Rainwater harvesting

Also called: RWH · Topic: Water Resources of India: Availability, Use and Conservation · NCERT: Class 7, Ch 1 "Geographical Diversity of India"; Class 7, Ch 1 "The Story of Indian Farming"; Class 8, Ch 1 "Natural Resources and Their Use"; Class 10, Ch 3 "Water Resources"; Class 11, Ch 6 "Natural Hazards and Disasters: Causes, Consequences and Management"; Class 12, Ch 4 "Water Resources"; Class 12, Ch 9 "Geographical Perspective on Selected Issues and Problems"

Meaning

Rainwater harvesting (RWH) means catching rain where it falls and storing it to use later. It also means guiding rain into pits, wells and borewells so that it soaks down and refills the groundwater (water stored underground in soil and rock).

It is cheap and does no harm to the environment. Households, villages and towns can build it themselves. This makes it a key local answer to falling water tables and water shortage in cities, and it supports large dams rather than replacing them.

Explanation

How it works: cause → process → result

• Cause: rain is lost as runoff.
• Rain often flows away quickly over the surface. This flowing water is called runoff.
• In cities, roads and roofs stop rain from soaking into the soil.

• In cities, the demand for water has also grown faster than the supply.

• Process: catch, store or recharge.

• Rain is caught on roofs or land surfaces.

• It is either stored in tanks for direct use, or sent underground through pits, trenches, wells and shafts.

• Result: more water and a higher water table.

• The water table is the top level of underground water. Recharge pushes it back up.

Rooftop rainwater harvesting, step by step (NCERT Class 10, Fig. 3.3)

1. Rain falls on the roof.
2. Pipes carry the water down to a filter made of sand and bricks. The filter removes dirt.
3. The clean water flows into an underground sump (a storage tank) for daily use.
4. Extra water goes into a well, an old abandoned dugwell or a hand pump. This recharges the groundwater.

Methods of recharge (NCERT Class 12, Fig. 4.3)

• Rooftop collection into tanks
• Recharge pits and recharge trenches
• Recharge through dug wells and hand pumps
• Recharge shafts: pits or bores that carry surface water straight down to deeper aquifers. An aquifer is an underground layer of rock or sand that holds water.
• On land, RWH is linked to watershed management. A watershed is an area of land that drains all its rain into one stream, river or lake. In a watershed, rain is held back by:
• Check dams: small, low walls of stone, earth or concrete built across a stream or gully. They slow the water, so silt settles and water soaks down.
• Percolation tanks: ponds that hold runoff long enough for it to percolate (seep slowly down) and refill nearby wells.
• Johads: traditional crescent-shaped earthen dams that catch runoff.

Benefits (NCERT Class 12)

• More water is available. Stored rain adds to supply in the dry season.
• Water tables stop falling so fast. Recharge pushes the water table back up.
• Water quality improves. Fresh rainwater mixes with groundwater and dilutes harmful fluoride and nitrates.
• There is less soil erosion and flooding, because less rain rushes off over the land.
• It stops saltwater intrusion in coastal areas.
• Heavy pumping lowers fresh groundwater near the coast.
• Seawater then seeps into the aquifer.

• Recharge keeps the fresh water level up and pushes the seawater back.

• It saves energy. When the water table is higher, pumps lift water a shorter distance and use less electricity.

Conditions for success

• Upkeep: a structure helps only if it is cleaned and repaired.
• Community participation: the people who live in the area must plan the work, do it and look after it.
• Terrain and storage: steep slopes, hard rock and thin soil let little water soak in. In such places, storing the water matters more.

Where it occurs

• Tamil Nadu
• The first Indian state to make rooftop RWH compulsory for all houses and buildings (2001–03). People who do not comply face penalties.
• NCERT Class 12 says no building can be built there without RWH structures.

• Groundwater levels in Chennai rose after the law. Later, many structures were not maintained, and Chennai still faced a severe water crisis in 2019.

• Model Building Bye-Laws (MBBL) 2016, issued by the Ministry of Housing and Urban Affairs

• Building bye-laws are local rules that control how buildings are built.
• Every building on a plot of 100 sq m or more must include a complete RWH proposal.

• 33 States/UTs had adopted this feature by 2021 [4].

• Shillong (Meghalaya)

• Cherrapunji (Sohra) and Mawsynram, about 55 km away on the southern slopes of the Khasi Hills, get the highest rainfall in the world.
• Even so, Shillong faces an acute shortage of water. Its slopes are steep, its rock is hard and its soil is thin.

• Nearly every household has a rooftop RWH system. It meets about 15–25% of the household's total water need.

• Gendathur (Mysuru district, Karnataka)

• About 200 households harvest rooftop rain.
• The village gets about 1,000 mm of rain a year.

• With 80% collection efficiency and about 10 fillings of the tank a year, each house collects about 50,000 litres a year.

• Ralegan Siddhi (Ahmednagar district, Maharashtra)

• A drought-prone village in the rain-shadow (dry) zone of the Deccan. Anna Hazare led the change from 1975.

• The village repaired a leaking percolation tank. The groundwater rose, and seven wells below the tank filled with water again.

