Calculating Amount of Time to Put Down 1"

[Smolenski7]

So I have a bunch, probably about a dozen, home made in-ground pop-up sprinklers spaced around my yard. All together, they cover about 85%-90% of my 20K. Each one is calibrated a bit differently and they are not made up of the same sprinkler head. I know I can use a tuna can to approximate how long it will take each of them to put out 1" of water, however, I always question this method only because it seems to really depend on where the tuna can is placed. Too close to the head and it will fill up quickly, too far away and it will take hours! Then there is the wind. So with this in mind, I turned to math.

I found a website that says to use the following formula, it's called the Total Area Method and it finds the amount of water put down per hour: Precipitation Rate (Pr) = 96.25 x Total GPM / Total Area. The site claims that 96.25 is a constant that converts gallons per minute (GPM) to inches per hour. So, using the water monitor I installed over the winter, I know that most of the heads around my house put out 3.5 GPM. The Total Area that each head is responsible for is different. My problem is that some of the larger areas equate out to having to leave the water running for 6+ hours just to put down 1" of water. Does this seem typical? At that rate, it's just not realistic to put out 1" of water.

[ENVY23]

That could be about right, depending on how your zones are laid out and how many heads you have per zone. I have an area that's 5500 sqft, 3 zones with 4 heads per zone, so 12 heads total. Each zone runs for approx 2hrs, so in total, that's about 6hrs to put 1" on that particular 5,500 sqft area. Ideally though, you don't want it to run for 6hrs straight, that would lead to runoff and waste water. My system does a "cycle and soak" where, for example, the 1st zone runs for 30min, then the 2nd zone runs for 30min, then the 3rd zone runs for 30min, then it's back to the 1st zone which has had 1hr to let the water soak into the soil while the 2nd and 3rd zone ran.

[Smolenski7]

Wow! I don't have zones like a professionally installed irrigation system though. I just have several homemade pop-up sprinklers that I have buried that seem to cover most of my yard. When I sat down to do the math, I was expecting to get an answer of about 2 hours to put down 1" of water. I wasn't expecting 6+ hours for my largest area and about 2 hours for my smallest area. If I actually watered like this, my water bill would be in the thousands.....and my divorce lawyer would be even more!

[ken-n-nancy]

Yes, many hours for 1" of coverage sounds about right.

Irrigation systems should be laid out with "head-to-head" coverage to mitigate the issue of more water delivered near the sprinkler head and less further out, as you describe. If your sprinklers are not giving "head-to-head" coverage, more water is generally delivered near the heads and too little far from them.

[Green]

I would just measure with the cans, and lay them out so they're spaced in between the heads, equally. If you have enough, you could put them in all different places, but I think the differing amounts in the cans are enough to be frustrating.

There is no reason that a larger sprinkler zone needs more watering time if all else is equal...that's like saying you need higher fertilizer rates on larger areas...it's simply not true. Yes, you need more fertilizer (or more gallons) but not higher rates. If you're using one or two rotating heads at maximum radius and rotation angle to cover a larger area, then of course you'll need a longer watering time, but a lot of that is because of the larger number of degrees you're rotating. And some of it is because the sprinkler is being pushed to the limits. It'll definitely water unevenly in that situation.

[northeastlawn]

I 2nd measuring with cans.

I am using a DIY impact sprinkler system, I just used tuna cans to time how long its takes to get to an inch. To be honest its about 90min to get an inch with the impact sprinklers.

One of my projects is to try a regular pop-up sprinkler head from a big box store, I figured they can just screw into the bases I have, and see if it is more efficient.