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Why you should harvest rainwater?

With the onset of climate change, water scarcity is quickly becoming a major concern for many countries. World Wildlife Fund estimated about 1.1 billion people worldwide lack access to water, and by 2025, two-thirds of the world’s population is expected to suffer from water shortages. Climate change is altering the local weather patterns leading to droughts in drier areas and flooding in other areas. In 2018, Cape Town (South Africa) introduced the concept of “Day Zero” whereby most of city’s tap water supply was shut down to curtail water usage. In July of 2019, signs of a similar “Day Zero” surfaced in parts of India. On the other hand, the annual rainfall in India for that year was above normal compared to the previous years as reported by the India Meteorological Department. Therefore, the water scarcity during the year can be attributed to mismanagement of the rainwater. Proper planning and rainwater harvesting systems can help in not only recharging the groundwater but also reducing the municipal water consumption of the households.

What is a rainwater harvesting system?

Rainwater harvesting system also called the rainwater collection system or rainwater catchment system, is collecting the run-off from a structure or other impervious surface (catchment) to store it for later use or recharging the groundwater. Simply put, rainwater harvesting is where you collect and store (as well as purify) rainwater for future human use or groundwater recharge. Rainwater harvesting systems can vary from simple rain barrels to more intricate structures with pumps, tanks, and purification systems.

Following are the various ways to collect rainwater:

  • Capturing runoff from rooftops
  • Capturing runoff from local catchments
  • Capturing seasonal floodwaters from local streams
  • Conserving water through watershed management

For simplicity, we will be focusing on the most common method of rainwater harvesting which is rooftop rainwater harvesting.

What are the different methods to collect rainwater?

There are essentially two types of rainwater harvesting systems. The two systems are called a “dry” system and a “wet” system.

  • Dry System: This method has a smaller storage volume. The collection pipe “dries” after each rain event as it empties directly into the top of the tank. This system is relatively less expensive, easier to maintain, less complicated and great for climates where the rainfall is infrequent. An example of such a system would be a rain barrel as shown in Figure 1. This is the most common method of rainwater harvesting where you just place a barrel at a gutter downspout. The only disadvantage of such dry systems is that you need to place the storage tank near the house.

Figure 1 Rain barrel– a type of dry system
  • Wet system: This method is more complicated with a larger ability to collect water including collecting from multiple gutters and downspouts. The tank is located away from the house below the lowest gutter on the house. The rainwater fills the underground piping and the water rises in the vertical pipes until it spills into the tank. This can be illustrated through the Figure 2 below.

Figure 2 A wet system with a large tank

How to create a system and what are the components?

There are five components to a rainwater harvesting system. The same components and principles can be used for apartments and small houses.

  • Catchment – This includes the surface where the rainwater would fall. It can be roofs, ground or balconies.
  • Conveyance system – This includes pipes or drains which transport the water from the catchment to the storage location. Both the sides of the pipes or drain (openings) need to have mesh to filter out debris and large pollutants.
  • First Flush – First showers often carry with them particulate materials such as pollutants and toxic agents from the air. Since this water is not suitable for consumption, there needs to be a system in place which allows for this water to be taken away elsewhere.
  • Filter – Generally, a filter system will have layers of gravel, sand and mesh filter called netlon which also forms the topmost layer of the storage tank. This will remove debris, turbidity, some microorganisms and color from the water.
  • Tanks and recharge structures (storage) – The storage tank can be bought based on the nature, amount, and quality of rainwater that is collected (ADDA.IO, 2017).

The stored rainwater can be used in two ways: direct use and to replenish natural aquifers. Based on the location of the storage tank you can place a motor and transport the water to the faucets via pipes. And secondly, the water can also be used to replenish groundwater reservoirs like borewells, trenches or percolation tanks.

You can apply the same principles in rural areas as well. Small scale dry systems of rooftop harvesting have been popular in rural regions. People have also harvested monsoon runoff by capturing water from swollen streams and stored it in various forms of water bodies. Harvesting water from flooded rivers by making temporary channels is another method used in rural areas to collect rainwater.

How does a rainwater harvesting system work?

Figure 3 An illustration of a rainwater harvesting system with its components.

Figure 3 shows the components of the rainwater harvesting system and how it works. The rain falls on the roof catchment which is directed to the guttering. The guttering directs the water to one side due to its slight slope.

The water then moves through the downpipe with the help of gravity. The downpipe has mesh on both sides to filter out debris and large contaminants. This requires regular cleaning as blockages can be created. In the downpipe, there is a valve which deviates the dirty first flush water away from the storage tank.

The remaining water is then taken directly to a storage tank or the plastic tank as seen in Figure 3. The tank is equipped with a filter system, netlon. As previously mentioned, if you aim to replenish groundwater you can direct the water to the underground and you can skip the storage tank. Otherwise, you would have a storage tank on the surface or underground depending on your situation. The storage tank is connected to a pump. This pump then sends the water to the faucets when necessary.

What can you use the water for?

This type of system does not provide drinkable water, but you can add more filtration systems if you need it. However, the water hence harvested can be used for the following purposes:

  • Irrigation
  • Washing vehicles and/or pets
  • Industrial processes
  • Washing driveways and sidewalks
  • Refilling swimming pool
  • For lawns and gardens

What should you be cautious about?

Following are the things you need to keep in mind while installing a rainwater harvesting system:

  • As previously mentioned, the water available through this system is not drinkable and you should avoid taking bath in it as well. There is an issue of bacteriological water quality which means that the rainwater can become contaminated by pathogenic bacteria. To avoid this, you can regularly clean the catchment area, keep the tank away from trees and make sure first flush functions properly.
  • You need to seal all the tanks to prevent insects from entering. Mosquito proof screens should be fitted to all openings. This is essential to prevent the breeding of mosquitoes to take place in the tanks.
  • There are three main things to consider before deciding on the size of the systems- the amount of rainfall, the size of the catchment area and the cost of the initial installation.
  • Depending on the size of the catchment system, the cost can vary significantly. For example, when building a system for a gated community or large buildings the cost of installation can go very high due to the number of pipes involved. You would need some technical skills for installation as well. Such a system would also have a high maintenance cost. But on the other hand, building a rain barrel system for a house in a rural area can be completed for very cheap and with almost no manpower.

What do you gain from building a rainwater harvesting system?

Having a rainwater harvesting system in your house or apartment will have a direct impact on your demand for water and a reduction in the water bills. It will prevent your home from getting flooded when there is an intense downpour and ensure some water security when there is a drought. It acts as a great backup water supply for emergency situations. As mentioned above, you can use the water for irrigation without any need for filtration. If you are using it to recharge the groundwater, it can improve its quality as well. And lastly, it promotes both water and energy conservation. Building small dry rainwater harvesting systems in your house can be very easy and cheap. You can get payback on your investments very quickly and be a more conscious citizen in the process.


Published by Pranshu Patel

I am an ambitious environmental science graduate who is passionate about climate change and decarbonisation. I enjoy researching and writing about sustainability and climate change.

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