Infiltration and Soil Water Processes

Abstract Water at the soil surface is drawn into the soil pores under the influence of both suction and gravity. Suction (or tension/capillary) forces are largest in the small pore spaces between soil particles. Therefore, small pores fill first, and large pores empty first. As a soil becomes saturated, suction forces approach zero. As a soil dries, suction forces dominate, and there is little or no gravitational flow. For a dry soil during the early stages of wetting, the effect of soil pore suction is predominant. However, as the depth of wet soil increases, the suction gradient decreases (the same difference in suction is spread over an ever increasing depth interval) and the gravitational head gradient becomes the main driving force. Infiltration processes cannot easily be separated from the processes involved in movement of water within soil because the rate of infiltration is partly controlled by the rate of soil water movement below the surface. Soil water movement continues after an infiltration, as the infiltrated water is redistributed. Infiltration and soil water processes play a key role in surface runoff, groundwater recharge, ecology, evapotranspiration, soil erosion, and transport of nutrients and other solutes in surface and subsurface waters.