Effect of Roots on Water Flow in Unsaturated Soils

Abstract

In recent years, beneficial effects of plants on the remediation of large volumes of slightly contaminated soils have been addressed by various researchers. Water is the main agent moving soluble contaminants in the soil profile, and an important one in transferring contaminants to root surfaces and subsequently to the shoot. Although there are a number of models for water flow in cropped soil, the parameters of the models, which often include rooting density, root permeability, and root water potential, are chosen by trial and error such that an overall model fits the data. Such methods can be tiresome and impractical. Therefore, the purpose of this research is to study the root water interaction of crops by developing a simple predictive technique that is usable in a wide range of applications, such as in-situ plant remediation, for contaminated soil near the surface zone. The simulation model is developed by incorporating a time-specific root distribution model into a vertical unsaturated soil water flow equation. The model validation for the time-specific root distribution is carried out using field data for cotton. A good agreement is observed between the simulation and field data. Based on constant daily water flux to individual roots under well-watered soil conditions, the cumulative amount of water consumed over time and the water uptake rate at each growth stage for cotton are investigated for the preliminary study of proper irrigation schedules. These results are included in upper boundary conditions in computer simulations of the water flow model. Soil hydraulic properties, which are important parameters in water flow model, vary as soil water content changes. Variations in the parameters of the model results in a nonlinear partial differential equation. The water flow model is solved by the Crank-Nicholson numerical scheme. The simulation results show the mechanism of plant root influence on soil water conditions across time and soil depth, which is in close agreement with results obtained in other studies. Usage of the present model is restricted to annual crop growth in soils frequently irrigated under favorable conditions. However, this simple predictive technique, coupled with a root growth and distribution model will allow for practical use in a large variety of agricultural studies. Additionally, since the model describes the variation of water flow and moisture content throughout the soil profile, it will provide necessary input for calculating the transport of solutes in the soil profile.