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 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.
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.
|Area of Terrace
|Height of rainfall
||1.30m (1300 mm)
|Volume of rainfall over the Terrace
||Area x Height of Rainfall 100 sq.m. x 1.30 m : 130 cu.m. (1,30,000 litres)
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.
Take example of a multi-storied building with a terrace area of 500 sq.m.
|For the annual rainfall of 1300 mm, assuming that 60% of the rain water is harvested, the total rainwater harvesting potential for this building will be:
||500 x 1.3 x 0.6 cu.m.= 390 cu.m.= 3,90,000 litres
If there are 16 flats in the building the per day water availability per flat will be 67 litres/day.
|Name of the building
|Quality of rain water harvesting potential assuming that 60% of the rainfall can be harvested
||3,500 x 1.3 x 0.6=2730 cu.m.=27,30,000 litres
|Water availability per student per day
||365 x No.of students
From the above illustration it is quite clear that the rain water harvesting has a vast potential to meet the water needs of individual house holds and institutions.
If we take the Chennai Metropolitan area for knowing the rain water harvesting potential
Area = 173 sq.km. = 173 x 106 m²
Rain fall = 1.30 m
Quantity of water which can be harvested (assuming 60% rain fall can be harvested) = 173 x 106 x 1.30 x 0.6 = 135 x 106 m² = 135000 ML = 370 MLD
In the following table the monthly average rainfall data of Chennai and the possible rainwater collection in percentage from the terrace area of 100 sq.m is detailed below: