Effects of streamflow isotope sampling strategies on the calibration of a tracer‐aided rainfall‐runoff model

Abstract

Isotopes are increasingly used in rainfall‐runoff models to constrain conceptualisations of internal catchment functioning and reduce model uncertainty. However, there is little guidance on how much tracer data is required to adequately do this, and different studies use data from different sampling strategies. Here, we used a 7‐year time series of daily stable water isotope samples of precipitation and streamflow to derive a range of typical stream sampling regimes and investigate how this impacts calibration of a semi‐distributed tracer‐aided model in terms of flow, deuterium and flux age simulations. Over the 7 years weekly sampling facilitated an almost identical model performance as daily, and there were only slight deteriorations in performance for fortnightly sampling. Monthly sampling resulted in poorer deuterium simulations and greater uncertainty in the derived parameter sets ability to accurately represent catchment functioning, evidenced by unrealistic reductions in the volumes of water available for mixing in the saturation area causing simulated water age decreases. Reducing sampling effort and restricting data collection to 3 years caused reductions in the accuracy of deuterium simulation, though the deterioration did not occur if sampling continued for 5 years. Analysis was also undertaken to consider the effects of reduced sampling effort over the driest and wettest hydrological years to evaluate effects of more extreme conditions. This showed that the model was particularly sensitive to changes in sampling during dry conditions, when the catchment hydrological response is most non‐linear. Across all dataset durations, sampling in relation to flow conditions, rather than time, revealed that samples collected at flows >Q50 could provide calibration results comparable to daily sampling. Targeting only extreme high flows resulted in poor deuterium and low flow simulations. This study suggests sufficient characterization of catchment functioning can be obtained through reduced sampling effort over longer timescales and the targeting of flows >Q50.

Assessing the influence of water sampling strategy on the performance of tracer-aided hydrological modeling in a mountainous basin on the Tibetan Plateau

“…The uncertainty of CRC estimation kept decreasing until the data series length reached four years and two years, for the aspects of water source and runoff pathway, respectively. This was consistent with the finding by Stevenson et al (2021) that the benefits from isotope plateaued after a certain year number, which was five for that study.…”

“…It can be expected that more data help to provide more constrains on identification of model parameters. Nonetheless, water sampling in high mountains on the TP is restricted by environment accessibility, financial and human costs (Stevenson et al, 2021. It is therefore highly needed to find optimal strategies of collecting water samples that balance well between data adequacy for model running and affordable sampling cost (Sprenger et al, 2019).…”

“…Some water sampling works in large mountainous catchments were conducted in a single field campaign (e.g., Xia et al, 2019;Dong et al, 2018), which is, although helpful to understand the generations of shortterm runoff events, not suitable for the calibration of tracer-aided models in a multi-year simulation period (Knapp et al, 2019;Zhang et al, 2019). An exception is Stevenson et al (2021) who utilized a 7-year dataset of stream water δ 18 O in a 3.2 km 2 catchment to analyze the effects of stream water sampling strategies on the calibration of a tracer-aided hydrological model. Challenges arise when transferring their findings to the application of tracer-aided hydrological models in large high-mountain basins: First, it is questionable that whether sampling stream water at one site can adequately represent the isotope signature of stream water over the whole basin, considering the strong spatial variability of hydrological processes caused by the heterogeneity in meteorological factors and land surface conditions in mountains (Wang et al, 2021;Li et al, 2020).…”

Precipitation fate and transport in a Mediterranean catchment through models calibrated on plant and stream water isotope data

“…One such approach is through using StorAge Selection (SAS) functions (e.g., Benettin et al, 2017;5 Fang et al, 2019;Rodriguez and Klaus, 2019), which represent the relationship between the age distributions of water stored in a system (i.e. a catchment or other control volume) to the age distributions of the fluxes, such as evapotranspiration (ET) and discharge (Q), leaving the system (Botter et al, 2011;Harman, 2015;van der Velde et al, 2012). However, a challenge in using SAS functions is that values of the parameter representing the storage selection of ET fluxes are often 10 unidentifiable.…”

“…While such simplistic transport assumptions often violate field observations, they were made in over 2/3 of 20 the tracer-aided modelling studies over the last decade (Sprenger and Allen, 2020). Such simplification can be problematic beyond simply constraining the ages of water contributing to ET, as it has been previously demonstrated that assumptions regarding ET and shallow subsurface mixing also affect calculated runoff travel time distributions (van der Velde et al, 2012). This feedback between subsurface water sustaining the ET flux or leaving the catchment via Q has been 25…”