ABSTRACT
Like groundwater, the soil zone is an important store. The depth of the soil and its other characteristics such as its texture, determine how much water can be stored in the soil and whether it can be held there against gravity or whether it percolates downwards to become groundwater recharge. Similarly, these characteristics, together with those of the soil surface, determine how quickly rain infiltrates. Collectively, these properties determine the way in which the soil zone absorbs, retains or releases water, thereby regulating runoff. Although soils can be saturated, most often they are not and understanding soil moisture movement in unsaturated conditions is more challenging than when saturated. In this chapter we develop an understanding of the fundamental principles of soil moisture movement and provide an overview of the main methods of measurement and estimation, including by remote sensing.
