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Evapotranspiration: Definition, Formula & Calculation

Instructor: Michael Quist

Michael has taught college-level mathematics and sociology; high school math, history, science, and speech/drama; and has a doctorate in education.

It happens every day all around us, but not many people realize how much is involved with the evaporation of water into the atmosphere. In this lesson, we'll discuss the definition, formula, and calculations for evapotranspiration.

What is Evapotranspiration?

One of the most remarkable life-giving aspects of our planet is its water cycle. Every moment of every day, water is being drawn up from the ground, purified, and then returned to the earth in the form of rain, snow, sleet, etc. This cycle ensures that the plants, animals, and people of the earth have a consistent, reliable, constantly-renewed source of the fresh water that is necessary for life.

Evapotranspiration (ET) is the earth losing water to the air through one of two methods. One is familiar, evaporation, in which the water that has been soaked into the ground or is sitting on the surface turns into water vapor, which is carried into the air. Most people don't know as much about the other one, transpiration, in which the plants suck up water through their roots, send it through their stems and leaves, and then use it for food (combining it with carbon dioxide to make oxygen and sugar) or evaporate it off their surfaces. In this lesson, we'll take a look at the conditions that affect evapotranspiration and some of the calculations involved in measuring it.

Factors Affecting Evapotranspiration

The sun's radiation constantly strikes the earth during the day, energizing the water molecules and increasing the rate at which water evaporates. Plants use that energy to power their sugar production and growth, using water for their processes. Measured in megajoules (a joule is a small unit of energy and a megajoule is a million of those units) per square meter per day, radiation is monitored by local weather stations.

Warm air holds a lot more water than cold air, so another important factor is the average (mean) temperature of the air. Generally what you'll do is add the highest temperature during a 24-hour period to the lowest temperature during that same period, then divide that sum by 2. This will give you a good estimate for the average temperature during that time.

The movement of air, or wind speed, aids in the evaporation process and is measured in meters per second. Since the air density is higher at lower elevations, elevation plays a part in the evaporation and transpiration processes. Generally speaking, water is more likely to evaporate at higher elevations (lower air densities).

Another important factor is how much water is actually there to work with. If crops have little water available, they will close their pores and allow much less water to pass through their systems. As a result, the movement of air and radiation come into contact with less water, so they will accordingly cause less evaporation.

The soil's ability to transfer heat, the quantity of leaf coverage, and other factors are taken into consideration when you begin to calculate the ET for a certain piece of land.

Calculations

One of the more commonly used formulas for calculating ET is shown below, as developed by the American Society of Civil Engineers (ASCE). It was developed to help farmers and others interested in agriculture to calculate and understand the level of water usage for a particular crop area. Let's look at some of the calculations and measurements you'll need for this process.


Standard ASCE ET formula
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1. Mean temperature (Celsius) from the weather station. You add the high temperature to the low temperature and then divide by 2.


Average temperature
Temperature

2. Average solar radiation values (megajoules per square meter per day). If you have a complete set of hourly readings for a 24-hour period, you could use an equation like the one shown.


Net radiation calculation
Net Radiation

3. Wind speed (meters per second, normally 2 meters above the earth) can be measured at the field's elevation. This is an average, based on readings taken at regular times during the day.


Average wind speed
Wind speed readings

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