Rain Water Harvesting - An Overview

We all know that rain water is one of the purest forms of water available to us, as it is essentially distilled water.

Hence it is extremely precious, as it does not require any long and complicated purification process, and is, during the times of rain fall, readily available, free, and accessible indiscriminately to all. 

With the increases in population the world over, other conventional water sources are drying up, or are already dry or polluted beyond safe use.

So, we are now trying to collect or harvest the pure rain water that falls for a part of the year, and ensure that it is not wasted.

When the rain water hits the ground (in cities), it runs off down roads, into drains, mixes with dirt, garbage, etc, - becomes unpotable - and eventually recahes the seas. It will take another year for this water to complete the water cycle, and fall back on
the earth as potable rain water - in normal circumstances.

In un-urbanised areas, (that are not concreted over)the rain water will soak up into the soil and add to the existing water table and raise the levels of all the local wells and springs, and fill up the existing tanks.

But in cities, we have little space for tanks and large storage of water, and most natural wells are already dry, because of the phenomenon of boring, that has sucked out large amounts of water, and pushed the water table deeper. (Average depth of water table in suburban Mumbai presently: 200-300 ft)

So there are 2 basic aspects to rain water harvesting: 
1) preventing run-off into drains, and replenishing the ground water levels
2) actual collection and storage of water for a planned future use.

For replenishing the ground water, what is required is that we channelise the water that falls, and direct it to a particular spot on the site/plot, (usually the lowest spot, so that the natural slope assists the water flow) that is not paved over, and from there, the water soaks up into the ground. A small chamber or dug out is built at this spot, to aid in this process.

The benefits of this step are long term - as are most efforts and interventions with the environment.

Collection can be done in many ways - it's as simple as placing a bucket in the rain. For buildings, there are already roof gutters, and rain water down-take pipes from the terrace. These need to be connected to a storage tank, and water can then be collected and stored.

There are several details like understanding how much water to store, depending on the region that you are in, how to keep the water clean, how to use it, etc., and for this, there are several resource persons, web-sites, and Rain Centres (live-examples) that can help you with your situation.

Points to note:
Rain water harvesting is not a new phenomenon: the 2000 year old Kanheri caves in the National Park, have rock-cut water channels that guide the run-off of the rain water into storage cisterns. As do the Elephanta Caves, and most examples of rock-cut architecture. 

Several traditional settlements and buildings have incorporated rain water harvesting features into their design. For eg.: houses in Cambay in Gujarat, all have an underground storage tank, that stores the rain water run-off, and provides the households with year round supply of potable water, as well keeps the house cool, in this hot and arid region.

Hope this answers your question.

Tanya Mahajan

Centre for Environmental Research and Education (CERE)
Jaganath Shankar Seth Chowk Municipal School
3rd Floor - Room # 78
Nana Chowk
Mumbai - 400 007
INDIA
Tel: (+ 91 - 22) 2381 1581
Website:  www.cere-india.org