The term evapotranspiration combines two words: evaporation of water from the soil, and transpiration of water from plants into the air. Evapotranspiration means the total loss of water from a crop into the air. Water evaporates from any moist surface into the air unless the air is saturated. Water surfaces in contact with air, such as lakes, plant leaves, and moist soils, all evaporate water.
Plant leaves lose water though small openings called stomata that are found on the leaf surface. The water moves from the moist soil into the plant roots, up through the plant, and leaves through the stomata. Evapotranspiration is loosely called crop water use.
Crop water is important because it determines how much water must be provided by irrigation or rain. If there is too little water, the crop yield can diminish. If there is too much irrigation, then it will waste energy, water, and nutrients and unnecessarily deplete the aquifer.
Water has three functions in plants. It cools and hydrates them, and is also essential for the transport of nutrients. Less than 1% of water remains in the plant tissue. Considering water makes up 90% of the weight of most crops, this may seem surprising, but plants use the water for other purposes.
Weather is a major factor in evapotranspiration. The surface temperature of plants and soil is almost that of the air temperature. Brighter sunlight means that plants need to evaporate more water through evapotranspiration to keep their temperature near normal. If the air is dry and hot, with strong winds, then the crops will lose water at a faster rate. More water will evaporate from plants if the air is a higher temperature, if there is more solar energy and lower humidity, and if there is a faster windspeed.
Evapotranspiration is estimated by the use of many formulas. There are computer software programs available to help people estimate evapotranspiration. Radio, newspapers, and network services often give out reports on potential evapotranspiration figures. There is also crop referencing, which compares the evapotranspiration rate of a reference crop of plants to the same types of crops grown by other individuals.
Soil water and crop water are very important to crop evapotranspiration, and critical to irrigation management. The characteristics of soil determine how tightly it can hold water and how quickly that water can replace absorbed water in crop plants. The depth of the plant root controls the amount of water available to the crop for evapotranspiration. The main goal of irrigation is to keep the soil water availability from limiting evapotranspiration.