A SIMPLE, MECHANISTIC MODEL FOR SOIL RESISTANCE TO PLANT WATER-UPTAKE

A simple, mechanistic model for soil resistance (R(S)) to water flow towards plant roots is presented. This new R(S) model is derived from a single root, water flow model and assumes constant soil hydraulic conductivity within the rhizosphere for each combination of depth and time increment in the calculations. The new R(S) model takes into account root density, soil hydraulic conductivity, and soil-water capacity. The effect of the soil-water capacity is included by either i) using the slope of the curve representing the unsaturated soil hydraulic conductivity versus the soil-water potential in a log-log coordinate system as a dynamic model parameter or ii) if the unsaturated soil hydraulic conductivity is not measured, by using the slope of the soil-water characteristic curve in a log-log coordinate system as a dynamic model parameter. The latter approach is based on the validity of the so-called Alexander capillary tube model for water flow. The new R(S) model is compared with the traditionally used R(S) model based on steady-state flow. The two R(S) models give different results for both clayey and sandy soils. The differences are mainly due to the effect of the soil-water characteristic curves. The new R(S) model is mechanistically more appropriate than the steady-state based R(S) model because it includes the effect of the soil-water capacity, i.e., the soils ability to withhold water at given pressure gradient. The new R(S) model should be useful in obtaining simple, mechanistic, micro-computer models for plant uptake of water.