Sensitivity analysis of root water uptake reduction using the Bartholomeus model in shallow water table scenarios

A frequently used approach to estimate the reduction of the root water uptake (RWU) caused by oxygen stress in hydrological models such as SWAP is the empirical model of Feddes, which describes RWU using a piecewise linear function. Critical values associated with the threshold pressure heads defining oxygen stress (h₁ = -10 cm and h₂ = -25 cm) seem not to be able to represent properly this condition, because oxygen may start at more negative values of h. As an alternative, Bartholomeus et al. (2008) proposed a model based on physical and physiological soil and root processes to calculate the minimum gas-filled porosity of the soil at which oxygen stress occurs. We performed a sensitivity analysis of the Bartholomeus model focusing on two parameters, the threshold to stop root extension in case of oxygen stress and the air-filled root porosity in shallow water table scenarios cropped with soybean. We performed simulations for five soil types in combination with several water table depths. To do so, the water table was used in SWAP as the lower boundary condition. The sensitivity of the RWU and relative transpiration to combinations of parameters will be shown and discussed.