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Water Audit
to Save Water
© 1997, 1998 Streamline Publications
Evapotranspiration (ET) is the
term that describes the amount of water that is used in growing vegetation.
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It's a clear case of evapotranspiration, doctor, probably brought on by water bill shock. |
The ET number is based on the total
amount of water that evaporates from the soil and transpires through the plant
leaves. When you know the ET rate for your area, you can figure out how much to
water without under or overwatering. The amount of water needed is
influenced by the type of plant, temperature, wind, sun exposure, soil type and
other variables. Using the average daily ET rate for your area you can make
well-reasoned adjustments to your watering schedule. Lawn sprinklers are notoriously
inconsistent in their coverage. Adding sprinklers to an area may not necessarily
improve coverage. The best sprinklers are only 60 to 80 percent effective in
their evenness of application. There is an easy way to check. Place at least ten to fifteen empty,
straight-sided shallow cans randomly on the turf about three to five feet apart.
For very large turf areas you may want to do this in sections. Tuna cans are
good for this. The more cans you use the more accurate the test results will be.
Turn on your sprinklers for 30 minutes.
Measure the depth of water in each can and note it's location. If any one can
contains twice as much or more water than any other can, there is unacceptable
variation in the coverage. Sprinklers should be adjusted to reduce the
differences. Run the tuna can tests and make adjustments until you are satisfied
the coverage is even. When coverage is equalized, run a final
test. Add the water depth readings together and divide by the number of cans to
determine an average water depth.
NOTE: If adding and dividing
fractions is a problem, take measurements on the metric scale of your ruler so
you can work in decimals. One inch equals about 25 millimeters. This number (in hundredths of an inch)
represents the amount of water your irrigation system puts out in 30 minutes. If
you want to water for some other period, such as 20 minutes, divide by 30 to get
the quantity-per-minute rate then multiply by the desired run time. Once you have gathered the information
on your irrigation rate you can begin using the ET figures for your area. The
rates will be different for inland and coastal areas and at various altitudes.
First determine the ET zone in which the plot of grass lies. The local office of
the State Water Resource Board, water district, county agriculture office or
extension service may be sources of this information. When you know your zone
you can then use the daily ET rates to time your watering. In some areas ET rates are published in
the local newspaper on the weather page, or you may have to call a weather
service recording.
How to Work ET Let's assume that your average watering
rate for 20 minutes run time is 0.75 inch. Beginning with the last day you
watered, start adding the daily ET rates together until they are approximately
equal to 0.75 inch. Now it is time to water again. Run sprinklers for 20 minutes
to replace the 0.75 inch of water lost to the atmosphere since you last watered
through evaporation and transpiration through the grass leaves. Rainfall is the reverse of
evapotranspiration. Just as you add ET rates, you must subtract rainfall
amounts. For example, you have been adding ET rate figures and reach 0.45 inch.
Another 0.30 inch and it will be time to water. Rain begins and totals 0.20
inch. Now the cumulative ET figure should be reduced by rainfall. Subtract 0.20
from 0.45 for a new cumulative ET figure of 0.25 inch. Begin adding dry-day ET rates to 0.25
until you reach about 0.75 inch. Now it is time to irrigate for 20 minutes.
Variables The ET method has to be considered a
general guideline. ET figures represent averages. Since evapotranspiration can
vary within zones with grass type, soil, slope, shade, exposure to elements and
thatch buildup, you will probably have to make adjustments.
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