Rain water harvesting: Importance, techniques, pros and cons

Water harvesting is the process of collecting runoff from rain storms from a catchment (the area from which water falls into bodies) in order to provide water for irrigation right away or for later use by storing in above-ground ponds or aquifers. Water harvesting, to put it simply, is the direct gathering of rainfall.

What is rain water harvesting?

Rain water harvesting is a technique used to gather and store rainwater that runs off natural or man-made catchment areas such as rooftops, compounds, hill slopes, rocky surfaces, or artificially repaired impervious or semi-pervious surfaces. The process is carried out using artificially designed systems. The water collected through rain water harvesting can be filtered, stored and used for various purposes.

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Water harvesting techniques: Why is it important?

• To serve as domestic usage with appropriate filtration (drinking, watering garden).
• To act as unfiltered landscape irrigation, especially for dryland farming.
• To boost groundwater recharge, which will increase soil fertility even further.
• To lessen sewage treatment plant overloads, urban flooding, and stormwater discharges; keeps the clean, fresh surface water free of metals, pesticides, fertilisers, and other sediments.
• To lessen the influx of saltwater into coastal communities.
• Rainwater harvesting methods are affordable than other purifying or pumping methods, and ensure high-quality water.
• It reduces the demand for ground water. Having a rainwater harvesting system boosts the productivity of aquifer leading to increase in groundwater levels.

Rain water harvesting: Advantages

• This an easily accessible renewable water resource.
• Rain water harvesting reduces urban flooding.
• Rain water harvesting will prevent soil erosion.
• Rain water harvesting is very cost effective way of saving water.
• This is not labour intensive.

Rain water harvesting: Disadvantages

• The water that is received by rain water harvesting may not be suitable for drinking unless treated properly.
• It cannot be done in areas that have long dry spells.
• One has to maintain the storage facility properly or else it can result in contamination of water.  These can also become breeding grounds for insects.
• While its not expensive, the initial set up may be high.
• The rainwater harvesting system yield depends on the rainfall received and varies from season to season.

Rainwater harvesting techniques

Rainwater collection mostly uses two techniques:

1. Harvesting surface runoff

This technique works best in metropolitan areas where rainwater that runs down the ground during rainstorms is collected and put into a special water storage space. Small tributaries of rivers or reservoirs have their flow changed to accommodate the storage of surface runoff.

Ponds, tanks, and reservoirs designed for this purpose are used to store the surface runoff. Efficient and effective water conservation techniques are used to store rainfall while lowering evaporation. To maintain clean and sanitary water, several steps are required.

2. Harvesting rooftop rainwater

Individual households or schools may choose the rooftop rainwater harvesting method, in which rainwater is collected from roof catchments of residential or commercial structures, diverted, and stored in tanks.

To satisfy everyday demands such as flushing toilets, washing machines, washing cars, gardening, showers, sinks, and baths, harvested rainwater can either be kept in a tank or transferred to an artificial recharge system.

Rooftop rainwater harvesting techniques

This section provides examples of several rooftop rainwater harvesting methods.

1. Direct use storage

With this technique, rainfall that has accumulated on the building’s roof is directed to a storage tank. Designing the storage tank must take into account the catchment’s availability, rainfall, and water consumption.

Before connecting to the storage tank, each drainpipe should have a filtering system, a first flush device, and a mesh filter at the mouth. A method for excess water overflow should be present in every tank.

The recharge system might receive more water. Water from storage tanks may be utilised for extracurricular activities like gardening and washing. The most economical method of collecting rainwater is this one.

The primary benefit of collecting and using rainwater during the rainy season is not just to conserve water from traditional sources but also to reduce energy costs associated with water distribution and transportation. If groundwater is being drawn to satisfy demand when it is raining, this also conserves groundwater. Figure 5 is an example of a storage tank.

2. Groundwater aquifer recharge

Different types of structures can be used to replenish groundwater aquifers so that precipitation percolates into the ground instead of draining away from the surface. The following are typical recharge techniques:

•       replenishment of bore wells
•       filling up of excavated wells
•       refilling pits
•       trenches for recharge
•       shafts of recharge or soakaways
•       filtration tanks

3. Filling up bore wells

Through drain pipes, rainwater collected on the building’s roof is sent to a settlement or filter tank. Filtered water is transferred to bore wells after settlement to replenish deep aquifers. Bore wells that have been abandoned can also be recharged.

