Isotope and Temperature Index Modelling of Runoff Generation in the Snowy Headwaters of Western Himalayas, India

This study quantified the seasonal contributions of snow/glacier melt, rainfall runoff, and groundwater/baseflow to discharge in the Beas and Parbati headwaters of the Indus River Basin, western Himalaya, India. Long-term hydro-meteorological records, Moderate Resolution Imaging Spectroradiometer snow-cover data, the SNOWMOD temperature-index model, and stable-isotope measurements of δ18O and δD were integrated to examine runoff-generation processes. Hydro-meteorological records from 1986 to 2010 showed an increase in mean annual air temperature of approximately 0.05 °C year−1 and a decline in precipitation of approximately 11.30 mm year−1, coinciding with reduced annual discharge in the Beas and Parbati Rivers. SNOWMOD simulations for 2005–2011 indicated that rainfall runoff, snow/glacier melt runoff, and groundwater/baseflow contributed approximately 11–14%, 31–35%, and 53–54%, respectively, to annual discharge in the Beas River. Corresponding contributions in the Parbati River were 12–15%, 33–37%, and 51–53%, respectively. Stable-isotope mass-balance analysis for 2010–2012 estimated annual snow/glacier melt, rainfall runoff, and groundwater/baseflow contributions of approximately 32%, 18%, and 50% in the Beas River and 38%, 14%, and 48% in the Parbati River. The broad consistency between model-derived and isotope-based estimates indicates that snow/glacier melt is an important component of annual runoff, while groundwater/baseflow sustains low flows during post-monsoon and winter periods. Because the modelling and isotope analyses represent overlapping but non-identical periods, the comparison is interpreted as corroborative rather than as direct validation. The integrated framework provides a basis for assessing runoff-source variability and supporting water-resource planning in snow/glacier-fed Himalayan catchments.