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When I'm trying to figure out how much to water things in my garden I'll often see things like, "...they will need 1-2 inches of water each week."

From what I understand, this is the same as if there had been 1-2 inches of rain. My problem is that everything for drip irrigation systems seem to measure in gallons or liters per hour.

How can I go about converting inches to gallons?

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One US gallon is 231 cubic inches.

One square yard is 1296 square inches.

So "1 inch of rain" is 1296/231 or about 5 and a half US gallons or 21 liters of water per square yard.

The flow rate through your drip system obviously depends on the area it is watering and how many hours it is on each day, but that gives you a starting point.

Note: 1 imperial (UK) gallon is 277 cubic inches which gives 4 and three quarter UK gallons per square yard - but commercial UK irrigation systems will be specified in metric units so that is fairly irrelevant.

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I just went through the same thing myself when setting up my drip irrigation system. It's actually pretty easy.

The formula is as follows:

Pr = 96.25 x Total GPM of all emitters (gal/min)
     -------------------------------------------
                 Total area (ft²)

Where Pr is the "precipitation" rate in inches/hour.

To make it more clear, I'll take an example from my own garden. I have 6 slicing tomato plants planted in roughly an 18' x 3' area. I have 2 sections of drip tubing that stretch on either side of the plants which provide a total of 29 0.9 gal/hour (GPH) emitters within that area. I also have an additional single 1 GPH emitter for each plant.

So, to find the Total GPM of all emitters (gal/min) I do the following:

29 x 0.9 gal/hr = 26.1 gal/hr

6 x 1.0 gal/hr = 6 gal/hr

26.1 gal/hr + 6 gal/hr = 32.1 gal/hr

To get gal/min, divide by 60:

32.1 gal/hr / 60 = 0.535 gal/min

So, plug that into the equation and you get

Pr = 96.25 x 0.535 gal/min
     ---------------------
            18 x 3

So, Pr = 51.5 / 54 or .954 inches/hour

Let's say I want to give them 2 inches of water per week. At 0.954 in/hr, I'd need to water a total of 2.097 hours, or 126 minutes per week.

I compared this calculated method to an actual physical test (held tub underneath 2 of the drip tape emitters for 30 minutes), and it's pretty much dead on.

I highly recommend setting this up as a spreadsheet with columns for width and length, and then columns for the quantities of each speed emitter you have.

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An Imperial Gallon is about 4.5 litres. An inch is 0.025 metres. So an inch of water per week is equivalent to 0.025 cubic metres, or 25 litres per hour per square metre for one hour, or 5.6 gallons per hour per square metre for one hour. It's early morning here, so I hope I've got that right.

  • You forgot to say how much ground the "2.2 gallons per hour" will water. (Note to self: don't answer questions before being fully awake...) – alephzero Jul 22 at 9:12
  • @alephzero - I thought I might screw it up. I've now added "per square metre". I'll have you know that not only did I get the figures wrong, I also assumed the OP was Canadian (hence my reference to Imperial Gallons) when I saw he comes from Sacramento, CA. Whoops. – Peter4075 Jul 22 at 12:23
  • Just to be clear, "25 liters per hour for one hour" is the right number so long as the system is turned off for the other 167 hours in the week. But "an inch of rain in one hour" is a severe tropical storm, not the usual way to water plants! – alephzero Jul 22 at 20:09

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