The settlement tank/filtration tank’s appropriate capacity may be built based on the catchment area, rainfall quantity, and recharge rate. Floating debris and silt should be kept out of the recharging structure since they might clog it up.

To prevent contamination, the first one or two showers should be cleaned off using a rain separator.

See also: Water conservation projects and methods adopted in India: Tips to conserve water at home

4. Recharge pits

Small pits with a weep hole placed at regular intervals that are contracted with a brick or stone masonry wall are known as recharge pits. Perforated coverings can be used to cover the pit’s top. Filter media ought to be put in the pit’s bottom.

The catchment area, the intensity of the rainfall, and the rate of soil recharging may all be used to determine the pit’s capacity. Typically, the pit’s dimensions range from 1 to 2 metres in width to 2 to 3 metres in depth, depending on the depth of the preceding stratum.

Small dwellings and shallow aquifers can be recharged in these holes.

5. Recharge or soakaway shafts

Where the topsoil is alluvial or less porous, soakaway or recharge shafts are supplied. These are the 30 cm-diameter bored holes that may reach depths of 10 to 15 m, depending on the thickness of the previous layer. To avoid the collapse of the vertical sidewalls, the bore should be lined with PVC/MS pipe that has slots or perforations.

To capture runoff before it filters through the soakaway, the necessary size sump is built at the top of the soakaway. The sump should include filter media.

6. Filling up of dig wells

Wells that have been dug can act as recharge structures. After going through the filter bed, rainwater from the rooftop is directed to drilled wells. Regular cleaning and desalting of dug wells are necessary to increase the rate of recharge. One option is to employ the filtering technique indicated for bore well recharge.

7. Trenches for recharge

Where the upper impermeable layer of soil is shallow, a recharge trench is supplied. The earth is dug for the recharge trench, which is then replaced with porous materials like pebbles, boulders, or brickbats. It is often designed to gather surface runoff.

In order to improve percolation, bore wells can also be installed inside the trench as recharge shafts. Depending on the anticipated quantity of runoff, the trench’s length is chosen.

Small homes, playgrounds, parks, and roadside drains can all benefit from this technique. The recharging trench can range in size from 0.50 to 1.0 metres in width to 1.5 metres in depth.

8. Tank percolation

Percolation tanks are man-made pools of surface water that submerge a patch of land with enough permeability to allow for enough percolation to replenish the groundwater. These can be constructed on sizable campuses where there is accessible land and suitable topography.

It is possible to direct roof runoff and surface runoff into this tank. To increase the groundwater, water that has built up in the tank percolates through the solid.

Gardening and other direct uses of the stored water are both possible. Urban greenbelts, parks, and gardens should all have percolation tanks installed.

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Rooftop rainwater harvesting components

1. Catchments: The surface area that receives direct rainfall and supplies rainwater to the rainwater collecting system is known as the catchment. Terraces with flat RCC/stone roofs or sloping roofs, courtyards, and open land that has been paved or not are all possible.
2. Transportation: Water pipes or drains transport rooftop rainwater to the rainwater collection system. Each drain’s mouth should be covered with wire mesh to contain floating debris. Water pipes must have the appropriate capacity and be UV resistant.
3. First Flush: A mechanism called first flush is used to remove water from the first shower. To prevent contamination in storage from significant levels of atmospheric and catchment roof pollutants, this is crucial. Because of this, it aids in clearing up cracks and other debris that is dumped on a roof during dry seasons. At the exit of each drainpipe, preparations for the first rain separator should also be constructed. Rainwater is sent through filters after being flushed.
4. Filter: People are sceptical about rooftop rainwater harvesting because they believe that if the right filtering mechanism is not used or if underlying sewage drains are damaged, rainfall might contaminate groundwater.

Rainwater harvesting in India

Water is a state subject in India. But, the Central government supplements the efforts of states on water conservation and recharge, including rainwater conservation and its harvesting, through technical and financial support. The government has also been implementing various schemes for rainwater conservation and its harvesting in with convergence with schemes of states governments and with people’s participation.

One of the such major initiatives of the government include the Jal Shakti Abhiyan in 1,592 blocks of 256 water-stressed districts of the country during July to November 2019, to promote water conservation and water resource management.  The Jal Shakti Abhiyan: Catch The Rain – 2022 mission is also launched on March 29, 2022, in all districts of the country  for water conservation and rainwater harvesting. While the National Water Policy advocates water conservation and rain water harvesting, the Atal Mission for Rejuvenation and Urban Transformation (AMRUT) the Atal Bhujal Yojana also focus on rain water harvesting.

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