Catch water if you can

Rainwater harvesting can’t be just paid lip service. We ignore these practices at our own peril in cities...

Currently the water supply situation in almost every Indian city, including Delhi and Mumbai, is alarming. People waiting in serpentine queues for a trickle from the public tap is a common sight.
Rainwater harvesting has, therefore, emerged as a viable solution for the long-term. Though rainwater has been harvested traditionally, we stopped employing these native techniques as we urbanized. Now, as taps dry up, we need to turn to these methods once again.
Rain is a primary source in the water cycle: rivers, lakes, and groundwater are all secondary sources. We, of course, are all dependent on secondary sources for our water needs. And as these run dangerously dry, rainwater harvesting is by far the best solution for our water needs.
Water harvesting involves collecting rainwater and storing it—either for direct use or for groundwater recharging.
Several NGOs are trying to popularise rainwater harvesting. Even the High Court has ordered that flyovers be provided with rainwater harvesting structures. But this work needs to be taken up seriously, and not just lip service paid.
How much can you harvest?
The total volume of rainfall over an area is called the rainwater endowment of that area. Of this, the amount that you harvest is called the water harvesting potential of that area. An example best describes the enormous amount of water that you can harvest from your own home. You can apply this method to calculate the rainwater harvesting potential of any plot of land or rooftop.
Suppose your building has a terrace area of 100 sq metres. Assume that the average annual rainfall in your area is approximately 600 mm (24 inches). Of course, for ease of arithmetic, suppose also the terrace floor is impermeable, and that all the rain that falls on it is retained without evaporation.
Area of plot = 100 sq. m. (120 square yards)
Height of the rainfall = 0.6 m (600 mm or 24 inches)
Volume of rainfall over the plot = Area of plot x height of rainfall = 100 X 0.6 = 60 metre cube. One cubic meter is 1,000 litres of water.
Assuming that only 60 per cent of the total rainfall is effectively harvested, the volume of water harvested in this case could be as much as 36,000 litres.
If you assumed 50 litres per day to be the drinking water need of a family of five, that will amount to an annual need of 18,000 litres. The harvested volume here is about twice this annual drinking water need. Of course, this doesn’t account for other water needs which account for eight times as much as drinking water.
Rooftops, roads, and parking areas together account for more than 50 per cent of an urban area. If water harvesting is implemented in all these areas, then water shortages will just become a memory.
Maintenance
You need to undertake maintenance work once or twice a year. This doesn’t cost much.
At this point of time resident welfare associations must realise that rainwater harvesting alone can provide the answer to their water woes. The sooner societies put rainwater harvesting systems in place, the better.
The author is President, Facility Management Service, at AEZ Group.