Rainwater Harvesting for Garden Irrigation

Collecting roof run-off in a tank for garden irrigation reduces the demand on mains water. The system looks simple, but the filter, tank size and overflow route need proper planning.

In a dry summer, watering the garden can make up a large share of a private house's water use. Meanwhile, the rain running off the roof in autumn and spring often goes straight to the drain or the street. Rainwater harvesting links these two needs: water collected in the wet season goes to plants in the dry months. The system is not complicated, but if a few simple rules are ignored, the tank becomes dirty, smelly or causes flooding.

Catchment surface and water quality

The roof is the main source. Smooth coverings such as metal, ceramic tiles and concrete give clean water and shed most of the rainfall. Bitumen coverings and old or mossy surfaces can pass colour and odour into the water. Green roofs retain much of the rain, so they yield less for collection.

Run-off from paved areas, paths and parking picks up oil residues and dirt, so feeding it directly into the main tank is not recommended. That water can be returned to the ground through separate drainage or a soakaway.

Rainwater collected for garden irrigation is not used for drinking or cooking. If use inside the house is planned, for example for toilets, this requires separate pipework, labelling and more rigorous filtration, and it must never mix with the mains supply.

System components

• Gutters and downpipes collect water from the roof and should be fitted with leaf guards.
• A first-flush diverter: in the first minutes of rain, dust and bird droppings wash off the roof. A simple device diverts this first water away and lets the cleaner water that follows into the tank.
• A filter holds back leaves, sand and coarse particles before the tank and should be easy to reach for cleaning.
• The tank, above or below ground. Water should enter near the bottom through a calming inlet so sediment is not stirred up.
• An overflow sends excess water to a safe place, such as a soakaway, drainage or sewer, when the tank is full.
• A pump or gravity feed delivers water to the irrigation system.

Above-ground and underground tanks

Above-ground tanks are simple, relatively inexpensive and need no excavation. But they take up space, affect appearance, and in the sun the water warms and algae grow. Choose an opaque, dark or shaded tank. Where there is frost in winter, above-ground tanks and pipes must be drained.

Underground tanks are out of sight, keep water cool and are protected from frost. They require excavation, a body able to resist soil pressure, and attention to the groundwater level. Where groundwater is high, an empty tank can be pushed upward by the ground, so it must be anchored. When siting an underground tank, avoid driveways or choose a load-bearing lid.

How to think about tank size

Tank size depends on three factors: roof area, how rainfall is distributed through the seasons locally, and the water demand of the area to be irrigated. A small tank fills quickly in the rainy season and most water goes to overflow. An oversized tank may never fill and wastes space.

In a climate where rain falls mainly in autumn and winter and irrigation is needed in summer, the tank may not cover the whole summer demand. Rainwater then does not fully replace mains water, but it reduces part of it. Combined with drought-tolerant plants and drip irrigation, the effect becomes much more noticeable.

A practical approach is modular: start with a medium-sized tank and lay pipes and overflow lines so that a second tank can be added later. After a season or two of observation, it becomes clear how quickly the tank fills and empties.

Maintenance rules

• Clean gutters and guards regularly during the leaf-fall season.
• Check the first-flush device and filter after every heavy rain.
• Remove sediment from the tank bottom every few years.
• The tank lid should be child-safe, lockable and fitted with mesh to keep mosquitoes out.
• Protect above-ground pipes and the pump from frost before winter.

Zaferoğlu İnşaat plans and carries out site plans, excavation and yard works for private houses according to design. Tank, drainage and irrigation positions are best fixed alongside the house design, so contact us early.