How much rainwater can be collected in a day from a 100 sq.m. rooftop? Enough for a family of four for at least 3 days
Hyderabad and its surrounding areas have been receiving rain for the past few days. How much of it could have been used for domestic purposes? People working towards rainwater harvesting said that if 25mm rainfall is received, around 2,000 litres can be collected from a rooftop spanning 100 square metres (over 1,000 square feet).
On Friday (May 16, 2025) morning, the highest of 36.6 mm rainfall was received in Golconda, according to data from Telangana Development Planning Society. Stand alone apartments, gated communities, shopping malls and other structures with huge surface area can harvest several thousands litres of rainwater.
Amount of water needed in a day
Kalpana Ramesh, founder, The Rainwater Project said that a person needs, ideally, 130 litres of water a day. Around 2,000 litres of harvested rainwater would be sufficient for a family of four for at least three days.
How is rainfall that can be collected from a rooftop calculated?
Amount of rainfall that can be collected from a rooftop is calculated by multiplying surface area with rainfall
As type of roof surface (soft with vegetation or hard), slope, debris, rain intensity, precipitation and other factors impact the rainwater collection, runoff coefficient too is included in the calculation.
A 0.8 coefficient means that 80% of rainwater can be collected
Formula: Surface area (in square metres) x rainfall (in metres) x runoff coefficient
Example: If rainfall depth is 25mm over 100 square metres rooftop, then the rainfall collected is: 100 square metres x 0.025 metres x 0.8 = 2 cubic metres (2,000 litres)
Groundwater level in Hyderabad
The methods to recharge groundwater are under focus as the annual average water level in Greater Hyderabad Municipal Corporation (GHMC) continues to plummet dangerously: from 11.40 metres below ground level (bgl) in 2018-19, it dropped to 8.98 in 2024-25. And it was at 9.05 in 2024-25 till March. It continues to be in the red zone — which is below 20 metres bgl.
The areas with Outer Ring Road where the water table is in the red zone includes Malkajgiri, Bachupally, Dundigal, Serilingampally, Hayatnagar, Abdullapurmet, Saroornagar and Kukatpally mandals.
Filters can be used
Vishwanath Srikantahiah from Biome Environmental Trust said that a sump, built to store municipal water, can also be used to store the rainwater. He said it has to be ensured that the roof is clean. When there is a forecast for rain, the roof can be cleaned. Filters costing around ₹3,500 can be used to filter rainwater.
Ms. Kalpana said that inline filters are fitted in rainwater pipes. It weeds out leaves and other debris before collected rainwater is let into a sump or RWH pit.
Rain throughout the year
She said that as rainfall is not confined only to the monsoon, people will have sufficient water in non-monsoons too. According to TGDPS, the cumulative rainfall in GHMC from June 1, 2024 to May 15, 2025 was 996.5 mm against normal 811.8 mm.
A contractor said that it takes around ₹15,000 to set up a RWH pit three feet wide and four feet deep, which needs maintenance.
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