What is Evapotranspiration?

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  • Written By: Garry Crystal
  • Edited By: Niki Foster
  • Last Modified Date: 06 October 2019
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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.


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Post 3

@ZipLine-- There are businesses that calculate evapotranspiration for farmers for a fee. Not just farmers, but large estate owners and golf club owners need this information as well.

Farmers can calculate it on their own using the evapotranspiration formula, but like you said, a lot of factors have to be considered. Farmers need to find coefficients for different crops and then they need to consider the season and how much water the crop needs at that time.

Crops need different amounts of water when they are first planted and throughout their development. Of course factors like the weather also effect this calculation. For example, evapotranspiration is higher when there is rainfall.

I think many farmers have someone else calculate all this for them, especially if they plant more than one crop.

Post 2

Do local instiutions help farmers calculate evapotranspiration and how much irrigation will be necessary for their crops that year?

From what I understand, since crop water use depends on many factors like the soil, the crop, the climate and the stage of growth, general calculations won't really be useful for farmers. Estimating evapotranspiration also seems hard because these factors are changing all the time.

If I was a farmer, I don't know how I would go about calculating water use for my crops. Any farmers here who can tell me a little bit about how they do it?

Post 1

This explains why we don't have to water plants as much in fall and winter.

I've always wondered about this since I was young. My parents love growing fruits and vegetables in the yard, as well as potted plants and flowers inside our home. I've always helped them water and care for the plants in my free time.

One of the first things I learned was that plants don't need as much water in winter. In fact, once I over-watered several potted flowers and almost killed them. I didn't know that plants need less water in cool climate because they are losing less water through evapotranspiration.

Thanks for the information!

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