At least 523 million litres of rainwater will reach the rail tracks in Bangalore.
The Metro being built across Bangalore will become one of the prime modes of transport in the city. Apart from helping mass and fast transport of people, a potential benefit that can accrue to the city from its construction is rainwater harvesting.
The rail track offers tremendous potential for collecting rainwater. A total length of about 45 km and a width of about 12 metres means that the endowment of rainwater on the relatively clean track with Bangalore's annua rainfall of 970 mm is about 523 million litres.
Considering a coefficient of runoff of 0.9, the harvestable rainwater is around 471 million litres annually. This can provide about 13,000 people with their annual requirement of water at 100 litres per day. A substantial sum.
Since the Metro will intend to develop a small landscaped area between piers of about 25 metres and with a width of 2.5 metres, part of the rainwater can be used for this landscape purpose. The additional rainwater can be used to recharge the aquifers without causing any urban flooding.
A rain barrel should be attached to every pier to store rainwater above the ground. This will enable water to be drawn by gravity for drip irrigation of the landscaped area between the piers.
A sump has to be set up to store the overflow from the rain barrel. There can be a small pump to refill the rain barrel when empty and draw the water when required.
The sump should have an arrangement for a cowl which can be used to fill water from a private water tanker in case the sump goes empty without rain and during long stretches of dry days.
A recharge well to take the final overflow from the sump tank and recharge the aquifers so that the water table is made up. The recharge well can also be used as a regular well in stretches where the water table is high. This will also help improve the quantity and quality of groundwater along the stretch of the Metro as a positive externality.
A landscape designed for low water use, say for growing plants such as bougainvillea, will demand roughly four litres per square metre daily which can be distributed through drip irrigation systems.