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.
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.
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.
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.
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.
No, existing buildings can also implement RWH by modifying the existing plumbing and making additions, if necessary.
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.
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.
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.
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.
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.
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:
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.
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.
Recharge structures are constructed to allow rainwater to replenish groundwater. The various ways in which recharge can be done is through:
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.
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.
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.
The cost of digging a 3 feet diameter well, which is 20 feet deep, is approximately Rs 20,000.
3 steps to begin RWH:
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.
The benefits include:
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.
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.
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.
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)