Use rain to beat drought

An analysis by IMD proves that rainwater harvesting is the best way to overcome the continuous dry spells the country witnesses.
Mumbai in monsoon. (Source: Wikimedia Commons)
Mumbai in monsoon. (Source: Wikimedia Commons)

A recent analysis by the Indian Meteorological Department (IMD) determined the rooftop rainwater harvesting potential of the districts in Maharashtra in a year by calculating the average amount of rainwater in litres that can be caught over one square foot roof area. Head of IMD’s Climate Application Group, Dr Pulak Guhathakurta spoke to India Water Portal on the findings of the study.

What was this study about and why was it undertaken?

India is in the tropical monsoon zone and receives plenty of rainfall during the monsoon season. In spite of it, many parts of the country experience droughts and water scarcity every year. This year, 11 states in the country faced drought! 

Rainfall in India shows a lot of variability with short spells of high intensity in a number of places. But most of the water tends to flow away, leaving very little for the recharge of groundwater leading to drought-like conditions. For example, Maharashtra receives rainfall ranging from 6000 mm over the ghats to less than 500 mm in Madhya Maharashtra. The Konkan subdivision that includes coastal districts and western ghats receives the heaviest rains, more than 6000 mm and the plains receive 2500 mm of rain. Rainfall decreases rapidly towards eastern slopes and plateau areas where it is a minimum of about 500 mm. It again increases towards the east i.e. in the direction of Marathwada and Vidarbha and attains the second maximum of 1500 mm in the eastern parts of Vidarbha. However, all this water flows away rapidly with little chance for recharge (of groundwater).

Even with this amount of rainfall, water scarcity continues to be rampant in parts of Maharashtra. The annual droughts in the state have put immense pressure on the available water resources and the need to explore other decentralised and localised means of harnessing and conserving water is getting crucial day by day. This is why we decided to undertake this study to determine the rooftop rainwater harvesting potential in different locations in Maharashtra districts.

Why do you think rainwater harvesting is important?

Rainwater harvesting (RWH) is the process of collecting and storing rainwater for future productive use. If rainwater is harvested, the scarcity of water can be eliminated altogether. This is an ideal solution to overcome water problems where surface water sources are insufficient and groundwater supply is already stretched quantitatively and poor in terms of quality. Rooftop rainwater harvesting (RTRWH) is the technique through which rainwater is captured from the roof catchments and stored in reservoirs. This method is particularly important in drought-prone, hilly, urban and coastal areas.

The Maharashtra government is already promoting RTRWH under the Shivkalin Pani Sathwan Yojana, which proposes that all houses should have provision for rainwater harvesting without which the house construction plans will not be approved. Bombay municipal corporation and Pimpri-Chinchwad municipal corporation have already made RWH mandatory by enacting building bylaws.

We have been getting requests from a number of NGOs for information on how much rain can be harvested in Maharashtra during a good rainfall as there is no guarantee of a good rainfall every year. Under such conditions, it becomes important to make use of whatever water is available by harvesting it. We undertook this study to assess the rainfall-harvesting potential or determining the maximum amount of water that can be harvested in an area of one square foot on the roof of a house in different districts of Maharashtra by assessing the rainfall trends in the respective places.

How was the potential calculated?

The total amount of water that is received in the form of rainfall over an area is called the rainwater endowment of that area. Out of this, the amount that can be effectively harvested is called the water harvesting potential.

For calculating rainfall harvesting potential in Maharashtra, the normal rainfall data of 326 well-distributed rain gauge stations for the period 1951-2000 was used. The runoff, which is the amount of rainwater that flows off after it falls on a surface was calculated. The quantity of rainwater that flows off after falling on the surface depends on the area and the type of material on which it falls. For example, the runoff from a concrete roof will be very high as the concrete surface does not absorb much water as compared to maybe, earthen tiles that can absorb the water falling on its surface.

We calculated the runoff coefficient based on factors such as intensity and duration of rainfall, the surface on which the rain falls, the slope of the surface, the degree of absorption of water by the surface. The water harvesting potential of a site was estimated from the amount of rainfall in millimetres, the area of the catchment and the runoff coefficient. 

What did the study find?

The study found that if the state harvests rainwater during a good monsoon, it will be possible to save as high as 26 crore litres of water every year! Let us look at Pune as an example. A small house in Pune can harvest close to 44,717 litres of rainwater during the monsoons, a medium-sized house 89,435 litres while a big house can harvest 1.34 lakh litres of water.

The analysis found that some stations in Kolhapur, Nashik, Raigad, Pune, Sindhudurg and Ratnagiri have the highest rainfall harvesting potential in the state. Among the 20 stations that were studied in Pune district, Khandala, followed by Paud and Vadgaon Maval, were found to have the highest rainfall harvesting capacity during monsoon.

Same is true of cities like Mumbai. Andheri has the highest rainwater harvesting potential followed by Borivili during the monsoon. In places like Nashik, Igatpuri has the highest rainwater harvesting potential followed by Peint, the smallest tehsil of Nashik subdivision. Mahabaleshwar in Satara district has the highest potential of all in the state.

What do you think is the usefulness of this study?

At a time the groundwater resources are drying up due to excessive groundwater extraction, heavy contamination of surface and groundwater resources are making the available water unsuitable for use. Drying up of surface water sources such as rivers during the summer leads to severe water scarcity in certain parts of the state. With more and more unpredictable rainfall, we must be able to utilise rainwater to the maximum capacity.

I look at each building and structure as an opportunity to catch the rain when it falls on the ground which can play an important role in dealing with the increasing water scarcity that the state faces year after year. We thought of doing this study so that people can have information on the true rainwater harvesting potential of an area after which systematic efforts can be undertaken with support from the government as  well as non-governmental organisations to construct rainwater harvesting structures in the area. We have started calculating the potential for Maharashtra; we have done it for Madhya Pradesh and plan to do for all the other states as well. We plan to make small booklets that can be freely available for anyone to use. Public participation is very important in this process.

A detailed report on the findings of the study can be downloaded from below: