ABSTRACT
Water flow in soils may occur in both unsaturated and saturated conditions; however, clear differentiation between flows requires a review of definitions first.[1]
In the saturated zone, it is generally assumed that the pore space within the soil matrix is saturated with water, and that the hydrostatic pressure in the water is greater than atmospheric pressure. In contrast in the unsaturated zone, the pore space is only partly filled with water, resulting in a soil water pressure smaller than atmospheric pressure. The region between the unsaturated zone and the groundwater is called the capillary fringe, where the soil is satiated but where the soil water is held by capillary forces (Fig. 1). The difference between satiated and saturated water content is caused by the general presence of entrapped air within the soil matrix of saturated soils. The unsaturated zone is bounded by the soil surface at the top and merges with the groundwater of an unconfined aquifer in the capillary fringe of the water table or phreatic surface at the bottom. By definition, the phreatic surface is the soil depth at which the water pressure is atmospheric.
