How to Design a Residential Rainwater Collection System

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A residential rainwater collection system can mean different things. If you want to collect rainfall solely for domestic uses, such as cooking, washing or drinking, and the water is clean enough or you have a filtration system, you should install a cistern. If you also want to use the collected rainwater for a garden, you'll need to do a bit more work, but you will end up saving a lot of water.

Things You'll Need

  • Measuring tape
  • Cistern
  • Piping and gutters
  • 2 yardsticks
  • Clear plastic tubing
  • Duct tape
  • Shovels

Determining Cistern Size

  • Measure the roof area of your house with a measuring tape. Multiply the length times the width in feet to get the roof's square footage.

  • Look up the average monthly and yearly rainfall in your area. Use the rainfall statistics page linked in the resources section, or find a similar website. Convert the averages, which are probably in inches, into feet by dividing by 12.

  • Multiply the monthly rainfall totals, in feet, by your roof area, in square feet. The result will be the volume of water, in cubic feet, which you can expect to fall on your roof in a given month. Each cubic foot is equal to 7.48 U.S. gallons or 28.3 liters. This will give you an idea of how large your cistern needs to be in order to capture all of the rainfall.

Installing the Cistern

  • Watch your roof carefully during the next big storm, or simulate one with a garden hose. You want to find all the downspouts.

  • Find a good place for the cistern. Aside from obvious issues of space and aesthetics, look for a spot where most of the water leaves your roof so you don't need to install too much piping to get the water to your cistern.

  • Install gutters anywhere that water would normally fall to the ground. Direct the gutters straight to the cistern or to piping that leads to the cistern. The specifications of the piping and gutters will depend on the type of roof you have and the model of cistern.

Landscaping for Rainwater Collection

  • Observe your garden area during a rainstorm to see where the water flows. The goals of your water harvesting plan are to keep it on your property where it can be used by plants, slow its flow to prevent erosion, and collect it in shallow basins so it doesn't drown plants.

  • Build a bunyip, a simple device for comparing elevations. Tape, tie or glue a piece of plastic tubing to two yardsticks so the open ends face up and several meters of closed tubing connect the yardsticks. Pour water into one of the open ends of the tube and look at the markings on the yardsticks. The difference in waterlines shows how much the ground differs in elevation.

  • Begin digging trenches to move water from areas where it isn't being used to areas with plants that need it. Use the soil from the trenches to build berms (ridges of packed dirt) around big, flat basins. Trenches should be wide rather than narrow, and berms and trenches should be studded with pebbles to slow erosion.

  • Plan for worst-case scenarios. Make sure your basins have overflow points where water can escape during big storms.

  • Observe how your work performs. After the next storm, see whether there has been excessive erosion or damage to your earthen structures. There should be no standing water left after 24 hours, or 48 hours for really big storms. If the water isn't infiltrating fast enough, your basins are too small or you need to create a better overflow canal.

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