Bangalore

It is estimated that 40 per cent of the population of Bangalore is dependent on groundwater, whereas the remaining part is pumped from the Cauvery river through a distance of 95 kilometres and a head of 1000 metres.

Bangalore receives 970 mm rainfall annually and the number of rainy days is 59.8. Highest amount of rainfall is received during April to November, while the rest of the months receive scanty rainfall. Peak runoff is 50 millimeters per hour. Due to the availability of rainwater throughout the year, water is basically stored in these rainwater harvesting systems and used for non-potable purposes. Water from the rooftops is led into storage structures. First flushing is normally done by providing an extra length of pipe to collect the polluted 2.5 mm of rainfall. Filters are made of sponge and a mixture of sand, gravel and charcoal. After first flushing and filtration water is led into under ground sumps (which are very common in Bangalore) or to a new storage tank.

The overflow from this tank is taken to an open well to recharge the aquifer.The geological formations are predominantly granite and granitic gneiss, with joints and fractures in abundance due to intense chemical weathering of rocks. The depth of weathering varies from 0.2 m to 20 m. This geological set-up offers an immense scope for recharging of ground aquifers.

The undulating terrain with gentle slopes draining into lakes offer an ideal situation for water harvesting. In the urban area of Bangalore waterbodies cover about 5 per cent of land.

Potential of rainwater harvesting

Name of City	Proposed area for 2011(category and area in Sq km)	Annual water harvesting potential in billion litres
		100 per cent harvesting	50 per cent harvesting
Bangalore	Development area 597.0	579.10	289.55
	Green Belt 682.0	661.54	330.77
	Total area 1279	1240.64	620.32

Notes: Average annual Rainfall = In mm 970; Annual demand-supply gap 49.28 billion litres
Case studies:

The project:
The industrial unit of a 20 hectare

Breakup of the area:
Rooftop area: 29,961 Sq. m
Paved area : 43,095.66 Sq.m
Unpaved area : 129,286.98 Sq.m
The total rainwater harvesting potential of the site is 185 million litres.

A pilot project was set up in May 2000 covering about 1, 280 sq.m of roof area for the administrative block and the canteen building. With storage capacity of 42,00 litres, the unit collects about 1.05 million litres per year. The system is expected to pay back for itself in five years.