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Rainwater harvesting is a technology used for collecting and storing rainwater from rooftops using simple techniques such as jars and pots.



Rainwater harvesting is the accumulation and deposition of rainwater for reuse on-site, rather than allowing it to run off. Rainwater can be collected from rivers or roofs, and in many places the water collected is redirected to a deep pit (well, shaft, or borehole), a reservoir with percolation, or collected from dew or fog with nets or other tools. Its uses include water for gardens, livestock, irrigation, domestic use with proper treatment, and indoor heating for houses etc. The harvested water can also be used as drinking water, longer-term storage and for other purposes such as groundwater recharge.

What actions do you propose?

Roof Top Rainwater Harvesting

Rainwater can be used for potable water (drinking, cooking, and bathing) or nonpotable uses such as landscape irrigation, livestock watering and washing. Collecting and using rainwater has numerous benefits, ranging from improved water quality to reduced stress on underground aquifers.

In urban areas, buildings are usually constructed with rooftops of Reinforced Cement Concrete (RCC), Mangalore tiles, Asbestos / galvanized iron / zinc sheets etc. Construction of buildings with the above mentioned material requires roof top rainwater to be removed from building tops and currently been let off into storm water drains outside the plot area (which eventually goes away from the city).The rooftops being built significantly with hard material, large quantities of rainwater runoff and loss due to evaporation and percolation are very minimal. Thus, rooftop rain water harvesting can be put to good use by storing rooftop water on (a) roof itself (b) ground level (c) below the ground, by using storage devices like masonry tanks / ferro cement tanks / plastic or metal containers. Effective roof area for Rainwater Harvesting

Flat roof:

Calculation of Effective roof area.Divide the roof area into convenient grids and calculate

The area of each grid. Effective roof area is excluding the peripheral wall thickness and any other opening.

Sloping Roof:

Calculation of effective roof area

Divide the roof area into convenient grids and calculate each grid area by taking projected length & breadth.


Down water pipes made out of PVC, HDPE or cement pipes can be used for transporting rainwater collected from roofs to the filtration system before storing. The size of the down take pipe varies depending on the roof area, which is connected to the down pipe.


Rainwater collected on the roof is very pure and clean. However, there are many substances, which get mixed up with this pure water on the roof (leaves, bird droppings, dust etc.). These contaminants need to be filtered before the rainwater is stored. There are many filtration systems.

a. Sand bed filter

b. PopUp filter

c. Stabilization tank

a. Sand bed filter:

Sand bed filter is the traditional method where coarse riverbed sand, pebbles and aggregates are filled as layers one above the other in a confined masonry structure. Rainwater is allowed at the top from one end and filtered water is drawn from the other side.

b. PopUp filter:

A simple "PopUp Filter" designed by A.R. Shivakumar of KSCST and it is effectively working for residential buildings and smaller institutional or industrial applications. The "PopUp Filter" has three components (rainwater receptor, flush valve and filter element) rainwater receptor where the rainwater is allowed to flow from down pipes and a flush valve is provided to flush the first flow of the rainwater along with leaves, dust etc.

Water received in the receptor flows upwards against gravity through a filter element to filter most of the floating elements and allow water to stabilize in this filtration zone. Rainwater passing through this filter element (which is relatively cleaner), flows out through an outlet, which can be led to storage device. Filter element is mounted on a vertical stabilizer pipe with a friction fit. In the normal course, rainwater gets filtered and flows through outlet into the storage device. Filter element needs to be cleaned periodically during the rainy season to remove the filtered material and to keep the filtration system clean. In the event where the filter is not cleaned and the filter element is getting clogged, The "PopUp Filter" has a built-in safety feature it to push out the filter element from the stabilizer pipe and allow the water to flow out freely. This safety feature will avoid flooding of the rooftop because of clogged filter. The first indication of the filter getting clogged is rainwater flowing out of a vent hole provided on the top of the filter element.


These PopUp filters are simple in design and are very flexible to install in verifying field conditions.

