12. Interaction of Groundwater and Soil Water

Groundwater and soil water can interact if the groundwater is relatively close to the soil root zone. Transport of water between groundwater and soil water can significantly improve the supply to plants of water and nutrients. A groundwater depth of about two metres below the soil surface is assumed to be the critical depth that contributes markedly to the supply of water to the plant canopy. In this chapter, the vertical transport of water from groundwater to the root zone of the soil is quantified. It depends on the soil hydraulic properties and on the depth of the groundwater table below the hypothetical level of water extraction by plant roots and on the soil-water matric potential at the same level. Internal drainage of the soil profile as a result of a lowering of the groundwater table is quantitatively expressed. The drainage equations are derived to calculate the parameters of the systematic drainage system: the depth, horizontal distance and diameter of pipe drains in a homogeneous soil under conditions of steady and unsteady drainage.