Evaporation is the transferral of liquid water into a gaseous state and its diffusion into the atmosphere. In order for this to occur there must be liquid water present and available energy from the sun or atmosphere. Potential evaporation (PE) is that which occurs over the land’s surface, or would occur if the water supply were unrestricted, or the ability of the atmosphere to remove water from the surface through the processes of evaporation and transpirationassuming no control on water supply. Actual evaporation (Et) is that which actually occurs , i.e., if there is not much available water it will be less than PE. These definitions have been concerned with “evaporation over a surface”. In hydrology, the surface is either water (river, lake, ponds,etc.) or the land. The Et above a land surface occurs in two ways – either as actual evaporation from the soil matrix or transpiration from plants. The combination of these two is often referred to as evapotranspiration (ET), although the term actual evaporation is essentially the same (hence the t subscript in Et). Thus, potential evaporation over a land surface covered partly or completely by vegetation is also referred to as potential evapotranspiration (PET). If the actual evapotranspiration is considered the net result of atmospheric demand for moisture from a surface and the ability of the surface to supply moisture, then PET is a measure of the demand side. Surface and air temperatures, insolation, and wind all affect this. A dryland is a place where annual potential evaporation exceeds annual precipitation.

Three conditions are needed for ET, namely, available energy source (often quantified using net radiation), available water supply (often quantified using surface water or soil moisture) and dynamic and dry atmosphere which ensure water vapor can be received in the atmosphere (often quantified using vapor pressure deficit and atmospheric mixing)

ET can be measured using direct micro-meteorological method (such as eddy fluctuation, Bowen ratio) and indirect method based on the water balance techniques(such as evaporation pan, lysimeters). However, all these methods present problems and the results of which always difficult to incorporated into the hydrological model. Thus, many estimation methods such as Thornthwaite and Penman have attracted much attentions and evolve quickly. Although some new techniques such as remote sensing have provide new approach, their accuracy in local scales is still controversial.

Scientists consider these two types of evapotranspiration for the practical purpose of water resource management. Around the world humans are involved in the production of a variety of plant crops. Many of these crops grow in environments that are naturally short of water. As a result, irrigation is used to supplement the crop’s water needs. Managers of these crops can determine how much supplemental water is needed to achieve maximum productivity by estimating potential and actual evapotranspiration. Estimates of these values are then used in the following equation:
crop water need = potential evapotranspiration – actual evapotranspiration

References：
(1) https://en.wikipedia.org/wiki/Potential_evaporation
(2) https://en.wikipedia.org/wiki/Evapotranspiration
(3) http://www.physicalgeography.net/fundamentals/8j.html
(3) John Gerrard, 2008, Fundamentals of Hydrology,Taylor & Francis Group.