Simulation of Post-Irrigation Moisture Movement

The transpiration-induced moisture removal by the plant roots is represented by a negative source term in the Darcy equation. When the soil is wetted to near saturation, the moisture flow is initially downward due to gravity. As transpiration dries the upper soil layers, capillary suction reverses the Darcian flow. The upward water flux through the soil has a maximum, which occurs high in the root zone initially and gradually moves downward. Throughout much of the time periods studied there was downward flow in portions of the soil-root system. Very little water, if any, was obtained by the plants from below the root zone, but considerable moisture was often lost to this region. As the soil-root system dries, a point is reached at which the Darcian moisture flux in the soil can be neglected, and the moisture removal process can be described by local extraction alone. When this occurs, the root geometry assumes major importance in determining the water supply available to a plant.