Know your sprinkler precipitation rate

Even with the advent of smart controllers, you can’t ignore irrigation fundamentals like precipitation rate
Eric Santos
Eric Santos

In business, PR usually stands for public relations. In the irrigation world, PR stands for precipitation rate.

When you ask most irrigation professionals, “How much do you water turf in the summer?” the most common answer you will get is, “Well, it depends.”

What does it depend on? “The type of sprinkler.”

Very rarely do you hear, “It depends on the sprinkler precipitation rate,” because most irrigation professionals assume that all sprayheads and rotors have the same precipitation rate.

Any professional responsible for programming an irrigation controller should have a solid understanding of precipitation rates and their variation.

What’s precipitation rate?

Precipitation rate is the rate at which we apply water to a given area. Precipitation rate is measured in inches per hour. I often explain it in these terms: If you were to water an area for an hour, how many inches of water would these sprinklers apply?

For conventional sprayheads, precipitation rates typically range from 1.3 inches to 2 inches per hour. For gear drive rotors, precipitation rates typically range from 0.4 inch to 1 inch per hour. For rotary nozzles, precipitation rates typically range from 0.4 inch to 0.6 inch per hour. The table below illustrates the difference in precipitation rates for two leading manufacturers for the same size nozzle, both operating at 30 psi.

Why is knowing the precipitation rate important?

At BrightView, when we program an irrigation controller and are deciding how many minutes to run the zone, the first step is knowing the rate at which we are applying water — also known as the precipitation rate. If we are losing 0.25 inch per day to evapotranspiration (ET), we need to replenish a total of 1.75 inches of water depleted for the entire week. To know how many minutes it takes to replenish 1.75 inches of water, we need to know the precipitation rate of the sprinklers applying that water.

For example, let’s assume my sprayheads have a precipitation rate of 1.5 inches per hour. I do simple math to figure that if my sprinklers apply 1.5 inches per hour, I will need to run my sprinklers for a total of at least 70 minutes for the entire week to replenish the total 1.75 inches of water depleted by ET for that week.

Additionally, I may want to add more time to account for losses caused by wind or anything else impacting distribution uniformity.

But what if my sprinkler precipitation rate is 1.75 inches per hour and not 1.5 inches per hour? I would need to run these sprinklers a minimum of 60 minutes across the entire week to replenish the total 1.75 inches of water depleted by ET for the week.

The difference between watering for 60 minutes for the week and watering 70 minutes for the week doesn’t sound like a lot, but it’s nearly 15 percent.

If you want to save your customers 15 percent on their water bills, it’s important to know your precipitation rate to know how long you need to water to replenish what was lost to ET.

Once we know the total amount of time it would take to replenish the ET depleted for the week, we would continue on to determine specific cycle runtimes, cycle starts, soak times and frequency, all of which are influenced by soil infiltration rate, root depth, slope, a soil’s water-holding capacity and management allowable depletion.

With the advent of smart controllers, knowing precipitation rates becomes even more important. Accepting sprinkler default precipitation rates, as opposed to entering actual field precipitation rates, can mean the difference between saving water or using more water with new technology.

Now that we understand how sprinkler PR can impact your PR, let’s think twice before we program our controllers.

In an upcoming article, I will explain how to calculate sprinkler precipitation rate and what can cause it to vary. Stay tuned!

Santos is the vice president of irrigation services for BrightView.

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