1. What is Rain Water Harvesting

The concept of rain water harvesting lies in tapping the rain water where it falls. A major portion of rainwater that falls on the earth's surface, runs-off from streams to rivers and finally to the sea. An average of 8% of the total rainfall recharges the ground water aquifers. Therefore, most of the rainfall goes waste in the form of surface run-off. The technique of rainwater harvesting involves catching the rain from localized catchment surfaces such as roof of a house, plain and sloping ground surface etc. The rainwater that falls on these catchment is diverted into dugout ponds, vessels or underground tanks to store for long periods. Construction of small barriers across small streams to check and store the running water is also an example of small catchment water harvesting.

Water harvesting means to understand the value of rain and to make optimum use of rain water at the place where it falls. In scientific terms, water harvesting (broadly) refers to collection and storage of rain water and also other activities such as harvesting surface water, extracting ground water prevention of losses through evaporation and seepage.

In general, water harvesting is the activity of direct collection of rain water. The rain water collected can be stored for direct use or can be recharged into the ground water.

2. Why Rain Water Harvesting?

Rain is the first form of water that we know in the hydrological cycle, hence it is a primary source of water for us. Rivers, lakes and ground water are all secondary sources of water In present times, we depend mainly on such secondary sources of water. In the process, it is forgotten, that rain is the ultimate source that feeds all these secondary sources and remain ignorant of its value. Water crisis situation occurs only because, effective collection and storage of rain water has been ignored. The potential of rain to meet water demand is tremendous. Unless people are involved in conserving rain water from individual households to big industries/institutions, it would be very difficult to meet the looming water crisis.

We get a lot of rain, yet we do not have water. Why? Because we have not realised the value of each drop of rain. Ironically, even Cherrapunji which receives about 11,000 mm of rainfall annually suffers from acute shortage of drinking water. This is because the rain water is not conserved but allowed to drain away. Thus it does not matter how much rain we get, if we don't capture or harvest it.

Chennai situation:

Chennai City receives rainfall during North-East Monsoon (Oct - Dec) and South-West Monsoon (June - September). A major portion of the rainfall is during North-East Monsoon. Some times the city also receives rainfall during January and Februuary, but that is quite rare.

The annual rainfall in Chennai is in the range of 1200 - 1300 mm. This is higher compared to the India's average rainfall of 800 mm. However, this rainfall occurs in short spells of a few days - on an average we receive rainfall for 300 hours throughout the year. The characteristics of our rainfall demands not only to conserve large quantity of rainwater during these few days but also to store wherever it rains in Metropolitan cities like Chennai, preferably for direct use and alternatively as ground water. Failure to do so results in flooding of low lying areas and wastage by means of run-off into the sea during rainy season and water scarcity during summer months. Also, due to the fast rate of urbanisation, the city has become a concrete jungle and it is very difficult to find open surfaces which would enhance the recharge of ground water. Even the open space left is paved with concrete or bitumen which does not allow the natural recharge of ground water. This highlights the need to implement measures to ensure that the rain falling over a region is tapped as fully as possible through appropriate water harvesting techniques for recharging the ground water aquifers as well as for direct storage and use of rain water.

How much water can be harvested?

The total amount of water that is received in the form of rainfall over an area is called the rain water catchment/endowment of that area. Out of this, the amount that can be effectively harvested is called the water harvesting potential. The collection efficiency accounts for the fact that all the rain water falling over an area cannot be effectively harvested.

Illustration 1

Consider a building with a flat terrace area of 100 sq.m. The average annual rainfall in Chennai is approximately 1300mm. In simple terms, this means that if all the rain that falls on the terrace is retained, then in one year there will be rain water on the terrace floor to a height of 1300mm.

Assuming that 60% of the total rainfall is effectively harvested. Volume of water harvested = 1,30,000 x 0.6 = 78,000 litres

That means ( 78,000 / 365 ~ ) 213 litres of water per day will be available for the household. That much quantity of water is enough to meet the drinking and cooking needs of a household having around 8 to 10 members.

Illustration 2

Take example of a multi-storied building with a terrace area of 500 sq.m.

If there are 16 flats in the building the per day water availability per flat will be 67 litres/day.

Illustration 3

Quantity of water collectable in 3 months is :   8,80,000 x 75 x 1.2 x 2/3 x 1000

                                 = 52,800 million litres

If  75% of this water is harvested by the Households, Quantum of water harvested from Rooftop

                                            = 52,800 x 0.75 = 39,600 million litres

                                                   
                     or 110 MLD for one year