Introduction to Rainwater Harvesting

In Rainwater Harvesting by Jeremiah CasteloUpdated: Published: Leave a Comment

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It is no news that water scarcity is one of the major problems facing the world today.

There will be about 1 billion more mouths to feed worldwide by 2025 and global agriculture alone will require another 1 trillion cubic meters of water per year (United Nations University, 2012).

Because one cannot under look such statistics, it is important for us to find solutions to this problem. One of such solutions is Rainwater Harvesting (RWH).

Also referred to as Rainwater Catchment or Roof Water collection, Rainwater Harvesting is also used in developing countries. Rainwater Harvesting simply refers to the collection and storage of run-off rainwater for subsequent use. Usually, it includes the collecting rainwater from the roof of a house via gutters which will then channel the run-off water into some kind of storage container.

To make the process of Rainwater Harvesting easier, it is advisable to build some sort of collection system. But before delving into the various components of an RWH system, it is, first of all important, to understand how useful rainwater harvesting is as well as the benefits of rainwater harvesting.

In this article, we'll cover:

  • The advantages and practical uses of collecting rainwater
  • The different methods and systems of rainwater harvesting
  • All the components that make up an effective rainwater harvesting system


Over the years, rainwater harvesting has gained grounds especially in developing countries where water scarcity is frequent making rainwater one of the major sources of water for many.

This has caused rainwater harvesting to move from just an old farm cistern to a major source of water for households and businesses, not only in developing countries but even in developed countries such as Germany, Singapore, Australia, Japan, China, and even America.

Why then are more and more households and businesses turning to rainwater harvesting as an alternative source of water supply?

  • First and foremost, rainwater harvesting allows you to control your own water supply allowing self-sufficiency and water conservation.
  • Rainwater Harvesting is an environmentally responsible and socially acceptable source of water.
  • Also, rainwater is a free and relatively clean source of water.
  • Because of its lack of chlorine, rainwater is ideal for landscape gardens and plants.
  • Rainwater Harvesting also helps in reducing runoffs and solving drainage problems while giving you unrestricted access to free and clean water.
  • It is an inexpensive and easy-to-maintain source of water.
  • RWH systems can be adapted to fit into any existing structure or constructed while building a new home. Also, the flexibility of RWH systems allows for easy relocation, reconfiguration and/or expansion.

1" of rain x 1 sq. foot = 0.623 gallons

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Advantages of Rainwater Harvesting


The collection and numerous uses of Rainwater come with a plethora of benefits. Some of these benefits are discussed below.

It Acts as a Backup Water Source

Water supply systems each have their uncertainties and sporadically need repair and maintenance. Also, droughts may cause water shortage. Such major repairs or periods of drought may result in the unavailability of water for daily use. However, a well-built and proper RWH system in place will allow you access to an alternative source of water for other household/business purposes while saving the little potable water for drinking and cooking.

Ecological Benefits

Probably one of the most important benefits of rainwater harvesting is the role it plays in the fight against water scarcity. It is estimated that about 35% of water at home is used to flush the toilet. When you add that to the amount of water you use to water your plants, garden, lawn and wash cars, you see the humongous amount of clean and potable water being wasted. RWH can help you greatly reduce the amount of safe water used in performing these chores. This reduction in the use of potable water will, in turn, reduce the amount of water being pumped from lakes and rivers which will, in turn, reduce the chances of drying up these water bodies.

Low Initial Capital, Reduced Utility Costs, and Easy Maintenance

Installing an RWH system is a one-time job which does not require a lot of capital. Also, once the system has been installed, you will automatically notice a fall in your utility costs which will mean more money for food and other expenses

Also, because the water harvested using RWH systems is generally not used for cooking or drinking, these systems are relatively easier and cheaper to maintain. This is because money won’t be spent installing costly purification mechanisms. RWH systems are thereby a cost-effective alternative water source and can also be a cost-effective method of artificially recharging groundwater aquifers.

Helps in Reducing Flooding and Erosion

Most RWH systems used to provide water to buildings feature built-in catchment areas around the rooftop which usually are capable of collecting and storing huge amounts of water. By collecting this water, flooding and soil erosion is greatly reduced since there is a reduction in the flow of water.

