Rainwater Harvesting

Rainwater harvesting (RWH) is the collection, storage, and/or recharge of rainwater for groundwater replenishment or non-potable use. Indian Standard IS 15797:2008 provides design guidelines; the National Building Code 2016 + state-level building bylaws (Maharashtra, Karnataka, Delhi, Tamil Nadu) mandate RWH for plots above specified area (typically 200-500 m²). The principal driver: Indian groundwater levels have declined 15-30% in major cities over the past 20 years, and RWH is the most cost-effective response.

Two main RWH categories: (1) Surface storage — rainwater is collected from rooftops via downpipes, filtered, and stored in tanks (overhead, underground, or surface) for non-potable use (irrigation, flushing, washing). Tank capacity sized for 5-15 days of demand at design LPCD. (2) Groundwater recharge — rainwater is collected and infiltrated into the ground via recharge pits, recharge wells, or recharge trenches to replenish the aquifer. Recharge volume is typically 30-50 m³/year per 100 m² roof area in moderate-rainfall regions (700-1500 mm annual). The recharged water is later available via bore-wells.

Design components: (a) Catchment — typically the building rooftop, paved areas, or natural ground; (b) Conveyance — gutters, downpipes (typically 100 mm dia for residential, 150 mm for commercial); (c) First-flush diverter — discards the first 10-20 mm of rainfall (which contains most accumulated dust and contaminants); (d) Filter — sand and gravel filter to remove particulates; (e) Storage or recharge structure. For a typical 200 m² Indian residential roof at 1000 mm annual rainfall with 80% catchment efficiency: 160 m³/year potentially harvestable. The most-overlooked aspect of Indian RWH: maintenance — without annual cleaning of gutters, filters, and tanks, RWH systems fail within 3-5 years. The Tamil Nadu government's experience shows that mandated RWH installations are largely non-functional after 5-7 years due to absent maintenance.