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Determine Sprinkler Zone Run Time

What is e·vap·o·tran·spi·ra·tion?

This article discusses how to determine zone run times using evapotranspiration data from Hunter Industries. The formula below allows you to calculate how long you need to run your sprinkler system to compensate for the water lost in evapotranspiration. For instance, the formula is designed to calculate for specific crops watered with a system running at specific precipitation rates.

Formula For Determining Zone Run Times Evapotranspiration Data:

T= 60 x Eto x Kc
Pr x Ea


T = Run time in minutes

Eto = Evapotransporation rate, in inches

Kc = Crop coefficient, percent

Pr = Precipitation rate of the area, in inches per hour

Ea = Application efficiency of the sprinkler system, percent

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


To determine the total monthly run time for a soccer field with a total monthly ETo of 2.0 inches, a crop coefficient for turf during the warm season of 0.70, and a sprinkler precipitation rate of 0.50 inches per hour with an application efficiency of 75%, you would use:

T = 60 x 2.0 x 0.70
.50 x .75

T = 224 Minutes Of Total Monthly Watering Time

Therefore, if the system is programmed to water for 10 days during a month, the zone watering time would be 22.4 minutes (1/10th) of the total monthly requirement.

Evapotranspiration Rates (ETo)

The ETo refers to the rate at which plants lose water to evaporation. Therefore, humidity, temperature, and other factors are taken into account to determine the Eto rate at a particular time. Seasonal changes also cause ETo rates to change. Daily and hourly fluctuations in temperature and humidity will cause changes. However, for lawns and gardens, the average seasonal ETo rates are sufficient for efficiently scheduling watering run times.

ETo rates for your area are provided by many reliable sources, such as the National Climatic Center. The following chart was provided by the NCC in Asheville, North Carolina. It reports the total monthly ETo rates for the Austin area.


Crop Coefficient

Crop coefficient (abbreviated Kc) refers to ETo water loss for specific plants. This allows you to adjust for ETo variations between soft leafy plants, like ferns, which lose water faster than hard, waxy-leaved plants, such as groundcovers. Warm-season grass species, such as Bermuda, lose water differently than cool-season grass species like rye.

Application Efficiency (EA)

EA refers to how evenly your sprinkler system is watering- the uniformity of coverage. In addition, the most accurate way of measuring EA is to run a sprinkler system audit. This can be done using catch cans, a rain gauge, or any non-tapered cup or can. Use the link below to purchase an audit catch cup by Hydro Rain.

Other important factors are soil consistency and active root zone depth. All of these need to be considered when calculating a water-efficient schedule for your sprinkler system.

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