• Arvari basin (Alwar district, Rajasthan)

• In the semi-arid Aravalli hills, villagers worked with the NGO Tarun Bharat Sangh, led by Rajendra Singh, from the mid-1980s. They built johads.
• The aquifers were recharged, and the dried-up Arvari river began to flow again.

• In 1999 the villages formed the Arvary Pani Sansad, a "water parliament".

• Andhra Pradesh: Neeru-Meeru ("Water and You"). People build water harvesting structures with their own labour.

• Haryali (2003): a watershed project sponsored by the Central Government and carried out by gram panchayats (village councils).
• Jal Shakti Abhiyan-I (2019)
• It covered 1,592 blocks in 256 water-stressed districts. Water conservation and RWH was one of its five focus areas [5].

• It ran in two phases:

  • 1 July – 15 September 2019 for the south-west monsoon.
  • 1 October – 30 November 2019 for states that get the north-east (retreating) monsoon, such as Tamil Nadu [5].

• Jal Shakti Abhiyan: Catch the Rain

• Launched on 22 March 2021 (World Water Day) with the theme "Catch the rain, where it falls, when it falls" [2].
• It is now held every year in all districts during the pre-monsoon and monsoon seasons [2].

• About 16.23 lakh water conservation and RWH structures were built by March 2022 [2].

• Master Plan for Artificial Recharge to Groundwater (2020), CGWB

• Artificial recharge means helping rainwater reach aquifers through human-made structures.
• The plan proposes about 1.42 crore RWH and recharge structures to harness about 185 BCM (billion cubic metres) of water [3].

• The plan only gives advice. States build the structures with funds from schemes such as MGNREGS and PMKSY [3].

• Traditional roots: RWH continues India's old tradition of community water harvesting, which includes khadins, kunds, tankas and step-wells.

Don't confuse with

• Watershed management: RWH catches rain at one site, such as a roof or a pit. Watershed management plans the whole drainage area, which is bounded by a ridge (a line of high land), and in its broad sense covers land, water, plants, animals and people.
• Check dam vs storage dam: a check dam slows a stream so that water percolates and recharges groundwater. It does not store water to feed canals the way a large multipurpose dam does.
• Percolation tank vs ordinary storage tank: a percolation tank is built to recharge nearby wells. It is not meant to supply water directly.
• Heavy rainfall vs water security: Shillong gets plenty of rain nearby but is still short of water. Water security depends on storage, terrain and management, not on rainfall alone.

Prelims Hooks

• Tamil Nadu was the first state to make rooftop RWH compulsory for all buildings (2001–03).
• Under Model Building Bye-Laws 2016, rooftop RWH is mandatory for plots of 100 sq m and above. 33 States/UTs had adopted this feature [4].
• Shillong (Meghalaya) is short of water even though Cherrapunji and Mawsynram, about 55 km away, get the world's highest rainfall. Rooftop RWH meets 15–25% of household needs there.
• Place–state pairs: Gendathur is in Mysuru, Karnataka. Ralegan Siddhi is in Ahmednagar, Maharashtra. Arvary Pani Sansad is in Alwar, Rajasthan. Neeru-Meeru belongs to Andhra Pradesh.
• JSA: Catch the Rain was launched on 22 March 2021 (World Water Day): "Catch the rain, where it falls, when it falls" [2]. JSA-I (2019) covered 256 water-stressed districts [5].
• CGWB Master Plan 2020: about 1.42 crore structures to harness about 185 BCM [3]. Trap: the plan only advises, and the states build the structures [3].

Mains Points

• Why decentralised RWH is needed (GS-III)
• Groundwater tables are falling and cities are short of water.
• Coastal aquifers are turning salty, and groundwater has fluoride and nitrate problems.
• RWH is cheap and local, so it supports large multipurpose dams rather than replacing them.

• Policy examples: Tamil Nadu's 2001–03 law, MBBL 2016 (33 States/UTs) [4] and the CGWB Master Plan 2020 [3].

• Upkeep and community decide success

• Chennai's groundwater rose after the mandate, but neglected structures could not prevent the 2019 crisis.
• In Ralegan Siddhi, shramdaan (voluntary labour) and village rules (no open grazing, no sugarcane) made the work last.
• In the Arvari basin, johads and a "water parliament" revived a river.

• Lesson: institutions and maintenance matter as much as building structures.

• Regional fit and policy change

• Cities need rooftop RWH required by bye-laws, plus recharge shafts.
• Steep, high-rainfall North-East hills such as Shillong need rooftop storage.
• The semi-arid and hard-rock areas of Rajasthan, Saurashtra and the Deccan need johads, check dams and percolation tanks.
• Policy has moved from panchayat-led Haryali and Neeru-Meeru to Jal Shakti Abhiyan: Catch the Rain. Catch the Rain pools funds from existing schemes (convergence), geo-tags water bodies (marks their exact location on a digital map) and revives step-wells [2].

Related concepts

• Rooftop rainwater harvesting
• Shillong rooftop rainwater harvesting
• Tamil Nadu rainwater harvesting mandate
• Watershed management
• Check dam
• Percolation tank
• Haryali
• Ralegan Siddhi
• Arvary Pani Sansad

Read more

• Rainwater harvesting and watershed management