Insights into Northwest Himalayan water cycle from continuous atmospheric water vapor and event-based rainwater isotopes

The Himalayan region, a major source of freshwater for downstream river basins, exhibits strong sensitivity to climate variability due to its complex terrain and the interplay of multiple moisture sources, primarily the Indian Summer Monsoon and western disturbances. This complexity limits the interpretations of rainfall variability and underscores the need for direct constraints on moisture sources and precipitation processes. Continuous monitoring of atmospheric water vapor isotopes (δ²H, δ¹⁸O, and δ¹⁷O), together with meteorological observations, has been instrumental in investigating the moisture transport, condensation processes, and evaporative source characteristics over the region. In this study, we analyze high-resolution atmospheric water vapor isotope measurements obtained using a Picarro L2140-i along with event-based precipitation isotope measurements during the JJAS 2024 season from Manali to assess below cloud rain-vapor interaction, and associated fractionation processes. Distinct intraseasonal variability is evident in the vapor isotope signals. Variations in local and regional meteorology, moisture recycling and the relative contributions of distinct moisture sources are investigated to account for the pronounced isotopic depletion observed during extreme rainfall and cloudburst events. A Lagrangian back-trajectory analysis is used to trace moisture sources associated with precipitation over Manali. We used the specific humidity–δ¹⁸O diagnostic diagrams, constrained by theoretical Rayleigh distillation curves and two-component mixing hyperbolas, to interpret the drivers of intraseasonal isotopic variability. Overall, this contribution highlights the utility of stable isotope analyses for improving process-based understanding of moisture sources, hydrological dynamics, and climate variability across the Himalayan region.