Unveiling Baseflow Dynamics in Mountainous Catchments: Insights from Stable Isotopes and SWAT Modeling in the Upper Indus Basin

In mountainous catchments of the Upper Indus Basin (UIB), the role of baseflow in water balance remains insufficiently understood. Quantifying baseflow is pivotal for effective water resource management. This research elucidates baseflow dynamics and assesses spatio-temporal trends in baseflow fluxes within high-altitude glaciated and non-glaciated watersheds of the UIB. Leveraging stable water isotopes, the SWAT modelSWAT model, and digital filtering, our study unveils the critical role of baseflow in sustaining surface water resources. Notably, temporal variability in baseflow volume arises from seasonally dynamic aquifer recharge and discharge patterns. In the glaciated watershed (Liddar), baseflow substantially contributes to streamflow, reaching 71% and 55% during winter and autumn, respectively. Conversely, the minimum baseflow contributions of 25 and 28% occur in spring and summer. Likewise, within the non-glaciated watershed (Sandran), baseflow contribution peaks in winter (80%) and autumn (68%), while reaching a minimum in spring (39%) and summer (40%). Comparative analysis of tracer-based (δ18O, Cl−) and model-derived results reveals annual baseflow volumes of 54 and 39% for the glaciated watershed, and 57.68 and 33% for the non-glaciated watershed. This indicates an underestimation of baseflow by 15% and 24.68%, respectively, in model-based estimations compared to tracer-based outcomes. Furthermore, our study underscores the utility of stable water isotopes for understanding baseflow dynamics in ungauged high-altitude watershedsWatershed. The SWAT modelSWAT model demonstrates strong performance in non-glaciated watersheds, displaying superior model ranking when compared to glaciated watersheds. However, in contrast to tracer-based seasonal estimations, our findings reveal irregular and variable seasonal contributions of model-based baseflow volumes in these watershedsWatershed.