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How can I ensure even water flow to my garden?

  1. May 8, 2015 #1
    I would like to setup a subsurface irrigation system for my garden.

    I have several beds which, while I made each one perfectly level, are at different elevations (about 1 or 2 feet between each) .

    The source of the water will be a permanently mounted bucket, barrel or cistern of some kind, with a single valve to open to feed the system.

    I will have garden hose that goes from the cistern to the various beds in the garden. One inch below the surface of each bed will be a perfectly level PVC tube, with holes drilled in the top. The water source hose will go into one end of these tubes, and the other end will either be capped, or lead to a lower bed. Water will push out of the top holes to water the beds.

    QUESTION:
    Is there a simple way for me to ensure that the amount of water delivered to each bed will be the same? If I just run a hose to each bed, the beds at a lower elevation will get more water per minute than the higher beds, (due to greater water pressure at the holes in the tube), which I would like to avoid. Or, is this incorrect?

    Ideally I could let a higher bed’s PVC pipe feed a lower bed’s PVC pipe, so I don’t need to run a hose to EVERY bed, just every ROW of beds using a "mainline".

    More info: Other than my well pump, which is used to get water into the cistern, everything else is gravity fed. The bottom of the cistern will be about a foot above the highest bed.

    My thoughts so far: Could this be done by simply making the LAST hole in a bed’s PVC pipe larger, and use that to feed the next tube? If each bed has N holes, of radius R, how large would that last hole have to be to ensure the water that flows out of it is equal to the water flowing out of all the previous holes in the pipe? To complicate it, what if I have 2 or 3 beds below the first? Also, would a change in the water source pressure, as it drains, throw this hole-size ratio off?

    I am hoping to get an answer as simple/brilliant as: "drill holes" in this section of pipe, to let air in, or something like that.
     
  2. jcsd
  3. May 9, 2015 #2

    Simon Bridge

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    You can make sure the water flow is the same for all pipes by adding a tap as a throttle in the leadup to the irrigation tube. Measure the water flow rate before burying and adjust the tap accordingly.
     
  4. May 9, 2015 #3
    A throttle in each line is a good idea if the amount of piping/headloss differs significantly between the "mainlines", but it doesn't help with the other issue of chaining pipes together in series within each string. As soon as the flow reaches the first holes, pressure will start dropping, and there's no passive device that could keep it equalized all the way down the line.

    The magnitude of the problem can be reduced by mounting the gravity-fed source higher (so there's higher initial pressure, and less variation in total height between the uppermost and lowermost beds), using larger/smoother pipe (lower headloss per foot), and drilling smaller holes (so the pressure loss at each is lower). Rather than making the holes progressively larger as you move down the line (a tricky problem even for constant source pressure), I'd just increase their frequency, i.e. drill more in the lower pipes, and even perhaps even space them further apart toward the upstream end of each pipe if the sections are very long. You'll probably have to determine the spacing experimentally before burying the tubes.
     
  5. May 9, 2015 #4

    Simon Bridge

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    Oh yes, the pipes have to be fed separarely to keep it simple.
    Welcome to PF BrentS.
     
  6. May 10, 2015 #5

    CWatters

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    Make sure you filter the water or you will be digging up the pipes regularly to unblock holes.
     
  7. May 10, 2015 #6

    jack action

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    I would use a multi-level «waterfall-pool» set up. All you need is to put an open reservoir at one end of each of the bed pipes. The input flow should arrive horizontally at the top of the reservoir and there should be an overflow feeding to the next lower bed. This way the pressure in the pipe will be controlled by the depth of the reservoir, not by the height of the cistern.

    For it to work, you need either a valve at the entry of each reservoir controlled by a floater in the reservoir or one valve at the cistern controlled by a floater in the lowest reservoir.

    To better equalize the flow along each of your pipe (if they're really long), you can put more than one of this set up in parallel, like one reservoir at each end of the pipe or even add others between the two.

    It would look like this (the recirculating pump replaced by the floater(s) and valve(s)):

    wat_illd.gif

    Like I said, the only difference would be to make sure the waterfall would fall into a 90° elbow such that the flow arrives horizontally above the pipe entry (at the bottom of the reservoir) such as not to add the dynamic pressure of the fall onto the reservoir depth water level.
     
  8. May 10, 2015 #7

    jim hardy

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    If you wanted to get crazy complicated.....

    Can you get some old nutating disk meters from your water company's junkpile ?

    They're a positive displacement engine , so belting their driveshafts together should give equal flow.

    220px-Dakeyne_disc_engine_animation.gif

    have fun !
     
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