Determining Zone Run Times Using Evapotranspiration Data
The Handbook of Technical Irrigation Information produced by Hunter
Industries provides formulas and other information that can aid irrigators in
arriving at efficient watering schedules. The following information is derived from this book.
"The sprinkler run time formula calculates the number of minutes
required to apply enough water to replace the water lost by evapotranspiration
for a specific crop irrigated with a system at a particular precipitation rate
Followed by the formula below:
T= 60 x Eto
Pr x Ea
T = Sprinkler run time in minutes
Eto = reference evapotransporation rate, in inches
Kc = crop coefficient, percent
Pr = precipitation rate of the area, in inches per hour
Ea = application efficiency of the system, percent
60 = Constant for conversion of area, flow, inches per hour and inches
per day into common units.
Determine the total monthly sprinkler run time for an athletic field with an
total monthly ETo of 2.0 inches and a crop coefficient for the warm
season turf of 0.70. The sprinkler precipitation rate is .50 in./hr. with an
application efficiency of 75%.
T = 60 x 2.0
.50 x .75
T = 224 min. of total monthly watering time
Thus if the system is programmed to water on 10 days during a month, the zone
watering time would be 22.4 minutes (1/10th) of the total monthly requirement.
Eto or the evapotranspiration
rate is a value given to help determine the rate at which plants lose water
through evaporation. Humidity, temperature and other factors are taken in
account to determine the Eto rate at a given time.
These rates vary with the season and realistically they vary by the hour. But
for lawn irrigation purposes, average Et rates are more than enough to schedule
According to the National Climatic Center in Asheville, North Carolina, the
total monthly Eto rates for the Austin area are as follows:
crop coefficient is a value assigned various plants that lose water at different
rates. This helps adjust for soft leafy plants, like ferns, that would lose
water faster than hard waxy leafed plants, such as groundcovers. Warm season
grasses such a Bermuda lose water differently than cool season grasses such as
rye. See also Texas A & M
University's Potential Evapotranspiration data from a number of special
weather stations located in central and south Texas.
or application efficiency can only really be measured by a
thorough sprinkler system audit. This will measure the uniformity of coverage by
using a system of "catch cans" placed throughout each zone, running
the system, reading the results, and crunching the numbers. A rating of 0.75 is
considered a pretty good number and indicates an efficient system.
Do not overlook soil consistency and active root zone depth. All of these
are considered when calculating a scientific schedule for watering a landscape.