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 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.
Because water use varies widely from plant to plant, estimates are usually first derived for a single situation, such as grass, which we use here, and then adapted for other plants. Eastern Washington tree fruit growers should consult Tree Fruit Irrigation, edited by K. Williams & T. Ley, for such information.
Furthermore, 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.
The calculator estimates daily pET in inches of water. It requires average daily values of four measurements: sunlight, wind, temperature, and humidity. I stress average because we often think of typical daytime conditions as being average. Here average means over the 24 hour day.
Solar radiation (sunlight):
The required measurement for solar radiation is the amount of energy reaching the grass in the form of visible and near visible light. This is usually measured in watts/meter-sq and requires expensive sensing devices. If this information is available, it can be entered directly into the calculator. Only remember, this is the average daily solar radiation. Night time values of zero should be averaged with day time values.
However, the calculator will derive fairly close estimates from the time of year and the cloud cover. 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. I set up the calculator using typical conditions during the irrigation season from Wenatchee, Washington (47°N latitude; semi-arid climate). The values may differ for other locations.
To estimate sunlight, select cloud cover and time of year. The calculator will place an entry into the solar radiation box.
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. To use dew point, enter the value, and click on "Use." A rough approximation of dew point is the minimum overnight temperature.
Average wind speed:
An average may be difficult to obtain. You may enter the value directly into the box, or choose one from the popup window. Values from the popup window are based on the widely used Beaufort scale. 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, with increased wind on warm clear afternoons and evenings.
Select the "Calculate" button to evaluate the entered values. The pET from grass will be displayed in the bottom line.
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Washington State University
Tree Fruit Research and Extension Center
1100 N. Western Ave.
Wenatchee WA 98801