Evapotranspiration is the loss of water through a combination of two separate processes: 1) water is lost from the soil surface by evaporation, and 2) water is lost from the plant by transpiration.
First let’s look at evaporation…
Water evaporates from a variety of surfaces including lakes, rivers, pavements, soils, etc. Direct solar radiation from the sun plus the ambient temperature of the air cause evaporation. Additionally, the water vapor pressure at the evaporating surface compared to the surrounding atmosphere also drives evaporation. When the surrounding air becomes gradually saturated with water vapor from evaporation, the process slows down. However, if the wet air is replaced by dry air because of wind, the evaporation process will continue. So, climate parameters such as amount of sun, air temperature, air humidity and wind speed all affect the evaporation process.
When we are talking about soil surface as the evaporating surface, the degree of shading of the plant’s canopy and the amount of water available at the soil surface are additional factors affecting evaporation. When the interval between rains and irrigation becomes large, the water content in the topsoil drops and the soil surface dries out. When this happens, evaporation actually decreases rapidly and may cease almost completely within a few days. The limited availability of water exerts a controlling influence on soil evaporation.
In the most basic scenario, you can figure out how much irrigation water is needed by identifying the plant’s water requirements, the amount of water lost through evapotranspiration, and the effective precipitation amounts.
You can learn more about parameters of landscape water budgeting and computing irrigation requirements in our Science & Technology section.