Suppose you had 32 sq. ft. of raised bed growing space. Here's how to calculate how much water each 32 sq. ft. bed would use per day. Multiply the number of emitters times the rated gallon per hour (GPH) for the drip line. In a four four by eight foot raised, one 32 sq. ft. bed would have five lines with 11 emitters each. That's 55 emitters total. Each emitter drips .57 gallons of water per hour. Here's the math:

 

  • 55 emitters x .57 GPH = 31.35 gallons of water per hour
  • 31.35 gallons / 6 = 5.23 gallons of water for 10 minutes            
  • or
  • 31.35 gallons / 60 = .523 gallons of water per minute

Remember that an hour has six 10 minute intervals and 60 one minute intervals. Therefore, you can figure exactly how many minutes you need to water and note how much water you're using. Keep in mind that none of the water is being wasted! Every drop goes to the roots of your veggie garden.

 

It's also worth noting that drip lines are metered. Drip tape isn't. Drip lines are the solid lines with the built-in emitters. At any rate drip line operate under low pressure, usually between 15 and 25 psi. This means the emitter at the top of a hill will get the same amount of water as the emitter at the bottom, even distribution. This important because you don't want water flooding one end of the garden while the other end is bone dry.

 

Grow'em big!

Damon


6 Responses to “Drip Irrigation: How to Calculate Flow Requirements”

  1. Kris from CT Says:

    Hi Damon,
    Thanks for the info above I am going to use some kind of irrigation this year for the garden.  I have raised beds as well and am wondering how a drip or soaker hose system would impact how I should position my plants? For instance if I want to plant something small, like radishes, would this type of irrigation require me to only plant along the soaker hose or at the base of each emitter (drip system)? I usually fill each square foot to the max and switching to a single "row" of radishes would greatly decrease production.  I guess my question boils down to: would a drip hose water the entire square foot or only a few inches around the emitters?
    Thanks,
    Kris

  2. Damon Says:

    Great question. There are a couple way to approach drip irrigation by the square foot:

    (1) You could use bubbler-style emitters to water the area. Be sure that you use a good mulch as not to splash any growing medium up onto the plant, making them sick and to hold back potential fungal attacks. I tend not to favor this method.

    (2) You can soak the entire area by spacing your drip lines every six inches along the manifold. You’ll be surprised at how much water this puts out. In fact it puts out too much water, so I relaxed it a bit. In a typical four foot wide bed, I’ll have five drip lines spaces about a foot apart each. This suffices to wick the medium, keeping a constant level of moisture with only about 10 minutes of watering per bed per day.

    NOTE: In general you’ll find that with drip irrigation the surface of your raised beds will be dry. Don’t be alarmed at this. If you dig down an inch or two the entire medium is evenly moisten. This trains the roots to grow down instead of remaining near the surface and is much healthier for the plants. Remember just wicking the entire medium, and all that organic matter will hold onto lots of moisture, so there’s no need for think in terms of watering each plant.

    Grow’em big!
    Damon

  3. Justin Smith Says:

    Damon,
    Thanks for doing an episode on drip irrigation systems, I can't wait to install one in the garden this year. I am wondering where you decided to purchase your system. Did you buy the pieces individually at a big-box store or online? What do you recommend to us Greenhorns?
    Best,
    Justin

  4. Damon Says:

    Online. I buy everything from DripIrirgation.com, and they sell DiG Corporation products. I stay away from the big-box Wally-Marts and the sort because they’re drip products actually cost a lot more and have far fewer choices.

    My system is very simple. As you see in the photo, each bed had it’s own connection, thus allowing you to use a basic hosepipe to transport water to any point in the garden. This saves cash by reducing the amount of tubing needed.

  5. Laura Says:

    Hi. I cant seem to find any general info. I hope you can help me. I have a 55 gal rain barrel that I would like to use for irrigation. I want to hook up a small line (maybe 1/4"?) to it and have it feed a dozen or so individual yard plants/shrubs. Ive seen this done on a palm tree farm in FL with acres of 1/4" line but didnt pay any attention to the water source or whether the line tapered down somewhere.
    My questions 1) is 1/4" line ok to run several hundred feet with only pressure from the barrel? 2) Will the plants at the end of the line get as much water as the rest?
    Thank you for whatever advice/info you can give.

  6. Damon Says:

    Hey, thanks for asking! You guys really make me do my homework. So you’re talking about a gravity fed drip irrigation system. Several hundred feet? No. You’d have to have a true water tower to make that work. It all depends on the pressure. The only way to increase pressure is to have gravity add pressure for you. 10 to 15 feet is probably all you’d get from a 55 gallon tank if you raised just a foot off the ground.

    Here’s a nice article from dripirrigation.com. It’ll give you a basic understanding of how gravity-feed irrigation works.

    http://www.dripirrigation.com/drip_irrigation_chapters/9/drip_irrigation_pages/40

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