Simulating hydrological processes with a fully coupled surface-subsurface model for estimating catchment travel times

How water and solute are transported in catchments is the foundation for sustainable water management. The flow and transport processes can be described through the travel time of water summarizing the catchment functions of storage, mixing, and release. For a catchment scale, travel time is defined as the time a water particle needs to travel from when it hits the ground surface until it leaves the catchment as discharge or evapotranspiration. Recent studies treated travel time distributions as time-variant in order to reflect the temporal and spatial variability of atmospheric forcing and corresponding hydrologic dynamics through the Master Equation and StorAge selection functions (SAS functions). A challenge is that travel times cannot be directly estimated from data but are inferred from either conceptual or physically based hydrological models. In our study, we employ the integrated surface-subsurface hydrological model Parflow to simulate water fluxes in the forested Weierbach catchment in Luxembourg. However, there are challenges on model parametrization and optimization to build a robust model that is representative of the catchments processes. Our objective here is to setup a robust model for Weierbach catchment based on available catchment parameters. We will evaluate the model against observed streamflow at several sites and soil moisture data. Nevertheless, such model can be used to reveal the spatio-temporal heterogeneity of the hydrological processes at our catchment once it is constrained with the available field data. Future work will consist of directly estimating the travel time of both discharge and evapotranspiration using Parflow and particle tracking (such as EcoSLIM) and will be constrained with the observed stable isotope data.