Quoted from: Soylu, Mehmet Evren, Christopher J. Kucharik, and Steven P. Loheide II. "Influence of groundwater on plant water use and productivity: Development of an integrated ecosystem–Variably saturated soil water flow model." Agricultural and Forest Meteorology 189 (2014): 198-210. https://doi.org/10.1016/j.agrformet.2014.01.019 

The soil water flux algorithm of the pre-existing Agro-IBIS neglected the role of groundwater, which is similar to many other LSMs (e.g., Dickinson et al., 1993, Sellers et al., 1996). Even though earlier studies have incorporated saturated soil layers into IBIS to represent groundwater (Kim and Eltahir, 2004, Yeh and Eltahir, 2005, Soylu et al., 2011), using the θ-based form of the Richards’ equation can be problematic if saturated soil layers exist in the soil domain. The reason for this is that soil moisture within the saturated porous medium (i.e., below the top of the capillary fringe) does not vary in a way that is consistent with pressure head fluctuations (Celia et al., 1990, Pan and Wierenga, 1995). This physical inconsistency does not cause problems for fully unsaturated soil domains, but when the water table is close to the surface, capillary upward water movement cannot be properly calculated using a θ-based form of Richards’ equation. To overcome this problem, we fully coupled Agro-IBIS and Hydrus-1D models (AgroIBIS-VSF henceforth, VSF stands for variably saturated flow) by replacing the Agro-IBIS soil water flux and heat transport algorithms with the Hydrus-1D model, which allows the model to simulate the impacts of groundwater on the plant/crop system in a more complete, physically-based fashion (Fig. 1). Soil moisture and temperature within each soil layer are therefore calculated by the approach of the Hydrus-1D model. These soil water and energy quantities ultimately influence root water uptake, evaporation, and infiltration, which affect important plant physiological processes in Agro-IBIS part of the model such as photosynthesis, respiration, and stomatal conductance, as well as the overall water balance.

 

Fig. 1 Agro-IBIS dynamic agroecosystem model structure. The blue outlined box depicts unsaturated and saturated soil layers represented by Hydrus-1D model, which simulates volumetric water content, hydraulic heads and soil temperatures at each soil layer. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)