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Lawn Watering How-To: Converting Inches to Minutes

Our weekly watering advice tells you, in inches, how much water your lawn needs to stay healthy. But just how long does your sprinkler need to run to dispense that amount? Use our simple formula to find out!

If you’re a regular reader of the weekly e-newsletter or Garden Style San Antonio, then you know I provide weekly irrigation recommendations in ½-inch, ¾-inch and 1-inch increments. Many readers ask how that translates into minutes. That’s an excellent question. Welcome to the wonderful world of irrigation!

The application or precipitation rate of a sprinkler or in-ground irrigation system is a unit of measurement for the amount (inches) of water applied to the ground per hour. This one unit determines all run times.

The application rate is determined by many factors including age of the system, head type, nozzle type, spacing and pressure. The actual run time derived from the application rate is further modified by the five S’s (soil, shade, slope, species, and season). It is, therefore, impossible to provide individual run times for individual homes, let alone an entire city. To provide the answer, we make assumptions.

For a typical San Antonio in-ground irrigation system, we assume that a typical application rate for a typical zone is:

Pop-up spray heads = 1.65 inches/hour
Rotor heads = .75 inches/hour
Multi-stream heads = .45 inches/hour

The application rates above are calculated using the following formula:

Run Time = Amount of water desired x 60 (minutes) / Application (Precipitation) Rate

So let’s plug in the variables we know using the pop-up spray head application rate (1.65) from above to get the run time:

½-inch of water multiplied by 60 minutes divided by 1.65 equals about 18 minutes.

Again, that run time is based on typical situations. If that same spray head is in the shade or in beds, it should only run about 11 minutes because those plants require only 60 percent of the 18 minutes.

You can use the formula for any amount we recommend and for any irrigation head. You may see different calculations and formulas on the inter webs, or have irrigators explain more complex formulas to you, but we prefer to keep it simple.

After all, the point is to irrigate your plants effectively and efficiently — saving you money and water.

Mark Peterson
Mark Peterson
Mark A. Peterson is a conservation project coordinator for San Antonio Water System. With over 30 years of experience as an urban forester and arborist, Mark is probably the only person you know who actually prunes trees for fun. When not expounding on the benefits of trees and limited lawns, you’re likely to find him hiking San Antonio’s wilderness parks or expounding on the virtues of geography and history to his friends.
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