Determination of empirical parameters for root water uptake models

Detailed physical models describing root water uptake (RWU) are an important tool for the prediction of RWU and crop transpiration, but involved hydraulic parameters are hardly-ever available, making them less attractive for many studies. Empirical models are more readily used because of their simplicity and lower data requirements. The purpose of this study is to evaluate the capability of some empirical models to mimic the RWU distribution under varying environmental conditions predicted from numerical simulations with a detailed physical model. A review of some empirical models used as sub-models in ecohydrological models is presented, and alternative empirical RWU models are proposed. The parameters of the empirical models are determined by inverse modelling of simulated depth-dependent RWU. The simulated scenarios give more insight into the behaviour of the physical model, especially under wet soil conditions and high potential transpiration rate. The performance of the empirical models and their optimized empirical parameters depend on the scenario. The largely used empirical RWU model by Feddes only performs well in scenarios with low root length density R, i.e. for the scenarios of low RWU "compensation". For medium and high R, the Feddes RWU model cannot mimic properly the root uptake dynamics as predicted by the physical model. The RWU model by Jarvis provides good predictions only for low and medium R scenarios. For high R, the Jarvis model cannot mimic the uptake patterns predicted by the physical model. Incorporating a newly proposed reduction in the Jarvis model improved RWU predictions. The proposed models are more capable of predicting similar RWU patterns by the physical model. The statistical indices point them as the best alternatives to mimic RWU predictions by the physical model.