Basic Principles of Rainwater Harvesting
Rainwater harvesting is an old-fashioned and simple way of collecting rainwater. For years people have channeled rainwater into cisterns and water barrels. By connecting the downspouts from roof gutters to some form of drum-like water barrel, people can quickly and easily collect rainwater for later use. This same basic set-up, with a few adaptations, is useful in city gardens where getting water can sometimes be difficult.
Only a few simple items are required: rain barrels, gutters, tubing, hardware to connect barrels, and materials to label barrels. Exactly how many or much of each is needed will depend on your design. Each water harvesting system will vary with the given garden's access to roof run-off, the amount of space available for the system, and the amount of water you hope to collect.
As you think about designing a water harvesting system, there are a few basic questions you should answer or at least keep in mind:
Will you collect water from a roofed structure in your garden, or from the roof of an adjacent building?
A system that collects water from the roof of a casita or tool shed is the simplest. They usually do not have as much surface area, and so will not yield as much water as a building roof. A system that runs off a building's down-spout will usually yield more water, but will also require that you get permission from the building owner and, if you are going to fill a number of barrels, will require that you have a strategy for managing the overflow.
How much water will you harvest?
You will be surprised how much water you will be able to collect from event the smallest roof. The rule of thumb is 623 gallons of water per inch of rain per thousand square feet of catchment area. You will need to calculate the footprint of the building. In other words if your building is 25 feet by 50 feet the catchment area is 1,250 square feet (25 sq. ft. x 50 sq. ft.) Not all the rain that falls can actually be collected. Efficiency is usually presumed to be 75% depending on system design and capacity. Here is the basic formula for calculating the potential amount that can be collected:
[Catchment area of building] x [inches of rain] x [.75] x [623 gallons] / 1000
To lookup average precipitation in your area visit the National Oceanic and Atmospheric Administration at http://www.noaa.gov/. Keep in mind that during a drought there will be less rainfall.
How many barrels do you want to fill?
There is no limit to the number of barrels you can put in your system. Simple tubing can connect them all.
Will the barrels be used for dipping into with a bucket of some sort, or will the barrel have a spigot and/or hose attachment?
It is great to be able to hook to your water barrels for watering. Keep in mind, though, that you will be relying on gravity or waters tendency to always seek the lowest point to produce water pressure, so the nozzle of the hose will always have to be lower than where the hose connects to the barrel. It is good to have the barrels higher up. You can do this in a few simple ways:
- Simply situate the barrels at the highest point in your garden, if your garden has any slope to it.
- Build a berm and a retaining wall to set the barrels on.
- Build a frame to stack the barrels on.
Remember that even a water barrel that is elevated on some sort of platform wont give you pressure comparable to what you get from the hydrant.
What will you do with the overflow?
Chances are that no matter how many barrels you put in your system, there will sometimes be rain that is heavy enough to overflow the last barrel in the bunch. There are various ways of handling the overflow and integrating it into your garden plan:
- Put the barrel that will overflow in a bed of sand and gravel. These ground surfaces will absorb excess water.
- Build a mini streambed, lining it with rocks, to channel overflow to a low spot in the garden. The best way to figure out where to build a small streambed is to let the barrel overflow and watch where the water naturally flows. You will need to make sure that the water will be absorbed and not create a standing water situation.
- Design your system so that the overflow from the last barrel automatically goes back into a downspout and into the sewer pipe where the buildings roof water drains.
What are the Department of Health recommendations affecting rainwater collection, in regards to the West Nile Virus?
For instance, the following are some basic recommendations from the Department of Health's West Nile Virus Fact Sheet:
Dispose of tin cans, plastic containers, ceramic pots, or similar water-holding containers.
(Gardens may have water-holding containers but they must be covered with tightly fitting lids, with no standing water exposed.)
Make sure roof gutters drain properly. Clean clogged gutters in the spring and fall.
Eliminate any standing water that collects on your property.
When building your rainwater collection system you will need to take the following precautions:
- Use rain barrels with tight fitting lids and keep them closed at all times.
- Use tight fitting, fine mesh screens to cover small ventilation holes on tops of barrels.
- Cover the overflow pipe with a piece of fine mesh. Do not allow water to pool up in overflow area.
- Use a spigot that seals shut when not in use.
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