An important feature in filtering is the separation of first flush of rainwater from relatively cleaner and purer subsequent rain.

c. Stabilization tank:

For large volume of rainwater a unique design has been developed by the author to trap light and heavy impurities without having any filter media. Rainwater is allowed to flow through a series of small tanks and by providing an entry and exit for water at strategic positions, impurities can be trapped in the stabilization tanks for subsequent cleaning. Heavier impurities will get trapped in the first two tanks as the water flows out at the higher level.

Lighter and floating impurities get trapped in the third and fourth tanks as the water flows out at the bottom or lower level. Periodic cleaning of these tanks is required to remove the impurities.


Storage of harvested rainwater is possible at various levels.The storage structures may vary from permanent masonry tanks, ferro-cement tanks to plastic or metal tanks. The capacity of storage device can be decided by considering parameters such as roof area, water usage and space availability.

Water consumption in a house is throughout the year and water availability from rainfall is for a limited period restricted to number of rainy days in a year. To make rainwater available in non rainy days, storage device need to be designed with an optimum capacity to suit the need / requirement. In Rainwater Harvesting system, storage device is the single most expensive component. Optimum size of the storage device and cost effective methods to store water are the key issues for a viable roof top rainwater harvesting system.

Collection surface

Larger the roof area available for rainwater collection higher is the quantity of rainwater collected. Cleaner and better the quality of rooftop, cleaner and better will be the rainwater collected. Maintenance of the roof surface and keeping it clean has a direct bearing on the quality of water collected.

Rainwater Harvesting Storage facility and Capacity

Rainwater collected on the roof and guided through the down water pipes gets filtered in the filtration system and is available for direct use. Rainwater coming out of the filter may be guided to a storage device for future use. Positioning, size and capacity of the storage container may be decided considering the roof area and the requirement of raw-water. Higher the storage capacity more will be the rainwater availability during the non rainy days. In urban areas, like Bangalore, total rainwater storage capacity of around 8,000 to 15,000 lts would suffice for the requirement of secondary usage (gardening, vehicle washing, cleaning etc.).

Roof yield or potential rainwater from a roof

Roof yield or the potential rainwater from a roof is normally referred to the annual yield from a given roof area. Annual yield is the quantity of water in liters collected from a given roof over a period of one year covering all the rainy days. It is the product of roof area and the annual rainfall.

Rainwater Harvesting for those who does not have proper roof

Rainwater can also be harvested by those who does not have proper roof by creating temporary collection surface by using a clean cloth piece.

Four corners of the cloth piece may be tied with separate threads and stretched three feet above the ground and tied tightly to four supports (poles / supports / walls etc.) during a rainy day. As the rainwater falls on the outstretched cloth depressions in the middle will be formed and all the water will get collected at the center. Since the cloth is pours water will start getting filtered through the cloth and starts dripping / flowing down at the center. A vessel or a can be placed to collect this pure rainwater for further storage in an enclosed tank or a larger container for future use.

Operation and Maintenance

Operation and maintenance of the rainwater harvesting system generally depends upon the type and technology used for harvesting rainwater. Operation and maintenance expenses are low; in fact, on average every household spends US$ 5 and 6 hours per year on maintenance tasks of the system. 

Generally, the system should be also checked and cleaned after every dry period of more than one month. The outsides of metal tanks may need to be painted about once a year. Leaks have to be repaired throughout the year, especially from leaking tanks and taps, as they present health risks. Chlorination of the water may be necessary.

Removal of debris and overhanging vegetation from gutters and the roof is important to prevent the gutter being clogged. Tank maintenance consists of physical inspection and repairing cracks with cement. Several studies have shown that water from well maintained and covered rooftop tanks generally meets drinking water quality standards if maintained rightfully. Basic water quality testing is recommended during the first year, with further testing when water quality is in doubt.

Rooftop rainwater harvesting system in Nepal:

A water harvesting system in which rain falling on a roof is led through connecting pipes into a ferro-cement water collecting jar.

Many households in Nepal’s midhills suffer from water shortages during the pronounced dry season. The technology described here – harvesting roofwater during times of heavy rainfall for later use – is a promising way of improving people’s access to water for household use, especially for households with no or only limited access to spring or stream water. The technology has yet to be extensively adopted Nepal’s midhills.

