It rained hard in Delhi; what if all that water did not go waste

The volume of rainwater generated in the past week was almost 13 per cent of the demand for water in Delhi, estimated at 2,765 million litres per day or 644,225 million litres

By Sushmita Sengupta
Published: Friday 21 August 2020

Waterlogging in Delhi after the city received constant rain in this third week of August 2020 was due to unplanned urbanisation. The concretisation or pollution of lakes, loss of green areas and mismanagement of stormwater drains have been major reasons for heavy waterlogging whenever it rains in the national capital.

A massive volume of water was pumped out from roads across the city because of the almost 118.4 millimetres rain between August 13, 2020 and August 19 — a 143 per cent deviation from normal average rainfall — according to the India Meteorological Department.

This much rain can produce a volume of 87 million cubic metres (MCM) or 87,000 million litres of water for a city like Delhi.

The demand for water in Delhi was estimated to be 2,765 million litres per day or 644,225 million litres (at the rate of 175 litres per capita per day) annually, according to Delhi-based think-tank Centre for Science and Environment.

The volume of rainwater generated in the past week — calculated by multiplying the area by the rainfall received and the co-efficient of the rainwater runoff — was almost 13 per cent of this.

This was estimated taking into account the city’s 18.8 million population, according to the 2011 census, with a coverage of 1,483 square kilometres area. The co-efficient of the runoff was taken as 0.5, considering the city’s combination of hard and soft areas, according to standard rules.

A tehsil-wise assessment of groundwater resources by the Central Ground Water Board (CGBW) was carried out. The total annual groundwater recharge of Delhi was assessed to be 320 MCM, with annual extractable groundwater resources at 300 MCM, according to the board’s latest report published in 2019.

The total current annual groundwater extraction was estimated at 360 MCM, with stage of groundwater extraction at 120 per cent. Of the 34 tehsils assessed, 22 were categorised as ‘over-exploited’, while two were ‘critical’, seven ‘semi-critical’ and three ‘safe’.

The artificial recharge of groundwater from the rainwater received must take place instead of it being pumped out from the roads, if urbanisation does not allow a natural recharge.

The CGWB identified around 693 sq km with 444 MCM of surface water in the city for artificially recharging groundwater and suggested 23 percolation tanks and 10 check dams and nallah bunds for this purpose. Around 21,735 rainwater harvesting structures were suggested for Delhi’s urban areas.

Whenever the city receives such heavy rain, it must be used for recharging its groundwater. Stormwater drains that carry rainwater should be clean and well-managed for this purpose.

Mixing sewage and solid waste with rainwater in these drains must be banned. There should also be provisions for rainwater to be drained in nearby green areas and parks, where it can be recharged into the ground through rainwater-harvesting systems.

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