Rainwater harvesting

Rainwater harvesting collects rain where it falls, usually from roofs and paved areas, and either stores it for use or puts it into the ground to recharge groundwater. In much of India, rain comes in a few monsoon months, while groundwater levels have fallen in many cities, so both storage and recharge help.

A roof-top system has a catchment, the roof; gutters and downpipes; a first-flush device that diverts the dirtiest early runoff; a filter of sand, gravel or mesh; and either a storage tank or a recharge structure such as a recharge pit, trench or well. Tanks need covers, overflows and screens against mosquitoes, and stored water used for drinking must be treated and tested.

The water available is the catchment area times the rainfall times a runoff coefficient, which allows for losses by splashing, evaporation and first flush; hard roofs have higher coefficients than gravel or green roofs. Storage is sized from the gap between supply and demand month by month; it is rarely economic to store a full year's harvest, so many systems store what is needed for a few weeks and recharge the rest.

Many Indian cities require rainwater harvesting for new buildings above a set plot size in their building bye-laws. IS 15797 gives guidelines for roof-top systems. Recharge must be kept away from sewage and contaminated areas so that groundwater is not polluted. Gutters, filters and first-flush devices need cleaning before each monsoon. Overflow from a full tank should lead to a recharge pit or the storm drain, never against the building wall.