Abstract
Tracer-aided hydrological models integrating water isotope module into the simulation of runoff generation are useful tools to reduce uncertainty of hydrological modeling in cold basins that are featured by complex runoff processes and multiple runoff components. However, there is little guidance on the strategy of field water sampling for isotope analysis to run tracer-aided hydrological models, which is especially important for large mountainous basins on the Tibetan Plateau (TP) where field water sampling work is highly costly. This study conducted a set of numerical experiments based on the THREW-T model to evaluate the reliance of the tracer-aided modeling performance on the availability of site measurements of water isotope in the Yarlung Tsangpo River (YTR) basin on the TP. Data conditions considered in the numerical experiments included the availability of glacier meltwater isotope measurement, quantity of site measurements of precipitation isotope, and the variable collecting strategies for stream water sample. Our results suggested that: (1) In high-mountain basins where glacier meltwater samples for isotope analysis are not available, estimating glacier meltwater isotope by an offset parameter from the precipitation isotope is a feasible way to force the tracer-aided hydrological model. Using a set of glacier meltwater δ18O that were 2 ‰~9 ‰ lower than the mean precipitation δ18O resulted in only small changes in the model performance and the quantifications of contributions of runoff components (CRCs, smaller than 5 %) to streamflow in the YTR basin; (2) strategy of field sampling for site precipitation to correct the global gridded isotope product of isoGSM for model forcing should be carefully designed. Collecting precipitation samples at sites falling in the same altitude tends to be worse at representing the ground pattern of precipitation δ18O over the basin than collecting precipitation samples from sites in a range of altitudes; (3) Collecting weekly stream water samples at multiple sites in the wet and warm seasons is the optimal strategy for calibrating and evaluating a tracer-aided hydrological model in the YTR basin. It is highly recommended to increase the number of stream water sampling sites rather than spending resource on extensive sampling of stream water at a sole site for multiple years. These results provide important implications for collecting site measurements of water isotope for running tracer-aided hydrological models to improve quantifications of CRCs in the high-mountain basins.
Highlights
Catchments located in mountainous regions generally provide important water resources for downstream regions (Viviroli et al, 2003)
Dominant characteristic of mountainous catchments on TP is the multiphase of water sources that generate runoff and the complex hydrological processes, highlighting the importance of accurately quantifying the contributions of runoff components (CRCs) to streamflow for better understandings of the runoff dynamics under changing climate
In comparison to simulations constrained by stream water isotope data from multiple sampling years, results constrained by stream water isotope data from multiple sampling sits yielded lower Mean absolute error (MAE) and standard deviation (STD) for the quantified CRCs
Summary
Catchments located in mountainous regions generally provide important water resources for downstream regions (Viviroli et al, 2003). Dominant characteristic of mountainous catchments on TP is the multiphase of water sources that generate runoff and the complex hydrological processes, highlighting the importance of accurately quantifying the contributions of runoff components (CRCs) to streamflow for better understandings of the runoff dynamics under changing climate. The isotopic compositions of runoff components strongly differ in high-mountain basins resulting from the following two reasons: One is the significantly more depleted δ18O of meltwater compared to that of rain, due to the altitude and temperature effects, and the fractionation effect during melting processes (Xi, 2014; Boral and Sen, 2020) Another is the damping and lagging isotopic variability of subsurface runoff pathway, compared to that of surface runoff, as a result of the catchment hydrological functions of storing, mixing and transporting water (Bowen et al, 2019; Birkel and Soulsby, 2015; McGuire and McDonnell, 2006). Three specific questions were addressed: (1) how does the estimated isotopic composition of glacier meltwater influence the performance of tracer-aided hydrological modeling when no glacier meltwater samples were available, (2) how does the collection strategy of site precipitation samples for the correction of iGCM influence the model performance, and (3) how does the sampling strategy of stream water influence the model calibration and evaluation?
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