Soon after infiltration, the soil surface is wet so plenty of water is available at the surface. In this early stage, the atmospheric demand or energy available to convert water to vapour are most likely to limit the evaporation rate. As the soil dries out, the ability of the soil to move water upward to the soil surface limits the evaporation rate. Water moves from soil to plant roots and on through plants as a result of potential differences, just as it does elsewhere. Water potentials in plants and the conductivity of the plant parts change rapidly and vary over large magnitudes depending upon the atmospheric conditions and the physiology of the plant.
Plants are capable of removing large quantities of water from soils.
Details at the microscopic scale of individual roots is difficult to study, but this basic work is being carried out. For most water management purposes, it is sufficient to quantify the average rate of water loss per unit area. This is often called the evapotranspiration rate or ET since it combines direct evaporation from the soil surface and evaporation of water taken up by plants and transpired through the plant system. Numerous methods have been developed to measure and estimate these evapotranspiration rates. Potential evapotranspiration rates or PET values are generally calculated for a reference crop growing under well watered conditions.