How Much Water for My Landscape? (cont.)

How Long Do I Run the Irrigation?  (Part 2 of 2)

By Lori Palmquist

We learned in part one of this post about the irrigation water needs of plants during the seasons when it's not raining. If you missed part one of this post, you can read it here.

How do we go from knowing the number of inches of water plants need to the number of minutes per week the irrigation controller needs to be scheduled? Inches are easily converted into run time minutes on the controller when we know the speed of the application of water for each zone in the landscape.

A good thing to make clear here is the fact that the irrigation needs to be a uniform method of application in order to use inches and an application rate. Uniform methods of irrigation have application rates, and non-uniform methods do not.

A uniform method would be either sprinklers or drip grids. Non-uniform would be irrigation that's applied sporadically, like point source drip. Point source applies a little water here, a little there, and so on. It only lends itself to watering in gallons, while uniform methods like drip grids and sprinklers apply the water in inches.

I’ve seen attempts made at assigning point source drip an application rate, like taking the wetted pattern of the drip emitter and measuring the square footage of it. This is inaccurate at best, since the wetted pattern of emitters will change as soil types and degrees of compaction vary throughout the landscape. Also, the emitter flow rate and the spacing on those emitters wouldn't be regular enough to establish an application rate.

So, we've established that the method of irrigation, in order to go from inches to run time minutes, would be either sprinklers or drip grids. The method of converting number of inches of irrigation needed to number of minutes per week of run time, is similar to the way GPS units calculate the amount of time it will take you to drive from point A to point B.

With GPS, we tell it the address of our destination. It then knows how many miles need to be driven. It also knows the speed limits on all of the streets and highways we'll have to drive on to get there. So it divides the number of miles by the speed we'll be driving, and comes up with the number of driving minutes. If we need to drive 60 miles, and we’ll be driving 60 mph to get there, it will take one hour (60 minutes) to get there.

Much like GPS, we take the number of inches of water required by the plants, and divide that by the speed of the water as it's applied, to determine the number of minutes each zone needs to run. This speed, otherwise known as application rate or precipitation rate, is measured in inches per hour. If the plant water need is 1 inch, and the irrigation applies water at a rate of 1 inch per hour, it will require a run time of 1 hour (60 minutes) to deliver the needed amount of water.

Finding out the application rate of the irrigation for each zone in the landscape is a fairly straight-forward process. There are three ways to determine application rate:

1. Do a sprinkler audit (catch can test).
2. Look it up in the manufacturer’s product catalog.
3. Use the precipitation rate formula to calculate it. You will need to know the gallons per minute or gallons per hour flow of the zone, and the area in square feet of the irrigated area of the zone. This is a bit more involved.

For this post, we'll simply use average application rates of different kinds of sprinklers and different drip grids.

This table shows average precipitation rates (application rates) of various sprinklers and drip grids. This list is from an online scheduling application I developed several years ago that can be found at  http://waterwonk.us/irrcalc.

Using the value for fixed spray sprinklers above of 1.8 in/hr, we can now determine the run time for our lawn, as an example. If it's July, and we know that the lawn needs 4.8 inches in July, we'd plug in the following formula:

Run time minutes for July = 4.8 inches / 1.8 inches per hour x 60 minutes
Run time in July = 160 minutes
Weekly run time minutes = 160 / 31 days x 7
Weekly run time minutes = 36 minutes per week in July

This has been a basic introduction to irrigation scheduling and knowing how much water in inches that landscape plants need. If you’re interested in taking a deeper dive into this and other irrigation topics that I teach, check out the online classes that we offer on this site.