Rainwater Harvesting Basics

Roofs

• Rooftop catchments are the most basic form of rainwater harvesting. The rain collects in gutters that drain into a down-pipe and collection vessels such as barrels. Roofs made of tile, slate, galvanized corrugated iron, aluminum and thatch are suitable for this purpose. Painted metal roofs and bamboo roofs are not suitable because they transfer impurities into the water. Some bamboo shoots are mildly toxic to humans and animals. Ingested rainwater that has flowed over bamboo can cause stomach irritation.

Ground

• Ground catchment techniques for rainwater harvesting involve the retention of water flow from small creeks and streams into small ponds or reservoirs. On sloping ground, shallow drainage trenches can be dug to drain into ponds at different elevations along the slope. Water loss can occur through seepage into the ground. Ground or vegetation clearance assists the flow but leaves a danger of soil erosion during times of heavy rainfall.

Storage

• Harvested rainwater needs storage in tanks that are protected from human, animal or chemical contamination. They need a cover to prevent algal growth or mosquito accumulation. Storage containers used for drinking water must have a tight lid. The storage tanks are made usually of ferrocement, a mixture of cement and sand that is reinforced with wire mesh. In regions of low rainfall over either the winter or summer months, a domestic water storage tank should be of sufficient capacity to cover household demands for two to three weeks.

Conveyance

• Conveyance systems transfer water from the catchment to the storage tank. The onset of rainfall pushes dirt and debris through these conduits even when care is taken to maintain them. This means that an initial volume of dirty water has to bypass the storage system into the ground or drains. As rainfall continues, the rainwater becomes cleaner and can be stored. Allowance must be made for excess water overflow from the tank so that any blockages do not contaminate the water supply. All pipes should be PVC. Rainfall is mildly acidic due to dissolved carbon and sulfur oxides and may corrode metal piping.