The value of water isotope data on improving process understanding in a glacierized catchment on the Tibetan Plateau

Abstract. This study integrated a water isotope module into the hydrological model THREW which has been successfully used in high and cold regions. Signatures of oxygen stable isotope (18O) of different water inputs and stores were simulated coupling with the simulations of runoff generations. Isotope measurements of precipitation water samples and global precipitation isotope product, as well as assumed constant isotope signature of ice meltwater were used to force the isotope module. Isotope signatures of water stores such as snowpack and subsurface water were updated by an assumed completely mixing procedure. Fractionation effects of snowmelt and evapotranspiration were modeled in a Rayleigh fractionation approach. The isotope-aided model was subsequently applied for the quantifications of runoff components and estimations of mean water travel time (MTT) and mean residence time (MRT) in the glacierized watershed of Karuxung River on the Tibetan Plateau. Model parameters were constrained by three different combinations of observations including a single-objective calibration using streamflow measurement solely, a dual- objective calibration using both streamflow measurement and MODIS estimated snow cover area, and a triple- objective calibration using additionally isotopic composition of stream water. Modeled MTT and MRT was validated by estimate of a tracer-based sine-wave method. Results indicate that: (1) the proposed model performed quite well on simultaneously reproducing the observations of streamflow, snow cover area, and isotopic composition of stream water, despite that only precipitation water samples were available for tracer input; (2) isotope data helped to estimate more plausible contributions of runoff components (CRCs) to streamflow in the melting season, and improved the robustness of MTT and MRT estimations; (3) involving isotope data for the model calibration obviously reduced uncertainties of the quantification of CRCs and estimations of MTT and MRT, through better constraining the strong competitions among different runoff processes induced by meltwater and rainfall. Our results inform high value of water isotope data on improving process understanding in a glacierized basin on the Tibetan Plateau.