Rainwater catchment is the collection and storage of rainwater from roofs and other surfaces for domestic, agricultural, and livestock use. A typical rainwater catchment system consists of a collection surface (roof), gutters and downspouts, a first-flush diverter, piping, and storage tanks. Rainwater is naturally soft, low in dissolved minerals, and suitable for Irrigation, livestock watering, and with proper treatment, potable household use. A 200 square meter roof in a region receiving 1 meter of annual rainfall can collect approximately 180,000 liters of water per year.
Rainwater harvesting has been practiced for thousands of years. Ancient civilizations in India, the Middle East, and Rome built sophisticated cistern systems. In the Americas, the Maya constructed large underground cisterns called chultuns. The practice declined in the 20th century with centralized municipal water systems but has experienced a revival since the 1970s due to water scarcity concerns and the back-to-the-land movement.
The collection surface is typically a metal, tile, or asphalt shingle roof. Metal roofs are preferred for rainwater collection as they shed contaminants more effectively than asphalt. Gutters are sized to handle the maximum expected rainfall intensity, typically 10 to 15 centimeters (4 to 6 inches) per hour. A first-flush diverter diverts the first 20 to 40 liters of rainfall per 100 square meters of roof, which carries the majority of dust, bird droppings, and debris.
Storage tanks are made from polyethylene, fiberglass, concrete, or galvanized steel. Above-ground tanks are common for non-potable use, while underground cisterns provide cooler storage and protect water from freezing. Tank sizes range from 1,000 to 50,000 liters for homestead applications. A filter screen at the tank inlet prevents mosquitoes and debris from entering.
Rainwater is generally clean but can pick up contaminants from the roof surface and atmosphere. Simple filtration through a 50-micron sediment filter is sufficient for irrigation use. For potable use, a multi-stage system including sediment filtration, activated carbon, and UV sterilization is recommended. Stored rainwater should be tested annually for coliform bacteria and chemical contaminants.