Drip Irrigation By the Numbers 1

The drip irrigation system is in and working. Now we can take time to do some calculations.

There are nine faucets we can use for ordinary watering with a hose, and there are three “zones” of drip irrigation. Zone 1 (we call it “Veg Hill East”) serves the seven eastern rows of Veg Hill plus Amanda’s herbs and the small stack of mushroom logs that rest just outside the deer fence. Zone 2 (“Veg Hill West” – are we creative or what?) serves the five western rows of Veg Hill (which tend to be longer rows) plus nine blackberries on the west border of the fence. Zone 3 is the line of blueberries and (eventually) the greenhouse.

This would be simpler if I could tell you the area of Veg Hill. And one day perhaps I will be such a man of leisure that I will. For now, though, I am intimidated by the fact that Veg Hill is an irregular polygon rather than one of those nice, neat rectangles on Mr. Archer’s geometry tests in high school, so I will simply say it’s about 1/4 of an acre, or about 11,000 sq. ft.

We use T-tape 515-30-250; I just learned that T-tape is now owned by John Deere, and on the John Deere website I see that our T-tape dispenses .34 gallons per 100 feet using emitters spaced 12″ apart. So the 2,004 feet of drip tape on Veg Hill East sucks down about 2,004 / 100 x .34 = 6.8 gallons per minute. If we water Veg Hill East for two hours, that means we’re blessing Veg Hill East with about 817 gallons of water.

So how does that equate with rainfall? That 817 gallons of water times 231 cu. in. per gallon constitutes 817 x 231 = 188,873 cu. in. of water. Veg Hill East has about 5600 sq. ft. of surface area, or 5,600 x 144 = 806,400 sq. in., so 188,873 / 806,400 = .23 inches of rainfall. But wait. That would assume that the irrigation water is evenly distributed over all that surface area, which is clearly not the case. One of the wonderful advantages of drip irrigation is that it puts the water right at the roots of the plants we want to grow and deprives the weeds around them. The water drips along two channels per 4-foot row. It doesn’t flow into the aisles or even much into the middle of a row. So maybe the rainfall equivalent for our “target” crops isn’t .23 inches of rainfall but more like three times that, or .7 inches of rainfall. While we’re at this stage, let’s take the time to calculate effective inches of rainfall equivalent per hour of drip irrigation. If two hours results in effective rainfall equivalent of .7 inches, then each hour is .35 inches.

So THAT tells me that if we plan to water on even-numbered days, we don’t need to be watering for two hours. That would equate to .7 x 365 / 2 = 128 inches of rainfall per year, in addition to the roughly 50 inches that God provides! Instead, let’s think in terms of 1 inch of drip irrigation rainfall per week. An average week has 3.5 watering days, so each watering day should be the equivalent of 1 inch / 3.5 watering days = .285 inches of rainfall equivalent. .285 inches per watering day / .35 inches per hour = 81% of an hour, or about 48 minutes. So if we water on every other day and we want to provide the equivalent of 1 inch of rainfall per week in addition to ambient rainfall, each watering needs to be 48 minutes.

Because I have made so many assumptions, particularly the one two paragraphs above where I magically multiply the rainfall equivalent by three, we must confess that all of this is very approximate. Maybe the right number is 25% above or 25% below the figure I’m calculating. Fortunately, one of the wonderful things about growing plants in general is that plants growing in good soil are extremely forgiving, so we assume that if we get in the right ballpark we’ll be okay.

And I don’t expect we will need to keep second-guessing the amount of time each zone receives. For example, I know we got .27 inches of honest-to-goodness water from the sky rainfall yesterday, but that doesn’t mean I need to change the timing on the drip irrigation at all. The drip system is all about deep moisture to the roots, a need that doesn’t change much with a shower here and there.

I could go through this same process for Veg Hill West and for the blueberries, but I won’t, because (a) it’s time for me to fetch Amanda’s coffee and (b) the relationship should be roughly the same anyway.

I’ll set our controller to provide 48 minutes each on Veg Hill East, Veg Hill West, and the blueberries, on every even-numbered day. And then we’ll step back and see how everything responds.

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One thought on “Drip Irrigation By the Numbers

  • Jonathon Meeks

    Deep moisture for your raised beds(actually any beds that you won’t till with a tractor) by building an underground “well” basically some solid container buried two or three feet down and filled with gravel with length of pipe leading to the surface for watering. With this method, almost no water is lost to evaporation or running off to the water table. It just sites there waiting for you plants.