What you actually lose
Most rainwater harvesting systems lose more water than they keep.
A 10,000 litre tank fills in one storm, then overflows for three more hours. The rest of the season repeats that pattern. Tens of thousands of litres run out the overflow pipe and into the storm drain.
It is not just a tank size problem. Bigger storage helps, but it does not solve the design flaw. The tank is treated as the end of the chain. Water in the top, tap at the bottom, surplus gone.
Treat the overflow as the start of a second chain instead.
Run the numbers
100 m² of roof. 900 mm of annual rain. That is 72,000 litres a year off the gutters.
Tank holds 10,000 litres. Household and garden use 40,000. The remaining 22,000 litres exits as overflow.
Sent into the storm drain, it is gone. Sent into the landscape, it recharges groundwater, soaks tree roots, and feeds rain gardens. Cost: a length of pipe and a half day of earthworks.
Cascade your tanks
The cheapest way to capture more is to add storage in series, not swap to one giant tank.
The chain. Tank A sits high, next to the house at gutter level. Its overflow pipes into Tank B downhill. Tank B's overflow runs further down to a landscape feature. Tank A fills first, holding the highest-pressure supply for gravity feed. Only when both tanks are full does water exit the system.
Use 90 mm pipe or larger between tanks. Narrow pipe backs up and overtops Tank A in a real storm. Cap every open end with mosquito mesh.
Want two tanks to equalise instead of cascade? Connect them at the base with a large diameter pipe. They behave as one tank.
Land it in the landscape
When overflow leaves the last tank, it needs a job.
Slow, spread, sink. Same principle as any earthwork, at the scale of one pipe.
On slope. Pipe to the uphill end of a contour swale. The level trench spreads the water along its length and soaks it into the berm's root zone.
On flat ground. Build a rain garden. A shallow planted depression that takes the inflow, filters it through mulch, and infiltrates. Size it to hold a typical event: full tank plus one hour of heavy rain.
Tight site. Dig a soakaway pit, one to two cubic metres, fill with coarse gravel, pipe the overflow in. Cover with geotextile and a thin layer of soil. It disappears.
Tree basins work too. Build an earth ring around a fruit tree, pipe the overflow into the basin, let its overflow feed the next tree downhill. Three trees in a row become a cascading infiltration line.
Stop the gully before it starts
A 90 mm overflow pipe on a full tank moves 50 to 100 litres per minute in a hard storm.
Hit bare soil with that and you get a hole, an undermined tank pad, mud up the wall, and sediment in the beds. Every event makes it worse.
Splash pad. Flat rock or concrete slab directly under the outlet. The falling water breaks into a sheet and loses its energy on the stone.
Rock channel. From the splash pad, line a short channel to the destination. 100 to 200 mm rocks set into the soil, smaller gravel in the gaps. Extend the lining at least one metre past where the flow lands in the swale, rain garden, or pit.
Heavy flow or steep ground. Use a gabion basket. A wire cage of rocks at the outlet disperses flow through its full volume and is close to indestructible. Pair it with vetiver grass or native sedges immediately downstream. The root mass binds the soil, the stems slow the sheet flow.
One hour at install time saves years of repair.
See also
- Rainwater Harvesting Basics: the system this sits inside
- Tank Sizing: right-size storage to cut overflow at the source
- Swales: the best receiver for sloped sites
- Rain Gardens: planted basins for concentrated inflow
- Earthworks: the slow-spread-sink toolkit