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
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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
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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
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4. The saturation vapor pressure slope is a complicated-looking equation that is based on the average temperature you used earlier and e, the natural logarithm base.
Slope of saturation vapor curve
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5. Atmospheric pressure is the weight of the air above the field, based on the elevation (z) above sea level.
Atmospheric pressure
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6. The psychrometric constant is based on atmospheric pressure, latent heat of vaporization, specific heat at constant pressure, and the ratio of water vapor to dry air at the given altitude. It can be a complex calculation, but is usually estimated based on the atmospheric pressure.
7. The soil heat flux density (G) is a measure of the energy used to heat the soil (and given off again when the soil is cooling). It is generally ignored for a daily ET, because it tends to zero out between the daytime and nighttime hours.
8. Our saturation and actual vapor pressures can be calculated using the temperatures and relative humidities measured during the day.
Saturation vapor pressure
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9. Finally, the C values in the equation represent the crop or foliage leaf coverage and the amount of time we're talking about, generally based on lookup tables, like the one shown.
Once you have all the pieces, you can plug them into your calculator and go for broke. ET calculations can get a little crazy, but they can also give you a good measure of the water usage of a piece of property.
Lesson Summary
Evapotranspiration is the measure of water loss due to evaporation (water evaporating from surfaces) and transpiration (water pulled from the soil and used by plants, then evaporated) for a particular area of ground. ET depends on various conditions, including temperatures, altitude, and leaf cover, and it can be calculated to determine the amount of water being utilized to keep the soil moist.
