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

Yi Nan,Lide Tian,Lili Shao,Fuqiang Tian,Zhihua He

doi:10.5194/hess-25-3653-2021

Abstract

Abstract. This study integrated a water isotope module into the hydrological model THREW (Tsinghua Representative Elementary Watershed) 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 generation. Isotope measurements of precipitation water samples and 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 quantification 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 calibrated by three variants with different combinations of streamflow, snow cover area and isotopic composition of stream water. Modeled MTT and MRT were validated by estimates of a tracer-based sine-wave method. Results indicate that (1) the proposed model performs well on simultaneously reproducing the observations of streamflow, snow cover area and isotopic composition of stream water, despite the fact that only precipitation water samples were available for tracer input; (2) isotope data facilitate more robust estimations on contributions of runoff components (CRCs) to streamflow in the melting season, as well as on MTT and MRT; (3) involving isotope data for the model calibration obviously reduces uncertainties in the quantification of CRCs and estimations of MTT and MRT, through better constraining the competitions among different runoff processes induced by meltwater and rainfall. Our results inform scientists on the high value of water isotope data for improving process understanding in a glacierized basin on the Tibetan Plateau.

Highlights

Glacierized catchments in mountainous regions are generally headwater catchments, which are of great interest because of their complex runoff-generation processes and important role on supplying water sources for downstream regions (Immerzeel et al, 2010)
Results indicate that (1) the proposed model performs well on simultaneously reproducing the observations of streamflow, snow cover area and isotopic composition of stream water, despite the fact that only precipitation water samples were available for tracer input; (2) isotope data facilitate more robust estimations on contributions of runoff components (CRCs) to streamflow in the melting season, as well as on mean travel time (MTT) and mean residence time (MRT); (3) involving isotope data for the model calibration obviously reduces uncertainties in the quantification of CRCs and estimations of MTT and MRT, through better constraining the competitions among different runoff processes induced by meltwater and rainfall
Our result indicated that the two model-based methods produced consistent results, which were similar to the lumped method, indicating the robustness of MTT and MRT estimation through a tracer-aided model without defining any prior distribution functions

Summary

Introduction

Glacierized catchments in mountainous regions are generally headwater catchments, which are of great interest because of their complex runoff-generation processes and important role on supplying water sources for downstream regions (Immerzeel et al, 2010). Stable isotopes in water (δ2H and δ18O) are powerful tools for investigating the water cycle and hydrological processes (Gat, 1996; Bowen et al, 2019). Isotopic composition of water changes with multiple ecological and hydrological processes and is affected by several environmental factors (Zhao et al, 2012; Wang et al, 2013); it is frequently used to track the storage and transportation of water. Isotopic compositions generally distinguish between different water bodies and phases (Xi, 2014) and is widely used to determine the relative dominance of water sources, especially in the glacierized catchments (Kong et al, 2019; He et al, 2020). Water isotope data have the potential for improving the understanding of hydrological processes in glacierized catchments

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Discussion

Conclusion