Potential Evapotranspiration (PET)

To understand Potential Evapotranspiration (PET) you first need to define Evapotranspiration (ET). Evapotranspiration is an important component of the water cycle. It describes the sum of evaporation and plant transpiration water losses (see Figure 1) from the earth's land surface to the atmosphere, where:

• Evaporation is the loss of water to the atmosphere from sources such as the soil, canopy, and water bodies
• Transpiration is the evaporation of water into the atmosphere from plants

Potential Evapotranspiration (PET) represents the evapotranspiration rate of a reference crop, usually pasture. The most widely used method for calculating PET is the Penman-Monteith equation.

PET is higher in the summer, on clear days, and closer to the equator, because of the higher levels of solar radiation that provides the energy for evaporation. PET is also higher on windy days because the evaporated moisture can be quickly moved from the ground and plants, allowing more evaporation to fill its place.

To convert the PET to an actual crop ET, a crop coefficient and stress coefficient must be applied. However, when irrigating, the stress coefficient is not essential as most irrigation strategies aim to prevent the crop from experiencing stress.

An excellent reference for crop evapotranspiration is the Food & Agriculture Organisation (FAO) Irrigation and drainage paper 56. Crop Evapotranspiration—Guidelines for Computing Crop Water Requirements (Alan et al. 1998).