need to to harvest water? Harvesting water? That's a funny term, right? Wrong. That is the correct term today. With most of India, both urban and rural, going through one of the worst summers and with experts insisting that conditions of drought will only worsen in the years to come, it is time we all understand what 'harvesting rain' meant.
The word 'harvesting' will confuse people. But it has nothing to do will crops and irrigation. Simply put, it is collecting whatever rain falls on the ground. It could be on your rooftop, in your garden, in your driveway or pathway, in your backyard, stormwater drains, roads and pavements, wherever. The idea is to channelise this water into some storing place which could either be an external storage device like a tank or, as it happens these days, an underground pit.
With underground recharge, the logic is that if we are sucking out water from below the surface, it is imperative that we put back as much into the ground. In the past such a thing was not needed primarily because of two reasons. First, most people then depended upon rivers, wells and lakes for their water needs. Second, use of electric power to suck out water by the help of tubewell in a very recent phenomenon. Moreover, the volume of water sucked is far higher than in the past.
How much of water can one capture? This is dependent on how much rainfall a particular day gets and what your rooftop and plot sizes are.
Plot area (including rooftop) x rainfall (mm) = volume of water
This amount is generally multiplied to a fixed quotient called runoff co-efficient which takes into consideration the gradient of the place, amount of rainfall, etc. This will determine the amount of water that actually flows. So if the gradient is sharp more water will flow in the required direction.
So your actual water calculation will follow this formula:
Rainfall (mm) x Area of catchment x Runoff coefficient = water harvested
How does one know whether one needs to store water externally or think in terms of recharging the groundwater? This is dependent again on the amount of water the place gets. In semi-arid regions such as Delhi, Rajasthan, etc there are just 40 days of rainfall and underground recharge is favoured method. On the other hand, in places like Mizoram, Meghalaya and Kerala, which get rainfall throughout the year barring the dry spell, external storage is preferred.
What will you require? What you'll need to put a harvesting system in place (the most basic type) will include pipes (PVC or galvanised iron, mesh or a sand filter, a sump (which will have to constructed), and aquifer receiving the water. It could be a dugwell, tubewell, a recharge well or simply a recharge pit.
How much will it cost? Typically, installing a water harvesting system in a building would cost between Rs. 2,000 to 30,000 for buildings of about 300 sq. m. This estimate is provided by Delhi-based Centre for Science and Environment. But cost could differ, depending on the size of the plot.