Every monsoon, thousands of litres of clean rainwater fall on your roof and flow straight into the drain. Rainwater harvesting captures that water to store or recharge groundwater. Many Indian cities now require it for new buildings, and it can noticeably reduce tanker and borewell dependence.
The simple formula
Water collected (litres) = rainfall (mm) × roof area (m²) × runoff coefficient
One millimetre of rain falling on one square metre is exactly one litre. The runoff coefficient accounts for water lost to splashing, evaporation and absorption. Hard RCC or tiled roofs typically shed around 80 to 90% of rainfall.
Worked example
A 1,000 sq ft (about 93 m²) RCC roof in a city with 700 mm of annual rain can collect roughly 93 × 700 × 0.85, or about 55,000 litres a year. That is the daily water use of a family of five for nearly three months.
Two ways to use harvested water
- Storage: filter the water and store it in a tank or sump for flushing, gardening, washing or, with proper treatment, drinking.
- Recharge: direct it into a recharge pit or an existing borewell through a filter, so it replenishes groundwater. This is often the most practical option for houses with limited storage space.
Parts of a simple system
- Clean roof and gutters or rainwater pipes.
- A first-flush diverter that discards the first dirty rain of each shower.
- A filter, often layers of gravel, sand and charcoal, or a commercial mesh filter.
- A storage tank or a recharge pit or well.
- An overflow to the drain for heavy rain.
Frequently asked questions
Is rainwater safe to drink?
Not without treatment. Filtered and disinfected rainwater can be safe, but test it first. For most homes, using it for non-drinking purposes or recharge is simplest.
Does recharge really raise the water table?
One house makes a small difference, but when many homes in an area recharge, borewell levels can recover noticeably.