Evapotranspiration is a gallimaufry, salmagundi kind of word. The evaporation part is easy: it is the mechanism by which water is converted into an invisible vapour and absorbed by the atmosphere - the reason why puddles disappear after it has been raining, and why the washing dries when hung out on the clothes line. But the concept of transpiration is more difficult.
If you are old enough, you may recall that in a more genteel era than ours, nature decreed that "horses sweat, gentlemen perspire and ladies glow"; by the same token, plants transpire - they emit moisture through tiny openings on their leaves, little pores we call stomata.
Water drawn from the soil is distributed by a plant to its every shoot and leaf to be used in photosynthesis. It is vital for the health of the plant that it retain adequate moisture. When there is a plentiful supply, it transpires the excess through the stomata; but as the soil dries up, plants experience increasing difficulty in extracting water from it, and respond by closing their pores and retaining all the moisture they can. Little or none is released into the air.
Meteorologists, naturally enough, have an interest in the moisture content of the atmosphere, and in the various ways in which water is transferred into, and out of, the air around us. It loses water through precipitation - like rain and snow - and through the formation of hoar frost and the deposition of dew. The water content of the air is replenished in the case of open water surfaces and moist soil by evaporation. But in the case of vegetation, both evaporation and transpiration play a part; evapotranspiration is the key.
Evapotranspiration is now an important input for the computer models that simulate the global climate. Until relatively recently, climate models treated the Earth's vegetation rather like a vast damp cloth - a surface that evaporated water at a more or less constant rate. But the reality is more complex.
It so happens that it is also through the stomata that plants absorb carbon dioxide, and as the proportion of CO2 in the air increases in line with current trends, they should be able to satisfy their needs in this respect more easily, and the stomata may remain closed for longer periods than they do at present.
With closed stomata, the plants would exude significantly less water vapour through evapotranspiration than they do now - and the reduction in available moisture might well have profound effects on rainfall patterns all around the world.