The technology was introduced in the Jhikhu Khola watershed to demonstrate an alternative source of water for domestic use (mainly drinking water). This technology is appropriate for scattered rural households in mountainous areas. The harvesting system consists of a catchment roof, conveyance pipes, and a storage jar. The pipes include a gutter system made from longitudinally split polythene pipe which has a flushing system that allows the system to be periodically flushed clean.

The collected water enters a 500 or 2000 litre capacity ferro-cement jar made using a mould (see photo). A preconstructed mould made from iron rods and polythene pipes is installed on a concrete base plate. Metal wires are extended from the base plate over the main mould to the top. Chicken mesh is then wrapped over the mould and tied securely with thin wire. A cement coating is applied over the metal structure.

The jar is finished with three coatings of cement and the opening is covered with a fine nylon mesh to filter out undesired coarse matter. A tin lid is placed over the top.

A tap is fixed about 20 cm above the ground. This height allows for water to be collected in the typical 15 litre local water vessels (gagri) and avoids collection of too much water in bigger vessels as well as minimising the dead storage of water (Nakarmi et al. 2003). Trained masons can easily install the entire system. Provided all the materials and the mould are available, the entire system can be put together in about a week.

The main maintenance task is to keep the roof clean, especially after long dry periods. This is done using the gutter pipe flushing system in which the first dirty water from the roof is diverted away from the jar.

Who will take these actions?

 Government, private sector, and individual. It can be done at individual as well as in a community level. This way we can be self sufficient in terms of domestic water requirements and not just dependent on the actions initiated by government or any other local body.

Where will these actions be taken?


How will these actions have a high impact in addressing climate change?

Depending upon water use.

What are other key benefits?

The key benefits are:

  • By harvesting rainwater, homeowners can reduce their water bills.
  • It also lessens the burden of soil erosion in a number of areas, allowing the land to thrive once again.
  • It provides an independent water supply during regional water restrictions and in developed countries is often used to supplement the main supply.
  • It provides water when there is a drought, can help mitigate flooding of low-lying areas, and reduces demand on wells which may enable groundwater levels to be sustained.
  • Rainwater harvesting helps utilities reduce peak demands during summer months.
  • Rainwater harvesting system would be more energy efficient because water capture and storage requires no energy.
  • Rainwater is free of sodium.
  • Rainwater is superior for landscape use and plants thrive on rainwater.
  • The water is practically free: the only cost is to collect and treat it.
  • The zero hardness of rainwater helps scales from building up on appliances and so extends the life of appliances.

What are the proposal’s costs?

Cost of Rainwater Harvesting in Nepal:

Roof yield or potential rainwater from a roof

Roof yield or the potential rainwater from a roof is normally referred to the annual yield from a given roof area. Annual yield is the quantity of water in liters collected from a given roof over a period of one year covering all the rainy days. It is the product of roof area and the annual rainfall.


a. Roof area of 100 Sq. meters with annual rainfall of 1000 mm (milimeters)

Roof yield = 100 X 1000 = 1,00,000 liters/year

b. Roof area of 1000 sq. feet with annual rainfall of 40 inches

Roof yield = 1000 X 40 X 2.36 (constant) = 94,400 liters/year

Examples to illustrate (Roof yield) Rainwater harvesting can be many and the cost involved also vary from case to case. The parameters that influence the cost of rainwater harvesting are:

  •  How much rainwater to collect
  • Type of surface from where rainwater is collected
  • Existing structure and fittings for rainwater collection and flow
  • Type and capacity of harvested water storage device
  • Method and system of rainwater retrieval system for use from storage device

Considering the above facts, and using type of materials used for Channelization, Filtration, Storage system as well as depending upon  water collection surface and quantity of water harvesting ,  rainwater harvesting can total cost as little as US$ 20/- and may go up to US$600/- for a residential building.

Time line

Within 1 years installation of roof top rainwater harvesting system

Above 1years cost benefit analysis.

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