Frequently Asked Questions (FAQs) - Rainwater Harvesting (RWH)

RWH explained in simple Q &A, for people to carry out this exercise in their homes and communities with ease.
Roof RWH system at Patkhori High School in Mewat
Roof RWH system at Patkhori High School in Mewat

This simple guide lists out the most popular questions related to rainwater harvesting to help understand this important topic better.

Please click on a topic to view more detailed information.

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What is Rainwater Harvesting (RWH)?

RWH is the technique of collecting, storing and distributing rainwater for multiple uses. The collected water can be stored for direct use or diverted for borewell/groundwater recharge.

In simple terms it is a way to capture the rainwater when it rains, for later use.

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Who can practice RWH?

You, me and everybody! It will not only provide you with water in times of acute water shortage, but will also recharge the groundwater and increase its level.

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Why should I implement RWH?

Rainwater is the ultimate source of all the fresh water that we use. In India, rainfall occurs in short periods of high intensity, allowing the rain falling on the surface to flow away fast. This leaves little scope for recharging the groundwater, which results in water scarcity in most parts of the country. Through RWH, this erratic rainfall can be conserved, stored & used as per convenience, either directly or for recharging groundwater.

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Where can RWH be implemented?

RWH can be done in homes, apartments, societies, schools, institutions, commercial premises and any other space as long as there is a catchment area in the form of a roof or open space to capture the rain.

Domestic rainwater harvesting is a relatively simpler affair, where even a rain barrel can serve as a storage unit for rooftop RWH. Individual homes have successfully implemented this easy and eco-friendly method of augmenting household-level water availability. Farmers also have implemented RWH to transform a barren piece of land into a self sustainable, lush green farm.

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Is RWH only feasible for new buildings?

No, existing buildings can also implement RWH by modifying the existing plumbing and making additions, if necessary.

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What quantity of rainwater can be collected? 

The rainwater harvested depends upon the catchment area, the rainfall pattern in the area and the drianage/ collection system used. 

To understand the potential for rainwater harvesting, lets take the example of a house in Delhi with a terrace area of 100 sqm. Taking the average annual rainfall in Delhi as 600 mm, and assuming 70% harvesting efficiency (as some rainwater will be lost due to evaporation, collection etc.), we can calculate the amount of water harvested thus:

Volume of water harvested = 100 x 0.6 x 0.7

                                              = 42,000 litres

This volume is more than twice the annual drinking water requirement of a 5-member family, whose average daily drinking water requirement is 10 lpcd.

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What is the cost involved?

The cost will vary depending upon the catchment area and the conveyance/ storage structures finalised. RWH can be installed at a very low cost in large plots where public buildings, schools & colleges are located, and this cost is negligible to the total construction cost, if integrated with the building design.

If planned in an existing building, the cost is higher due to extra plumbing involved, but the returns are rich in terms of recurring benefits. 

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What types of filters are needed for RWH? Which type is needed if the rainwater is to be used for flushing toilets?

Various kinds of filters are used in RWH. If the rainwater is to be used for flushing toilets alone, there is no need for any filter but the roof needs to be kept reasonably clean. If necessary, a grating can be fixed at the inlet point to the loft from the roof.

For more: Need horizontal, stainless steel filter for RWH

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Can the stored rainwater in storage tanks be used for cooking and drinking?

The rainwater that falls on the roof is pure, but since it comes in contact with various surfaces on its way to the storage units, some dust and leaves may get carried away with it. This can be reduced if the terrace is swept before the rains. However, even if some dust or leaves go into the sump, they do not cause any harm as long as the water is boiled before consumption.

Various filters can be utilised to remove such suspended pollutants from the rainwater collected to make it safer for consumption.

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What are the various types of RWH?

Rainwater harvesting can broadly be divided into 3 categories based on the types of usage, the area in which harvesting is carried out and the people involved.

  • Storage or recharge: Based on the type of usage, structures can either be used to store the collected water for direct use or to recharge groundwater.
  • Urban-rural difference: Urbanization has resulted in the shrinking of open spaces as well as unpaved areas. This has resulted not only in flooding of cities but has also caused water scarcity due to groundwater depletion in general and saline water intrusion in coastal cities. While rural harvesting is mostly traditional and is carried out in surface storage bodies like rivers, tanks, ponds, lakes etc., urban harvesting, due to lack of open space for capturing the runoff, is mostly in sub-soil storage as groundwater recharge.
  • Rooftop & driveway harvesting: When we say rainwater harvesting, the first thing that comes to our mind is the terrace. This greatly restricts the scope of rainwater harvesting as a considerable amount of water that falls around the built-up area is let out of the building as run-off. Driveway run-off water should not be led into a sump for immediate use or to a source well, but it can very well be directed into recharge wells.

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What are the basic components of a RWH and conservation system?

  • Catchment areas that include roofs of buildings and open spaces.
  • Storage units that can be a barrel, a tank or even a sump.
  • Conveyance mechanism which transports the water falling on the catchment area to the storage unit.

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What are the characteristics of a good RWH system?

