Assessment of the ISBA Land Surface Model soil hydrology using four closed-form soilwater relationships and several lysimeters

Abstract. Soil water drainage is the main source of groundwater recharge and river flow. It is therefore a key process for water resource management. In this study, we evaluate the soil hydrology and the soil water drainage, simulated by the Interaction-Soil-Biosphere-Atmosphere (ISBA) land surface model currently used for hydrological applications from the watershed scale to the global scale. This evaluation is done using seven lysimeters from two long term model approach sites measuring hourly water dynamics between 2009 and 2019 in northeastern France. These 2-meter depth lysimeters are filled with different soil types and are either maintained bare soil or covered with vegetation. Four closed-form equations describing soil water retention and hydraulic conductivity functions, are tested: the commonly used equations from Brooks and Corey (1966) and van Genuchten (1980), a combination of the van Genuchten soil water retention function with the Brooks and Corey unsaturated hydraulic conductivity function, and, for the very first time in a Land Surface Model (LSM), a modified version of Van Genuchten equations, with a new hydraulic conductivity curve proposed by Iden et al. (2015). The results indicate a good performance by ISBA with the different closure equations in terms of soil volumetric water content and water mass. The drained flow at the bottom of the lysimeter is well simulated using Brooks and Corey (1966) while some weaknesses appear with van Genuchten (1980) due to the abrupt shape near saturation of its hydraulic conductivity function. The mixed form or the new van Genuchten hydraulic conductivity function from Iden et al. (2015) allows solving this problem and even improves the simulation of the drainage dynamic, especially for intense drainage events. The study also highlights the importance of the vertical heterogeneity of the soil hydrodynamic parameters to correctly simulate the drainage dynamic, as well as the primary influence of the parameters characterizing the shape of the soil water retention function.