The earth is a dynamic place, where the movement of things – some fast, some slow, some whimsical, some railroaded by tremendous forces – plays an important role in planetary processes, including those required to sustain life. Tectonic plates move to relieve and accumulate stress, ocean currents redistribute nutrients, volcanoes pump minerals up, trees fix minerals into the soil.
Evapotranspiration is one kind of movement that is part of a larger planet-wide rhythm called the water cycle. The term is an amalgam of evaporation – which is how the soil loses water – and transpiration – which is how plants do it. In particular, transpiration accommodates both the movement of water up through the plant and its loss into the air from parts exposed to the atmosphere.
Evapotranspiration is an amalgam of these terms conceptually, and it is the first part of the water cycle, when water from terrestrial surfaces moves into the atmosphere. A number of factors affect the rate of evapotranspiration, including solar radiation, the length of day, the amount of soil moisture, the ambient temperature, the winds, and the amount of water vapour that the air already holds.
The word is at least 86 years old, and was first published in hyphenated form. The American climatologist Charles Warren Thornthwaite later defined it in 1944. An important way in which it remains relevant to this day is for farmers, who use it to estimate how much water their crops need to be fed.
