Example of calculating irrigation run-time by meter reading and area

If you have a drip irrigation system, or if you want to use an alternate method to test your spray or rotor system, you can use your water meter to get your precipitation rate. This method requires additional math, because it relies on the meter reading and the area (in square feet) of the irrigation zone you are testing. First, use a measuring tape or measuring wheel to get a measurement of the total area irrigated by the valve you are testing, in square feet.

Next, find your water meter box, and carefully open it with gloved hands (black widow spiders often live in them). You will need a screw driver or other tool to open the lid. Check the underside of the lid and the inside of the box for spiders to make sure the area is safe. Then, pull up the faceplate of the meter to reveal the numbers. You may need to clean off the faceplate to clearly read the numbers. Record, in cubic feet, the current numbers shown on the meter before starting your irrigation valve (this is referred to as “beginning meter reading”). Run your irrigation system and record the how long it ran. We recommend at least five minutes for sprays, ten minutes for rotors, and fifteen minutes for drip. Record the new number shown on the meter after running the valve for the area you are measuring. This is referred to as “End meter reading”. See a step by step example below.

We will be calculating the run-time for this zone in a young, mostly California native plant landscape.

Step 1: Measure the landscape zone and make a small drawing with measurements.

Example: Above is the largest of three landscape sections (Section 1) that all operate on a singe irrigation zone.

Step 2: Calculate the area of each section of the landscape that is part of the irrigation zone. To calculate areas of shapes that are not perfect, break up zones into a series of simple shapes, rectangles, triangles, and circles. Above, we have broken up Section 1 into two rectangles, to calculate the area of each one, then add them together to get the total area of Section 1.

Area of part A = 17’ x 5’ = 85 sq. ft
Area of part B = 22.5’ x 19’ = 427.5 sq. ft.
Total Area of section 1 = 512.5 sq. ft.

The same irrigation zone also has two smaller sections that need to be accounted for in the area calculation as well.

Drawings of the two smaller landscape sections, above.

Section 2: Area of a rectangle = length x width
Area of Section 2 = 4.5’ x 14.33’ = 64.5’ sq. ft.

Section 3: Area of a triangle = 1/2 x base x height
Area section 3 = 1/2 x 4’ x 12’ = 24 sq. ft.

Section 1: 512.5 sq. ft.
Section 2: 64.5 sq. ft.
Section 3: 24.0 sq. ft.
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Total: 601 sq. ft

Step 3 (Above): Calculate the total area of the irrigation zone you will be testing, in square feet, by adding up the area of all sections watered by the zone.

Step 4: Find your water meter box. They are often located in the parking strip planted area along the street. This property has no parking strip, so it is located in the sidewalk.

Step 5: With a gloved hand, use a tool to pry open the heavy concrete lid of the meter box through the center hole. Here a large flat head screw driver is being used.

Step 6: This is what it will look like inside the box. Before reaching in, check both the underside of the lid and inside the box for spiders. Black widow spiders like the dark dry conditions in meter boxes.

Step 7: Lift open the lid on the water meter so you can read the face. You may need to clean if off to be able to read. A wet paper towel usually works well. This, and most meters read “cubic feet” of water. The directions here are for calculations with this type of meter. To begin the test, first write down the number displayed on the meter (40,597 cubic feet). This is your “beginning meter reading.”

Step 8: Manually run the zone you are testing for at lease 10 minutes. In this example, we ran the zone for 20 minutes. Make sure no water is running inside the house (laundry, faucets, toilets, showers, etc) during that time. Write down how long you ran the zone. This number will be critical later.

Step 9: Check the meter one more time and record the numbers now showing. This will be your “end meter reading.” close the lid of the meter and carefully put the concrete lid back on the meter box.

You are now ready to enter your information into the run-time calculator by meter reading and area, here, to get your run-time for this irrigation zone.