The RWH system must ensure that not a drop of rainwater falling within the premises is let into the sewerage or wasted as runoff. This can be achieved only if the method adopted within the premises satisfies the following criteria:

  • Completeness: Both rooftop and driveway runoff water must be harvested.
  • Apportioning of water: To avoid overload of any one system, leading to overflow and loss.
  • Proper design: Volume of water likely to flow through and the nature of the soil in the area should be considered.
  • Maintainability: Design should incorporate features allowing for periodic maintenance of the structure.

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Can existing structures be used for RWH?

Existing unused structures like dried open wells, sumps etc can be used for RWH as also defunct borewells, instead of constructing recharge structures. This will also reduce the total cost.

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What does artificial recharge to groundwater mean?

It is a process by which the groundwater is augmented at a rate exceeding that obtained under natural conditions of replenishment. Any man made scheme or facility that adds water to an aquifer may be considered to be an artificial recharge system.

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What are the various types of recharge structures ?

Recharge structures are constructed to allow rainwater to replenish groundwater. The various ways in which recharge can be done is through:

  • Abandoned dugwell
  • Handpump
  • Recharge pit
  • Recharge trench
  • Gravity head recharge tubewell
  • Recharge shaft

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What is a recharge pit? Is it the same as a recharge well?

A recharge pit is a hole dug in the ground, usually filled with gravel or jelly to give it structural strength. However, a recharge well is not filled with gravel, but needs concrete rings installed in it to stabilise its walls.

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What should be the depth of a recharge well/pit?

For effective recharge in Bangalore like conditions, a 15-20 feet depth is needed. (Provided you do not hit rock before that depth). If you hit water while digging the pit, you need to ask the workers to be careful while they continue digging. The diameter of the pit you choose depends on two things: space available and the quantity of water you will send in.  A 3 feet dia X 20 feet pit will ‘hold’ around 4000 litres (for recharging the ground). A 5 feet x 30 feet structure can hold 16000 litres.

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What are the legal guidelines and their implication for recharge pit sizes?

As per the law, you need to store or recharge 20 litres for for every sq.m of roof area and 10 litres for every sq.m of non-roof (ground surfaces such as parking, backyard) area. This is for plot sizes of 40x60 sq.m or more.  For e.g. if your home has about 100 sq.m of roof area, and about 50 sq.m of non-roof area, then you will need to create capacity for at least 2500 litres of  water. If you are using a 1000 litre tank to store rainwater for immediate use, you can connect the overflow to a 4000 litre (3 feet x 20 feet) recharge well.

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How much do recharge pits cost?

The cost of digging a 3 feet diameter well, which is 20 feet deep, is approximately Rs 20,000.

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What are the basic steps to begin the process of RWH?

3 steps to begin RWH:

Step 1:  Ask yourself: Why Rainwater Harvesting? Your answers to the questions below will help determine the most appropriate RWH interventions. In many cases RWH is implemented to achieve all three of these objectives. 

    • Is it a source of supplemental water (thus reducing demand from your existing sources)?
    • Is it to recharge ground water sources as you are dependent on borewells/open wells?
    • Is it also as a flood control measure?

Step 2:  Understand the strategy for your layout.

Step 3:  Talk to an expert 

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How to deal with the technicalities of direct storage and groundwater recharge?

For answers to common yet crucial questions related to RWH that address the technicalities of direct storage and groundwater recharge, you may refer to this Technical FAQ.

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What are the benefits of RWH ?

The benefits include:

  • Flood mitigation: Appropriately designed recharge structures in open public spaces, will help keep the roads from flooding. When water is not allowed to leave the premises, the chances of it choking up the roads are minimal.
  • Increasing groundwater levels: Marked improvement of both the quantity as well as the quality of the groundwater in areas which have implemented rainwater harvesting
  • Greater water availablity: Rainwater collected in storage tanks is available as and when needed
  • Prevents soil erosion and flooding especially in urban areas

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Is RWH a new trend?

RWH can be traced back to thousands of years in India. Our ancestors traditionally harvested rainwater through tankas, johads, madakas and many such local innovative structures that can be seen even today, across the country. 

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Which of the states have taken action to promote rainwater harvesting ?

In compliance to CGWA (Central Ground Water Authority) directions, steps have been taken by states/UTs to promote and to make rainwater harvesting mandatory. Andhra Pradesh, Gujarat, Haryana, Himachal Pradesh, Kerala, Madhya Pradesh, Maharashtra, Tamil Nadu, Ranchi Regional Development Authority, Bombay Municipal Corporation and Pimpri – Chinchwad Municipal Corporation, Municipal Corporation of Ludhiana, Improvement Trust, Jalandhar, Jaipur Municipal Corporation, Mussorie Dehradun Development Authority, and Union Territories of Delhi, Daman & Diu and Puducherry have made necessary provisions in their building bye-laws to make installation of rain water harvesting systems mandatory.

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Are there some videos on RWH?

There are dozens of videos about RWH in multiple languages and contexts, to get a visual sense of the how and what of RWH, on youtube here.

 

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References

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IWP requests users to view the same as a starting point in collating information on rainwater harvesting and to add more suggestions, information as responses in this thread.

Access here: FAQ on Rainwater Harvesting (RWH)

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