 WSU-TFREC Orchard Calculator
Estimating Water Use by Plants
The following describes use of a calculator for estimating water use by plants
(potential evapotranspiration). Please read through the description
before attempting to interpret its use.
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.
Using the Penman calculator
The calculator is based on the Penman equation, a formula in wide use by
agricultural scientists around the world (see for example, An Introduction
to Environmental Biophysics by Gaylon S. Campbell).
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.
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. 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.
Calculate pET
Select the "Calculate" button to evaluate the entered values. The pET
from grass will be displayed in the bottom line.
Updated
July 24, 2004
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