ABSTRACT
Soil hydraulic Water held in soil storage is a fundamental requirement for the growth of plants on dry land, and thus it is an essential component of the food chains for most terrestrial animal life. Water adhering to the surface of soil particles forms a lubricating film that greatly reduces the effective mechanical strength. If free water remains ponded on the soil surface, or if the surface layers are continually wet, evaporation may occur, just as it does from a lake or any free water surface. Liquid water films move much more slowly under unsaturated conditions because many of the pores are not contiguous. Water held in soil storage below the saturation water content moves relatively slowly because of the exponential relationship between soil water content and unsaturated hydraulic conductivity. Plants obtain needed water and minerals from the soil in which they grow by absorption through their root systems.
