Markus Weiler

Understanding the transport processes and travel times of pollutants in the subsurface is crucial for an effective management of drinking water resources. Transport processes and soil hydrologic processes are inherently linked to each other. In order to account for this link, we couple the process‐based hydrologic model RoGeR with StorAge Selection (SAS) functions. We assign to each hydrological process a specific SAS function (e.g., power law distribution function). To represent different transport mechanisms, we combined a specific set of SAS functions into four transport model structures: complete‐mixing, piston flow, advection‐dispersion and advection‐dispersion with time‐variant parameters. In this study, we conduct modeling experiments at the Rietholzbach lysimeter, Switzerland. All modeling experiments are benchmarked with HYDRUS‐1D. We compare our simulations to the measured hydrologic variables (percolation and evapotranspiration fluxes and soil water storage dynamics) and the measured water stable isotope signal ( 18 O) in the lysimeter seepage for a period of ten years (1997–2007). An additional virtual bromide tracer experiment was used to benchmark the models. Additionally, we carried out a sensitivity analysis and provided Sobol indices for hydrologic model parameters and SAS parameters. Our results indicate that the advection‐dispersion transport model produces the best results. And thus, advective‐dispersive transport processes play a dominant role at Rietholzbach lysimeter. Our modeling approach provides the capability to test hypotheses of different transport mechanisms and to improve process understanding and predictions of transport processes. Overall, the combined model allows a very effective simulation of combined flux and transport processes at various temporal and spatial scales.

Water is a limiting factor for plant development. Therefore, understanding plant–water relations is vital for food security and ecosystem conservation. The stable isotopes of water (δ 2 H and δ 18 O) are widely used to study ecohydrological processes, such as root water uptake. However, obtaining water from plants to measure their stable isotopic composition requires water extraction techniques that are laborious and challenging. A method developed for soil pore water that circumvents water extraction is the water‐vapour equilibrium (WVE) method. In this study, we tested the capability and limits of WVE to determine stable water isotopes in different woody and non‐woody plant organs. For this purpose, we analysed roots, stems, twigs, leaves and fruits of various plants. We first tested the effect of various equilibration times (24–72 h) and preparation techniques (cutting or grinding samples) on the measured isotope ratios. Analyser‐internal variables that react to volatile organic compounds (VOCs) interfering with the measurement were also considered. Cutting samples and equilibrating for max. 24 h resulted in the most plausible isotope ratios, which was further tested as a proof‐of‐concept using controlled irrigation experiments. We could show expected progressive isotope enrichment from roots to leaves. Further, the isotopic composition of the water in fruit cores was more similar to irrigation water than that in fruit skins, and all the obtained results were consistent with the current process understanding of stable isotopes of water in plants, showing the feasibility of the chosen sample preparation. WVE seems to be a promising method to measure plant water isotopes with the challenge of dealing with VOCs likely influencing results.