Rainwater
What a household rainwater system does
A household system catches rain, sets aside the first dirty flow, and stores what remains. It does not calculate a tank size, and it does not make a dry year wet.
On this page
Rainwater harvesting, at household scale, is the practice of catching rain, moving it, setting aside the first dirty flow, and storing what remains for a chosen use. The system is a chain. A break in any link empties the promise of the others.
Catchment
The chain starts with a surface that rain falls on. Usually that surface is a roof. The material matters. Some roofs shed grit, leaves, and bird waste into the first flow. Gutters and downspouts move that water toward storage. They are part of the system, not a trim detail. A blocked gutter is a failed catchment.
How much a roof can catch depends on its area and on the rain that actually falls. Those are local facts. A note cannot know them for a reader. That is why this publication has no sizing tool. A tool with no one to keep the rainfall record honest would only pretend to answer. The inputs are catchment area, local rainfall, first-flush volume, storage volume, and the jobs the water is meant to do. They belong to the site.
What each part is for
| Part | What it does | What it does not do |
|---|---|---|
| Catchment | Collects the rain, usually on a roof. | Does not clean what lands on it. |
| Conveyance | Gutters and pipes move water toward storage. | Does not store it. |
| First-flush | Diverts the dirty opening flow. | Does not treat the water that follows. |
| Storage | Holds what remains, covered. | Does not create rain. |
| Outlet | Lets the household draw water. | Does not prove the water is fit to drink. |
First-flush and storage
The first water off a dry roof is the dirtiest. A first-flush device diverts that opening flow away from the tank. It does not purify the rest. It only keeps the worst of the roof wash out of storage. After the diversion, water enters a tank, a cistern, or a barrel.
Storage should be covered, screened at the inlet, and fitted with an outlet that does not suck the sludge off the bottom. Storage is a volume. The useful volume depends on how uneven the rain is and on what the household will use the water for. Toilet flushing, garden watering, and drinking are different jobs. Drinking needs a safer chain than a garden does. A tank does not decide that. The household does.
From roof to use
- Rain falls The catchment receives it.
- First-flush The dirty opening flow is diverted away from storage.
- Storage The rest is held, covered, off the sludge at the bottom.
- Use The household draws water for the jobs the system was built for.
- Drinking, if that is the job A test, and treatment the household can repeat, still sit outside this chain.
What the system does not do
A household rainwater system does not make a dry climate wet. If the rain does not fall, the tank stays empty. It does not replace a water test when the water is for drinking. It does not, by itself, remove chemicals that a roof or the air deposited. It does not tell you the tank size.
A general note can name the inputs. It should not invent the numbers. Area, rainfall, first-flush volume, storage volume, and intended use stay with the household that can measure them, or with a local record someone maintains.
A roof tank is not a well. Where a household is comparing the two, the well side of that comparison starts with how wells are made.