Influence of Hydraulic Conductivity Conceptualization and Unsaturated Flow Parameters for an Integrated Hydrological Model

In recent decades, integrated hydrologic models (IHMs) have advanced our understanding of hydrologic processes across catchment to continental scales. These models couple surface and variably saturated subsurface flow, incorporating land surface models to represent interactions within the critical zone. However, high computational costs hinder calibration and sensitivity analysis. Prior studies show that outputs such as runoff, soil moisture, and energy fluxes are highly sensitive to subsurface parametrization, particularly to the hydraulic conductivity (Formula presented.) in both saturated and unsaturated zones. Despite its relevance in soil-vegetation interactions, sensitivity to unsaturated parameters has been less explored, often limited to synthetic domains. To address this, we designed a deterministic approach consisting of 55 simulations, to explore (Formula presented.) and other parameters sensitivities. We employed the parallel, fully integrated model Parflow-CLM to simulate water and energy fluxes in a headwater catchment in the Odenwald, Germany. Simulations were evaluated against streamflow and soil moisture observations to ensure realistic results. Multiple combinations of (Formula presented.) -values, anisotropies, and van Genuchten parameters were tested, examining the impact of soil hydraulic properties on plant water uptake. Results show that increasing (Formula presented.) enhances baseflow and attenuates peak flows, while anisotropy in shallow horizons significantly affects runoff and groundwater dynamics. The shape of the soil water retention curve, represented by the van Genuchten parameters, as well as saturation at field capacity and wilting point, strongly influence simulated soil moisture during dry periods. Overall, our analysis provides insights into subsurface parameter sensitivities to hydrologic responses, supporting the design of calibration schemes for IHM applications.