ABSTRACT
Infiltration, the process of water entering the soil surface, is part of the water cycle (Fig. 1). Water infiltrates the soil because of absorptive (capillary) and gravitational forces,[3] which are strongly influenced by soil texture and structure. (For more detail, see the article Soil Water: Energy Concepts.) The infiltration water comes from rain, melted snow, irrigation, or upslope runoff or seepage.[9] As infiltration occurs, the wetting soil profile can be divided into several zones:[3,8]
saturation right at the surface to perhaps 1 cm deep, a transition zone of rapidly changing soil water content, a transmission zone with slowly changing soil water content, a wetting zone of rapidly changing soil water content, and a wetting front with a very steep hydraulic gradient.
