8 - Water Flow in Unsaturated Soil

Abstract

Unsaturated flow processes are complicated and difficult to describe quantitatively. This is so because they often entail changes in the state and content of soil water during flow. The flow of water in the soil is driven by a hydraulic potential gradient. The flow occurs in the direction of decreasing hydraulic potential, and its rate (flux) is proportional to the potential gradient and is affected by the geometric properties of the pore channels through which the flow takes place. These principles apply in unsaturated as well as saturated soils. However, the nature of the moving force and the effective geometry of the conducting pores may be very different. The water in an unsaturated soil is subjected to a subatmospheric pressure, or matric suction, which is equivalent to a negative pressure potential. The gradient of this potential constitutes a moving force. The most important difference between unsaturated and saturated flows is in the hydraulic conductivity. The transition from saturation to unsaturation generally entails a steep drop in hydraulic conductivity, which may decrease by several orders of magnitude as suction increases from 0–1 MPa. At still higher suctions, or lower wetness values, the conductivity may be so low that very steep suction gradients, or very long times, are required for any appreciable flow to occur at all.