MODELING ROOT WATER POTENTIAL AND SOIL ROOT WATER TRANSPORT .1. MODEL PRESENTATION

Plant water uptake has classically been supposed to depend on the leaf water potential (LWP) and LWP was assumed to control the stomatal conductance. A new approach to stomatal control has recently been proposed in which root water potential (RWP) determines stomatal aperture, and hence transpiration, by means of chemical messages. In such a case, models that provide precise calculations of RWP become essential for understanding soil-root water transport. The objective of this paper is to propose a theoretical approach to calculate RWP and water uptake by roots in the X-Y plane for any spatial root distribution. For the calculations of RWP, we assumed that the plant continuously adjusted its RWP so as to minimize the difference between the maximum evapotranspiration (MET) and the amount of water extracted by the root system. Water release from the roots in the soil was considered to be negligible, so roots in dry soil were temporarily removed from the pool of active roots. A Galerkin finite element method was used to solve Richards' equation. Examples of calculation are presented for a nonuniform root distribution under both wet and dry initial situations. Root water potential and the number of active roots varied greatly with time. Results suggested that no simple relationship existed between the plant water uptake and the root water potential. Furthermore, soil water-potential maps showed a close relationship between water uptake and root positions, and great differences between RWP and the mean soil water potential could be calculated. A greater or lesser water redistribution toward the roots occurred during the night. Finally, the proposed approach may be validated with precise soil and plant measurements.