After reading this article you will learn about the water flow pattern in soil with the help of diagram.
Between periods of rain, water held in the soil is gradually given back to the atmosphere by a twofold drying process. First, direct evaporation into the open air occurs at the soil surface.
Air also enters the soil freely and may actually be forced alternately in and out of the soil by atmospheric pressure changes. Even if the soil did not “breathe” in this way, there would be a slow diffusion of water vapour surface ward through the open soil pores.
Second, plants draw the soil water into their systems through vast networks of tiny rootlets. This water, after being carried upward through the stem and branches into the leaves, is discharged through leaf pores into the atmosphere in the form of water vapour. The process is termed transpiration.
In studies of plant ecology and hydrology, the term evapotranspiration covers the combined moisture loss from direct evaporation and the transpiration of plants. The rate of evapotranspiration slows down as soil water supply becomes depleted during a dry summer period, because plants employ various devices to reduce transpiration. In general, the less water remains, the slower is the loss through evapotranspiration.
Fig. 27.5 illustrates the various terms explained up to this point and serves to give a more detailed picture of that part of the hydrologic cycle involving the soil. The soil layer from which plants can draw moisture is the soil-water belt.
This belt gains water through precipitation and infiltration. As the minus signs show, the soil loses water through transpiration, evaporation and overland flow. Excess water also leaves the soil by downward gravity percolation to the ground water zone below.
Between the soil-water belt and the ground water zone is an intermediate belt. Beneath upland areas, water held in the intermediate belt lies too deep to be returned to the surface by evapotranspiration, since it is below the level of plant roots.