A mechanistic root water uptake model including soil and plant resistance

Quirijn de Jong van Lier

University of São Paulo, Brazil


Transpiration reduction functions are used in hydrology models to estimate actual transpiration as a function of soil water status. Empirical reduction functions are most frequently used due to the higher data needs and computational requirements of mechanistic models. Empirical models, however, lack a description of physical mechanisms and their parameters require extensive calibration. Modeling of water transport from soil to root to shoot is used to derive a process-based reduction function allowing the prediction of system potentials, resistances and water flows. An analytical solution for a special case of Brooks and Corey soils is presented. A numerical version of the reduction function for Van Genuchten soils was implemented in the SWAP hydrological model allowing predictions for layered soil profiles and root length density variations over depth. The analytic and numerical versions of the model allow an increasingly quantitative insight into the mechanism of root water uptake. The approach can be used for the estimation of threshold values for empirical reduction functions.