How long to put down 1" of water

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uncle_al
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How long to put down 1" of water

Post by uncle_al » July 31st, 2017, 10:02 am

I have been having some heat stress issues on some sections of my lawn. I usually run each zone for 2 hours. So, this morning I put out a tuna can and ran it for two hours. I was shocked to see only 1/2 inch of water in the can. Is this normal? They are Hunter I-20 heads.

Thanks

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ken-n-nancy
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Re: How long to put down 1" of water

Post by ken-n-nancy » July 31st, 2017, 12:42 pm

Short answer -- that's not unusual. Many people find that it takes far, far longer to deliver 1" of water through irrigation than they believed possible. Depending upon one's sprinkler system, it really could vary from as short as 15 minutes per zone to as long as 8 hours.

The only way to be sure is to do a "sprinkler audit" using tuna cans, or rain gauges, or whatever.

Even for a Hunter I-20 head, the amount of water delivered per hour is greatly affected by the specific nozzle being used, the pressure at the sprinkler head, and the degree coverage of the rotary sprinkler head.

The above factors are huge: nozzle size variation in an I-20 can range from 0.36 gallons per minute up to 14.8 gallons per minute. (Yes, that's a difference of a factor of over 40!!!)

Pressure at the sprinkler head can vary the GPM by almost 2 to 1.

A rotor adjusted for 90 degrees will deliver 4x as much water per square foot each minute as a 360-degree head, since the same amount of water is coming out of the sprinkler head, but only watering 1/4 as much area.

So, the amount of time needed to run an irrigation system to deliver 1" of water can vary radically. It will also likely vary even within your irrigation system from zone to zone.

So, the time needed to water 1" from your irrigation system could be radically different than mine. Using my times for your system could grossly underwater or overwater your lawn. I don't have my sprinkler audit results handy, but there's a lot of variation amongst my zones. I have one zone with 360-degree heads, which has to run 2x as long as another zone I have which is all 180-degree heads. I think your 2 hours for each zone to deliver 1/2" is pretty similar to our system.

greenrebellion
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Re: How long to put down 1" of water

Post by greenrebellion » July 31st, 2017, 2:15 pm

Certainly sounds plausible, I tested one of my backyard zones this weekend. 2 hours put down about 1/2" of water.

bmelz
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Re: How long to put down 1" of water

Post by bmelz » July 31st, 2017, 3:40 pm

I've read that one inch of water or is equivalent to 623 gallons per 1,000 square feet.

so my 2500 square foot yard will need 1557.5 gallons of water. With my current nozzle set up I think I am drawing about 11 gallons of water per minute. That would require my sprinklers to run for 141 minutes or 2 hours and 21 minutes to deliver 1 inch of water.

uncle_al
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Re: How long to put down 1" of water

Post by uncle_al » August 1st, 2017, 8:49 am

Thanks for the responses. Looks like my water bill will be much higher going forward!!


PW405
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Re: How long to put down 1" of water

Post by PW405 » August 4th, 2017, 9:33 pm

I was attempting to do some "math" (*disclaimer - it has been a while!) and here's what I came up with, using my largest (in area) zone as an example. This particular zone is about 2,000 sq ft, with dimensions of 40'x52' = 2,080 sq ft.

1 Gallon = 231 Cubic inches. Simple enough so far right?

So, we need to calculate volume for 1" X 40' x 52'. Let us first convert to square inches of the lawn. Easy enough:

480" x 624" = 299,520 sq. in.

Volume of 1" of water on lawn = LxWxH. H = 1", 480" x 624" x 1" = 299,520 cubic inches.

I figure this zone delivers about 15 gallons/minute based on my nozzle selection. In other words, this zone delivers (15 x 231) 3,465 cubic inches/minute.

Easy enough: 299,520 cubic inches needed. Putting down 3,465 cubic inches/minute = 86 minutes of watering this zone, or about 1,300 gallons.

NOTE - This method obviously has some flaws. For one, it assumes 100% perfectly uniform coverage. I know that doesn't occur in my case, as some areas get spray from multiple rotors, while other areas only catch a single rotor on 1 pass. While I've done my best to achieve uniform coverage, many times it isn't possible.

Additionally, as water works its way through pipes, it actually creates friction and loses pressure the further the pipe moves and the more turns/bends the water must make. Even with a 1.5 gallon/minute nozzle, I might not actually be getting that out of the end of the sprinkler. Or, I could be getting a little more.

If the spouse isn't happy with tuna cans strewn about the front lawn, I've seen some products such as this that seem like a great way to get an idea of how even coverage you've got, and duration to achieve 1":

https://www.amazon.com/Sprinkler-Bright ... 9KB9DWDP5Z

Despite the shortcomings of this method, I still feel as if it gives you a good enough idea of how long it will take to get the 1". It was much longer than I thought!

PS - if you are using rotor sprinklers, there is a good chance the installer used the default (usually 3 gallon/min) nozzles for all rotors, regardless of their swept area. Rainbird's rule of thumb is that you use a 3 GPM nozzle for a rotor that has a 180° path, you use a 1.5 GPM nozzle for a rotor that sprays a 90° pattern, etc. Makes sense to half the rate if one rotor is spraying half the area of another.

Lemme know if you have any questions!

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Re: How long to put down 1" of water

Post by mabehr » September 28th, 2017, 11:40 am

uncle_al wrote:
July 31st, 2017, 10:02 am
I have been having some heat stress issues on some sections of my lawn. I usually run each zone for 2 hours. So, this morning I put out a tuna can and ran it for two hours. I was shocked to see only 1/2 inch of water in the can. Is this normal? They are Hunter I-20 heads.

Thanks
Most of my I-20s are around .28 inches per hour, one zone is .21.

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