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Rain catchment system physics question

  1. Mar 11, 2015 #1
    I am volunteering at an orphanage in Guatemala. There is no well, and the city water is a relatively expensive. I would like to install a rain catchment system on our roof for use in the outhouse bathroom and garden. I've diagrammed it and attached the image.

    My question is this: Will the water tank completely fill up, or will the water level in the water tank stay at the water level of the red pipe? My education says that the rain will fill up the very bottom of the tank, then the flat part of the red pipe, and then continue to fill the tank and up the red pipe in such a way that the respective water levels are equal. However, common sense seems to be saying that the tank will not fill up past the level of the flat red pipe section because the water has to "push" its way up the tank.

    Will this system work? Is my common sense concern reflective of reality?

    Attached Files:

  2. jcsd
  3. Mar 11, 2015 #2
    The vertical part of the red pipe would have to have more volume then the tank for this to work. Im no expert, but I would place the red pipe at the top of the tank.
  4. Mar 11, 2015 #3


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    Yes that is correct. Assuming the top of the tank has a hole somewhere the tank can fill to the level of the red pipe inlet. ie the gutter height.
    leaks are twice as likely as a top filled tank and the pipe may be more vulnerable to damage on the ground, A stray spade strike and the tank empties.

    Why can't you go overhead? A hose tied to an overhead rope/cable strung between the house and tank works. Depending on catchment size, rain fall, flow rate structure strength, span length etc of course.
  5. Mar 14, 2015 #4


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    Welcome to PF.
    The red pipe and tank here form what is termed an “inverted siphon”.
    The top of the tank must have a small air vent.
    Place a stop valve at the tank where the red pipe attaches so you can service the red pipe without draining the tank.
    The tank only needs one drain/fill hole, the brown pipe can come from a 'T' in the red pipe.
  6. Aug 3, 2015 #5
    Thank you all!
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