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Determine Your Sprinkler System's Precipitation Rate

There are two ways to determine the precipitation rate (the rate at which sprinklers deliver water). The first method is to physically measure the water being delivered to your lawn. The second method is to refer to manufacturer performance charts for your specific sprinkler.

Tuna Can Test

  1. Place several tuna cans or similar containers around the zone you want to test.
  2. Run that zone for 15 minutes.
  3. Measure the water in the cans with a ruler and determine the average depth.
  4. Calculate the amount delivered per hour.

Using Manufacturer Performance Charts

The alternative to physically measuring the water your sprinklers are delivering is to refer to the manufacturer performance charts and apply the precipitation rate formula.

Pr =96.25 x Total GPM
Total Area

Pr = Precipitation rate in inches per hour.

96.25 = A constant that converts gallons per minute (GPM) to inches per hour. It is derived from 60 minutes per hour divided by 7.48 gallons per cubic foot. times 12 inches per foot.

Total GPM = The cumulative flow from all sprinklers in the specified area, in gallons per minute (refer to sprinkler charts)

Total Area = The area being irrigated, in square feet.

(Note: This formula comes from Precipitation Rates and Sprinkler Irrigation - Student Manual, (3/6/96) Courtesy of Hunter Industries, San Marcos, CA.)

Example: An area of 1000 square feet is receiving a total of 10 gallons per minute. Using the precipitation rate formula, we can determine the precipitation rate as follows:

Pr =96.25 x Total GPM
Total Area
Pr =96.25 x 10
= .9625 inches per hour

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