Isoscape of Surface Runoff in High Mountain Catchments: An Alternate Model for Meteoric Water Characterization and Its Implications

Water isotope-based hydrological and paleoaltimetry studies in high mountain areas are generally done using the isotopic composition of river discharge. However, rivers capture a basin averaged signal of regional precipitation and are less likely to register the meteorological processes intrinsic to distinct hydrological fractions at different altitudes. This has been observed in the Khumbu (Mt. Everest) Himalayan watersheds of Dudh Kosi Basin (DKB), where the δ18O values of snowpack and stream water vary non-uniformly with altitude while the δ18O values of river water show a uniform relationship. Snow exhibits the highest (+0.9 to −4.4‰/100 m) isotopic lapse rate (ILR), followed by streams (+0.2 to −0.4‰/100 m) and rivers (−0.05‰/100 m). Sublimation, catchment vegetation, diurnal temperature, cloud type, and insolation play a significant role in controlling the isotopic composition of snowpack and stream water. Similarly, the isotopic composition of small streams disproportionately represents the meteoric water composition of an area, as rainfall immediately joins the stream-runoff while the snow melts gradually around the year. To map the isotopic heterogeneity in DKB surface waters, we have modeled the isoscape for surface runoff using the isotopic composition of snow and stream water, and remotely sensed parameters. Accordingly, we simulate the isoscapes for snow and stream-runoff via multi-regression models which extrapolate the observed data as a function of the controlling factors. The amount-weighted summation of both the isoscapes (relative contribution (%) *δ18O value) constitutes the hydropool. The hydropool model incorporates spatiotemporal variation in ILR computed from the δ18O values of surface runoff.