Evapotranspiration can never be greater than potential evapotranspiration, but can be lower if there is not enough water to be evaporated or plants are unable to transpire maturely and readily. Some US states utilize a full cover alfalfa reference crop that is in height, rather than the general short green grass reference, due to the higher value of ET from the alfalfa reference.

Potential evapotranspiration is higher in the summer, on clearer and less cloudy days, and closer to the equator, because