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Calculating Water Use By Turf -pET Calculator

The use of water by plants can be estimated by calculating what is known as potential evapotranspiration (pET). This is the possible loss of water (or use) through evaporation and transpiration. Transpiration is the movement of water through a plant from the soil into the roots, up the stem, and out through the leaves. Simple calculations of pET can be based on sun, wind, temperature, and humidity.

True water use varies depending upon many other factors. When calculated daily, the accuracy of estimates will change from day to day. However, these will usually average out over a week to ten days, and the sum over that period may be quite accurate. Persons using pET to estimate water use and schedule irrigation also must adjust pET values based on their experience with their own specific conditions, especially including soil type.

Sky Conditions (sunlight):

The value of daily solar radiation is dependent on day length, angle of the sun from the horizon throughout the day, and the amount of clouds the sunlight must penetrate. To estimate sunlight, select cloud cover and time of year. The calculator will automatically calculate the solar radiation value.

Air Temperature:
Remember, this must be an average daily temperature. At least it should be an average of the maximum and minimum for the day.

Average Relative Humidity:
Enter the average daily relative humidity or have the calculator find the relative humidity from the dew point. A rough approximation of dew point is the minimum overnight temperature.

Average Wind Speed:
The effect of wind on pET reaches a maximum under moderately breezy conditions.
Once again, this is the average wind speed. We often think of the gustier winds as being average because they are most noticeable, but they are not average. Also this is a daily average, wind conditions vary through the day.

time of year:
sky conditions:
air temperature F
relative humidity %
wind condition:
inches per day


Lawn Sprinkler Tips & Facts
Did You Know?
Pop-up sprays distribute 2 to 3 times as much water as rotors.


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Spring Irrigation Co.  PO Box 485 Okemos, Michigan  48805