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 delta O-18 values of snowpack and stream water vary non-uniformly with altitude while the delta O-18 values of river water show a uniform relationship. Snow exhibits the highest (+0.9 to -4.4 parts per thousand/100 m) isotopic lapse rate (ILR), followed by streams (+0.2 to -0.4 parts per thousand/100 m) and rivers (-0.05 parts per thousand/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 (%) *delta O-18 value) constitutes the hydropool. The hydropool model incorporates spatiotemporal variation in ILR computed from the delta O-18 values of surface runoff.

Automated summary

Water isotope studies in high mountain areas typically use the isotopic composition of river discharge, but this may not capture the distinct meteorological processes at different altitudes. In the Khumbu Himalayas, snow and stream water exhibit non-uniform variations in isotopic values with altitude, while river water shows a uniform relationship. Various factors such as sublimation, vegetation, temperature, cloud type, and insolation play a significant role in controlling the isotopic composition of surface waters in the region.