“Raindrops keep falling on my head..." was a popular song of the 70s sung by B.J Thomas in the film ‘Butch Cassidy and the Sundance kid’. The song conjured visions of gentleness, idyllic surroundings and enjoyment of nature. Nature gives abundant rain and if the beautiful experience of raindrops falling can be given another dimension by attempting to hold every raindrop, in other words rainwater harvesting (RWH), it could well be the panacea for the water woes of Chennai. Just as solar power is a free source of electricity, so also is rainfall, for water. However, unlike solar energy that requires significant capital investment for conversion, RWH is inexpensive.

RWH is simple and inexpensive

Collecting every drop falling on residential/commercial building rooftops, open fields, playgrounds is the theme of RWH. Properly executed RWH will enable ground water recharge using Recharge pit, Trench, Open well, Dug well, Shaft, Tubewell. Simple devices like Roof catchment panels, Gutters, Downpipes, Rain water/Storm water drains, Filter chamber, facilitate rainwater collection into tanks, sumps for direct use through filters.

For a new building, RWH does not entail any additional expenditure or space; the roof drain pipes can be led into simple RWH pits below the car parking area. A typical 200 sq. m rooftop RWH system capable of generating 200,000 litres (200 cu.m) per year would cost ₹15,000 to ₹30,000; (depending on the urban/rural location); which is affordable for a house owner.

In Chennai, for a housing complex, with 140 flats the expenditure was only ₹1.5 lakhs which is eminently affordable by the residents, with payback periods as low as two months up to a year. The environmental benefits that accrue from RWH, namely: (a) increase in ground water level, (b) decreased run-off and choking of storm water drains (c) reduced flooding of roads and consequently lower maintenance costs (d) water quality improvement ( e) and lower soil erosion are substantial translating into high annual social rates of return of the order of +100 per cent to 1000 per cent.

RWH will not stress government finances unlike costly desalination plants that produce water at premium cost. Private residential dwellings and housing complexes can raise their own funds; government buildings and rural areas can be funded from the Central and State government schemes to meet the costs of civil work for check dams, run-off, storm water drains, wells; equipment such as tanks, filters and labour costs.

Alleviation of Chennai’s water problems through RWH

Data on ground water levels in Tamil Nadu indicate that while the water table in many areas is 2 to 5 metres below ground level, in majority of the areas it is 5 to 10 meters below ground level. Water table drops significantly during the summer, largely due to extraction of groundwater using wells, and pumping out for drinking water and farmland irrigation resulting in the rate of groundwater extraction becoming higher than the rate of replenishment from precipitation which is the theme of World Water Day 2022.

The quality of rainwater is excellent with minimum TDS (total dissolved solids). Unfortunately, this is being ignored and allowed to flow into gutters, roads and sea. On the other hand, if suitable RWH percolation pits to catch rainwater are provided, ground water gets recharged, water table increases and saline water intrusion reduces.

Greater Chennai’s area of 1189 sq. km and the average annual rainfall of 140 cm., translates to 16,64,600 million liters per year or 4,561 million litres per day (mld). Against this, for the city’s population of 11 million, the water requirement at per capita of 130 litres per day is 1430 mld OR 521,950 million litres per year. Thus, by exploiting less than one-third of the rainfall, Chennai’s water woes can be alleviated.

Meeting SDG

Out of the 17 UN mandated SDGs, to which India is a signatory and has committed to achieve by 2030, SDG 6, namely ‘provision of clean water and sanitation’ is a major challenge. Effective RWH can make India ‘water comfortable’ as exemplified by ‘water-scarce’ Israel.

This is also corroborated by research on the progress of SDG 6 in various Indian states where the top performer with an SDG score of 0.738 is Gujarat endowed with poor water resources but compensated by efficient water management. What is important is to view, all water systems as integrally linked with the hydrological cycle and contributing to the ecological system; in which context RWH assumes significance.

How to make RWH a Success story?

Tamil Nadu is a technology savvy State and its engineers are among the most competent in India involved in high end space missions, nuclear power plants and artificial intelligence. Paradoxically, RWH is no rocket science and does not entail any sophisticated technology or equipment.

The IIT Madras Alumni led Societal Impact Action Group (IITMAA SIAG) has conceived a simple process for RWH in Chennai. It involves (a) catching rain on building roof tops with filter arrangements for direct use (b) on-ground collection and channelizing into multiple distributed percolation RWH Pits (shown in sketches), filled with graded gravel (Blue Metal) for ground water recharge.

The design of pits is such that radial and longitudinal ingress of water is possible as opposed to only longitudinal flow in conventional pits with concrete ring and hollow space. The graded gravel RWH Pits, permit greater percolation of water into the ground are cost effective and enhance safety.

Both building rooftop and on-ground RWH have been implemented at residential complexes in Adyar, Saidapet, as also in a residential complex (Kamakoti Nagar) and community park in Pallikarnai; and industrial units/ office complexes in Chennai.

An 8-step plan

The major reason for the non-success of RWH that started in 2001 appears to be a lack of citizens’ will. Since the process is simple and inexpensive, what is required is a ‘Will Do’ mindset to make it all pervasive and in this regard the awareness campaign on RWH flagged off by the Chief Minister is heartening.

The following 8-step plan is suggested for bringing about a transformative change in a time bound manner.

(i) Create a sense of urgency by formulating it as a project with aggressive timelines. A Government order (GO) by the Special Program Implementation Department would be a logical starting point.

(ii) Identify 4 to 5 champions who are passionate about RWH who can form a guiding coalition for implementation

(iii) Articulate the vision of self-reliance through RWH both in English and Tamil

(iv) Communicate the vision through print, TV and social media

(v) Empower champions and project team-Institute a policy of incentives for compliance and penalty for non-compliance;

(vi) Create short-term wins among residential complexes, government building, industrial units and publicise them;

(vii) Set up a fool-proof data management system for constant monitoring and control of RWH progress as well as operational outcomes;

(viii) Document the changes taking place and put it up on the portal as repository for research and systemic design corrections wherever required.

The city of Chennai needs to get its act together on a war footing with regard to water in its quest to become a 1 trillion- dollar economy. Can Chennai in particular and Tamil Nadu emerge as the leader of RWH in the India and become role models for other metros and states to follow?

Dr. Suresh Mony is Advisor, Rajagiri Vidyapeeth and Retd. Founder Director of NMIMS, Bangalore; G. Ramkumar is Retd. Executive Director, Indian Oil Corporation and Rainwater Harvesting Expert. Both are members of the ‘IIT Madras Alumni Association – Societal Impact Action Group’.