Technical Information - Using Evapotranspiration Data

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 book provides the following information:

"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 and efficiency."

Followed by the formula below:

T= 60 x Et_o x K_c
_____________
P_r x E_a

_Where:

T = Sprinkler run time in minutes

Et_o = reference evapotransporation rate, in inches

K_c = crop coefficient, percent

P_r = precipitation rate of the area, in inches per hour

E_a = application efficiency of the system, percent

60 = Constant for conversion of area, flow, inches per hour and inches per day into common units.

Example:

Determine the total monthly sprinkler run time for an athletic field with an total monthly ET_o 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 x 0.70
.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.

Evapotranspiration Rates

Et_o 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 Et_o 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 watering.

According to the National Climatic Center in Asheville, North Carolina, the total monthly Et_o rates for the Austin area are as follows:

Crop Coefficient

K_c or 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.

Application Efficiency

E_a 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.

Don’t overlook soil consistency and active root zone depth. All of these are considered when calculating a scientific schedule for watering a landscape.

Technical Sections
[Programming] [How much to water] [Simple Method] [Evapotranspiration Method] [Precipitation Rates] [Water Audits]

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