Water Uptake by Plant Roots — a Multi-Scale Approach

Abstract

The soil-root-stem water pathway is a major component of the subsurface hydrological system. We model water uptake and water transport through the soil and through the root system as coupled processes. The model is based on a differential equation approach. The coupled non-linear equation system is solved numerically. On the macroscale, fluid flow through the soil matrix within and outside of the rooted area is modelled using a two-dimensional, axial-symmetric Richards equation. Flow through the root system is described by a Darcy equation. Water uptake and transport through the root system are considered to be passive processes; osmotic effects are neglected. In order to obtain the source and sink distribution of the root system, local soil water potentials (near a single root) are computed in a one-dimensional approximation in a further step. The model is capable of simulating hydraulic lift, which is the transport of water from moist into drier soil layers through plant root systems. The calculated amount of passively shifted water is consistent with experimental data. We discuss under which conditions hydraulic lift could be an optimised strategy for the plant and how water uptake patterns are modified by the multi-scale (local) approach.