There is a basic disconnect when you think in terms of, “How many inches of rain did we get last night?”  You think of precipitation in terms of “inches.”  But you irrigation controller wants you to think in minutes.  “How many minutes do you want zone 3 to run?”

Here’s how to do it.

If the sprinkler zone you’re dealing with is a spray zone, it precipitates at about 1.5″ per hour.  So, to “rain” one inch, you set your spray zones on 14 minutes, 3 times per week. (More on how I got that in a minute.)

If your zone or station is a Rotor zone, it precipitates at only about .5″ per hour.  So it needs to run for about 40 minutes, 3 times a week.

There you have it.

In order to get one inch of “rain” per week, just set your spray zones on 14 minutes x 3 days and your rotor zones on 40 minutes x 3 days.   But what if you want more or less than 1 inch?    All modern controllers have a setting that adjusts all the times at once.  It’s usually call “Seasonal Adjustment,” or “Water Budget” or something like that.  If you only need .5 inches this week, simply change your “Percentage Adjustment” to 50%.  If we are in the heat of August and need 1.5 inches this week, just set your “Seasonal Adjustment” on 150%.  Simple.

How and Why This Works

If you look in a sprinkler head manufacturer’s catalog, it will tell you the “precipitation rate” for each of their nozzles.  Many manufacturers come in at about one half inch per hour but you can find exact figures on the Internet.  If you need only one hour the math is to take two thirds of the hour or 40 minutes and divide it by the number of times you want to water (3).  40 minutes divided by 3 is not quite 14 minutes for one inch.

The same math for Rotors that precipitate at .5 inch per hour.  To get one inch it needs to run 2 hours per week, or 120 minutes.  120 divided by three gives us our 40 minutes, three times per week.

Why You Need To Change Your Controller

As our seasons change your landscape’s need for water changes too.  It’s up to you to give your lawn and beds enough water without wasting any.  It’s easy if you have set your controller this way and use the Percentage Adjustment setting to increase or decrease the water supplied by your controller.


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