The water considered in this analysis is above the water table and is available to the roots of plants. In a soil water budget, a column of soil is considered, and the St is expressed as the depth of the water that would be obtained if it were separated from the sediment. In this analysis, St is expressed as millimeters of water, and it is assumed that the soil can hold a maximum of 100 millimeters. Under this condition, the soil is said to be saturated.
Water is removed from the soil by evapotranspiration.
This is a combination of direct evaporation from the soil, and transpiration, which is evaporation from plants. Evapotranspiration reduces the quantity of soil water, which can decrease until it reaches 0. At this point, no more evapotranspiration can take place until precipitation adds more water to the soil. It is based on weather conditions such as wind and temperature and biological factors. In the winter, Ep is low in areas of temperate climate due to low temperatures and lack of leaves on deciduous plants.