Rainwater Harvesting

State groundwater legislation. Section 9 of the Maharashtra Act allows the Authority to require the owner or occupier of premises to provide rainwater harvesting structures for artificial recharge, and sections 41 to 45 provide the aquifer delineation and the plan within which recharge is designed.

The Central Ground Water Authority imposes harvesting and recharge as a condition of a no objection certificate for industrial and infrastructure abstraction in a notified area.

And the courts. In Intellectuals Forum, Tirupathi v. State of Andhra Pradesh, AIR 2006 SC 1350, the Supreme Court declined the prayer for the complete restoration and revival of two alienated tanks at Tirupathi and, relying on its expert committee, gave separate directions for each. At both, there was to be no further construction and no borewell or tubewell for any purpose, and piezometers were to be set up in consultation with the Central Ground Water Board to observe the effect of harvesting on the ground water regime. At the Avilala tank, each house already constructed by the Housing Board and the Development Authority must provide a structure for roof top rain water harvesting, all storm water in the built colonies was to be recharged to ground water through structures designed in consultation with the Board, and an area of forty acres reserved for the Government was not to be developed in a way leading to concretisation of the ground surface.

At the Peruru tank, each house already constructed by the Tirumala Tirupathi Devasthanam must provide roof top rain water harvesting, abstraction of ground water was completely banned, and a percolation tank with recharge shafts was to be developed on at least 50 acres. Facts and holding are worked in full in the chapter on the public trust doctrine extended; this is the operative part for the present purpose.

Storage or recharge, and how to choose

Storage puts the water in a tank or sump and uses it directly. It suits a place with a long dry season and a household able to maintain a filter and a first flush device.

Recharge puts the water into the ground through a pit, a trench, a shaft or an existing dug well, and takes it out later through the aquifer. It suits a place with a permeable formation and a depleted water table, and it is what a groundwater statute wants, because it repairs the resource rather than serving one building.

The first flush is the discipline in both. The first few millimetres of rain carry the dust, droppings and leaves off the roof and must be diverted to waste. A recharge pit fed without a first flush device and a filter chokes, and a choked pit is the commonest reason an urban harvesting mandate produces nothing.

Worked example

A housing society of forty flats in a Mumbai suburb, on a plot of two thousand square metres with twelve hundred square metres of roof, is required by the development control regulations to provide harvesting.

Step one, the yield. Twelve hundred square metres, about two metres of rain, coefficient 0.8, gives roughly 1,920 cubic metres a year.

Step two, storage or recharge? Storage of the whole is impossible; a tank of that size is a reservoir. So the design is a modest sump for direct use and recharge of the balance.

Step three, where does it recharge? Into a percolation pit or a recharge shaft in the plot, or into the society's own disused dug well, which is the cheapest recharge structure there is.

Step four, what is the legal consequence of not doing it? The occupation certificate is withheld, or a penalty is levied under the municipal law, and in a notified area the Groundwater Authority may direct it under section 9.

Step five, what does it contribute? It reduces the society's draw on the municipal supply, it reduces the flow to the storm drain, and it raises the local water table. In a coastal suburb the third of those also holds the salt water interface down, which links this chapter to the last one.

Distinctions

What it does NOT mean

It does not mean harvesting replaces a water supply. It supplements it, and in most Indian cities it can supply a substantial part of a household's need but not all of it in a dry year.

It does not mean any pit is a recharge structure. A pit without a filter and a first flush device silts up in one season.

And it does not work everywhere. In a hard rock formation with a shallow impermeable layer, or where the aquifer is already contaminated, recharge may achieve little or may spread the contamination. The site decides, which is why the Intellectuals Forum direction required the structures to be designed in consultation with the Central Ground Water Board.

Quick revision

Harvestable volume equals roof area multiplied by rainfall multiplied by a runoff coefficient, about 0.8 for a concrete roof.

Instruments: municipal building bye-laws and development control regulations; section 9 of the Maharashtra Groundwater Act; conditions in a Central Ground Water Authority no objection certificate; and court orders.

Tamil Nadu, 2003: compulsory for every building, existing and new, and the standing Indian example.

Intellectuals Forum, Tirupathi, AIR 2006 SC 1350: complete restoration of the two tanks declined, and separate directions for each. Avilala: roof top harvesting for the houses built by the Housing Board and the Development Authority, recharge of all storm water, design in consultation with the Central Ground Water Board, forty acres not to be concretised. Peruru: roof top harvesting for the houses built by the Devasthanam, a complete ban on abstraction of ground water, and a percolation tank with recharge shafts. Both: no further construction, no borewell or tubewell, and piezometers to monitor the effect.

Storage against recharge, and the first flush in both.

Test yourself

1. Calculate the annual yield of a two hundred square metre roof where the rainfall is one thousand millimetres. Two hundred square metres multiplied by one metre multiplied by a coefficient of 0.8, which is 160 cubic metres, or 160,000 litres a year.

2. Which instrument imposes harvesting on most Indian buildings? Municipal building bye-laws and the development control regulations made under the State town planning law, which make a harvesting structure a condition of the building permission and the occupation certificate.

3. What did the Supreme Court direct about harvesting in Intellectuals Forum, Tirupathi? Having declined the complete restoration and revival of the two tanks, it gave separate directions. At the Avilala tank, each house already constructed by the Housing Board and the Development Authority must provide roof top rainwater harvesting, all storm water in the built colonies was to be recharged through structures designed in consultation with the Central Ground Water Board, and forty acres reserved for the Government were not to be concretised. At the Peruru tank, each house already constructed by the Tirumala Tirupathi Devasthanam must provide roof top rainwater harvesting, and abstraction of ground water was completely banned. At both, no borewell or tubewell was to be allowed and piezometers were to be installed to monitor the ground water regime.

4. Why does a recharge system need a first flush device? Because the first rain carries dust, leaves and droppings off the roof, and a pit fed with that silt chokes within a season and stops recharging.