Rainfall-runoff modeling using HEC-HMS model in an ungauged Himalayan catchment of Himachal Pradesh, India

Assessment of environmental flow, that has to be maintained along a waterway to keep up health of riverine biological systems, is a key challenge in alleviating impact of establishing hydropower projects especially in mountainous ungauged catchment under limited data conditions. This study addresses the data scarcity issue by prediction of runoff from a Himalayan catchment, India, using HEC-HMS model and then estimating environmental flow based on daily rainfall data of 39 years (1980–2018). The soil conservation service–curve number method is employed for surface runoff estimation that utilizes spatially distributed maps of soil types, drainage, stream order, 4-year land use/land cover (1990, 2000, 2010, and 2020), and hydrologic soil group (HSG). Steep slopes (more than 60%), high annual rainfall (1377 mm), large area under C class of HSG (477.94 km 2 ), and moderate values of curve number (70.51, 70.36, 70.16, and 70.54) revealed high potential for surface runoff generation in the catchment. Predicted runoff depicted a gradually increasing trend during 1980–1995 and decreasing trend during 1995–2008 and 2011–2017. In addition, an abrupt change was observed in annual runoff values in years 1992, 1998, and 2018 when the peak rate of runoff crossed the value of 2000 m 3 s −1 . The HEC-HMS model is validated by close agreement between peaks and troughs of runoff and rainfall values, and with reasonable values of correlation coefficient (0.57) and coefficient of determination (0.33). The annual values of environmental flow is obtained as 75 and 55 m 3 s −1 from the flow duration curves at 70th and 90th percentiles, respectively. Findings of this study are useful for management of flood water in other ungauged mountainous catchment of Himalayan region as well as in other parts of the world under data scarcity conditions.