Performance assessment of a conceptual model to simulate fluxes in the unsaturated zone to better represent runoff and infiltration processes

In continuation to the previously presented methodological approach to estimate vadose zone boundary fluxes titled “A novel conceptualization to estimate unsaturated zone mass-fluxes and integrate pre-existing surface- and ground- water models” at the EGU GA 2022, and the performance assessment thereof showcased at the EGU GA 2023, titled “Performance assessment and Benchmarking of a conceptually coupled groundwater - surface-water model”, this study aims to assess the performance of the proposed methodology to couple surface- and ground- water models aims to investigate its local performance in a soil-column by comparing the results of a controlled simulation with that of HYDRUS-1D.   To recap, the initial study presented a conceptual numerical scheme that aimed to adequately estimate the in- and out- fluxes of the Unsaturated Zone (UZ) with the primary aim of coupling existing groundwater (GW) and surface-water (SW) models. It was expected that such a numerical scheme would provide a viable alternative to solving the computationally expensive Richard’s model for cases where description of fluxes within the UZ and the spatial description of the soil moisture were not in the interest of the modeller. Examples of such cases would be efforts to model the hydro(geo)logical effects of various climate-scenarios, efforts to estimate GW recharge dynamically, and efforts to design integrated watershed management design structures and systems, among others.   The model, and in effect, the methodology, established its capacity to simulate the fluxes of the UZ for the Tilted-V theoretical catchment setup during its comparison against the physically based ParFlow model, in the previous study. However, it also did demonstrate certain crucial shortcomings that arose from the nature of the coupling scheme (loose coupling - where the models ran consecutively until the end of a timestep, exchanged information, and continued so and and so forth until the end of the simulation period) used to couple the two GW and SW models.   In this study, the authors aim to more effectively assess the methodology, by attempting to simulate a real-world scenario of transport of water fluxes in the subsurface of a Spruce/Beech Stand in a Peatland experimental site in the Bohemian Forest region of Czechia. The model setup involves the simulation of fluxes in a 1D soil profile using the said methodology and also using the HYDRUS-1D modelling software and comparing the results of the two models and the results with observations. It is expected that such a setup should provide a robust assessment of the methodology. The discussion shall be an extensive analysis of the obtained results.   The authors also hope that the study fosters discussions to unify the polarising modelling approaches as outlined in Markus Hrachowitz er al., 2017.