydrology of large mountainous basins is sensitive to climate and land use change and impacts downstream availability in a diverse way. Our knowledge of the spatial and temporal variation of the water balance for large-scale mountainous basins like the Karnali (40,000 km²) is very limited.  Studies focus either on small alpine catchments or on major river basins of near continental scale. Studies focusing on the intermediate scale, where mountain water supply is directly linked to people and ecosystems downstream are scarce, but needed. In this study, we provide insight into the seasonal and spatial differences in meltwater contribution to streamflow, rain runoff, evapotranspiration and groundwater baseflow, with a particular focus on upstream-downstream dependencies. We use a high-resolution SPHY model, which we calibrate step-wise using satellite data of glacier mass balance and snow covers and observed river flow data. We explore the hydrological variability at the sub-basin scale, discuss the seasonal and spatial heterogeneity of the water balance components, and seek to understand the major drivers. Our results provide a baseline against which impacts of climate and land use changes will be assessed in a subsequent study.