Rainwater harvesting is simply a process of collecting, storing, and later using precipitation. There are many different ways to succeed in rain capture and different uses for your captured water.
Rain capture can be as simple as hooking a drum barrel to your house gutters and using the water for your garden or as complex as burying a tank in your yard to use gray water in your whole house.
Connecting barrels to your house rain gutters is a very quick and simple process and the benefits can be astonishing depending on how many barrels you have. Here's an example of a "pretty" version of a single barrel.
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The gutter connects at the top of the barrel through a cut slot, and the water is usually filtered through a screen (for leaves and twigs and things). Typically, you also want to have an angled runoff pipe. This will allow the excess water from alot of rain to slowly leak from the barrel so that the water does not back up into your gutter. You also will need to install a spigot to connect a hose to fill a watering can.
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If you typically receive a lot of rain and one barrel will not be enough for your area, you can make a series of connected barrels. Instead of having a excess runoff pipe in the first barrel, your runoff pipe connects directly to the second barrel - and if needed in runoff pipe from the second barrels can connect directly to the third and so on as long as your last barrel has a regular runoff pipe still. Then each of your barrels gets its own spigot.
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Here's how it would look to connect multiple barrels.
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The benefits can be overwhelming from rain capture. Unless you have a well on your property and only pay for the electric for the pump - you probably pay for incoming city water, and then you pay for that water a second time in your sewer water fee. What a financial waste!
Here's the kicker - most of that water waste is from toilets, laundry, filling swimming pools, washing your car, and watering your garden and flowers. Water that you can harvest from your impervious surfaces can be used for these purposes!
This is what a tank might look like when using the water for these purposes. This system would cost you around $1500
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Here's an example of an entire system in place. The pipes are a little harder to see, but if you look hard, you can follow them throughout the entire system.