Practical Uses of Rainwater Harvesting



Farming and landscaping are two very important human activities that use up enormous amounts of water. Rainwater Harvesting provides a less costly and more eco-friendly source of water for these activities. Rainwater is ideal for farming especially because of the absence of chlorine or any other chemicals which may prove harmful to plants and animals. It can also offer an alternative source of water for the watering of lawns, plants, gardens and filling up of swimming pools. Also, as has been discussed earlier, rainwater harvesting also helps prevent erosion and flooding by reducing the flow of water.

In Home Use - Drinking/Cooking

Rainwater Harvesting can also be a source of water for cooking and drinking. Rainwater is a natural source of potable water and with proper maintenance and construction, it is possible to build an RWH system that can provide water good enough for drinking. Water from an RWH system can be decontaminated through boiling, chlorination or other water purification techniques.

Fire Protection

Finally, it can also be used to protect against fires. This is especially the case in regions suffering from water scarcity or in areas not connected to the main water supply. This can be done by installing fire plugs which are innovative fire protection devices that automatically fill roof gutters with water in the event of a bushfire threat. In Fort Davis, TX, rainwater runoff is stored for fire safety purposes by the McDonald Observatory

Different Methods and Systems of Rainwater Harvesting


Dry System

Considered by many to be an upgraded version of the rain barrel system, this technique features a container larger in volume than the rain barrel usually located a few meters from the property. The roof gutter is channeled into the storage tank. This system is referred to as the “dry” system because, after rainfall, the pipes get dry since all the water empties into the top of the tank.

Advantages of this method include larger storage preventing flooding in case of heavy rainfall and easy implementation and inexpensive to install. This system is especially ideal for stormy areas.


Wet System

While the pipes channeling water in the dry system are located on top of the storage tank, the pipes here are located underground and beneath the storage tank causing the pipes to continually be full of water, hence the name “wet” system. With this method, a number of pipes are connected to multiple plugs on the building and channeled underground into an empty storage tank. In the absence of rainfall, the water level remains constant with the pipes constantly full of water.

However, because the pipes are always full of water, it is advised that pipes be watertight to avoid leakage into the soil. This method is the most expensive of all the three systems due to underground piping.


Rain Barrel

The rain barrel is probably the easiest and cheapest RWH system and is widely used by households. This system involves the installation of a barrel underneath the roof drain pipe such that all the rain falling on the roof will be guttered and funneled into the barrel. The barrel usually features a plug at the bottom which makes it possible for water to be drawn or for a hose to be connected.

One major advantage of this method is the ease of use. Barrels can easily be purchased or sourced whether new or recycled. If you choose to use a recycled barrel, it would be prudent for you to know what the barrel was formerly used for to be sure there are no chemical residues left. Also, find a lid for your barrel if you live in a mosquito-infested area.

The main disadvantage of this system is its limited capacity, usually resulting in overflooding when it gets full. However, barrels are relatively inexpensive and available for purchase at various locations.

Components of a Rainwater Harvesting System


Catchment Area

For most RWH systems, this is usually the rooftop. This is usually the area or structure from which the water flows down into the storage tank. It is important to take a number of factors into consideration when building a rainwater harvesting system:

  • First and foremost, it is important to take note of the material on the roof. Common asphalt shingle can be used (for landscape purposes). However, if you plan to build your system from scratch, a metal roof is the best material to use because water slides down immediately, avoiding the risk of bacterial festering. Avoid wood and materials that contain lead.
  • Another factor is the slope of the roof as it determines how quickly water will drain to gutters during rainfall. Steep roofs cause water to run off quickly making it easier to clean the roof and prevent contamination. Less-steep roofs, on the other hand, reduce runoff making it more possible for contaminations to stay on the roof
  • Size is important as it will determine the amount of rainwater harvested. Knowing the area of your catchment area can help you determine how much water you can harvest. The area can be calculated by adding the building’s area to the area of the roof’s overhang. The larger the size, the greater the volume of water harvested.



This simply refers to the gutters and roof drain pipes. These are the pipes that channel water from the catchment area to the storage tanks. Consider the following when selecting gutters and drain pipes:

  • The size of the gutters should allow adequate movement of rainwater harvested from a storm event spanning 100 years (with a 1% likelihood of occurring). Thus, storm-prone places require wider gutters than places with less-intense rainfall. Gutters should generally be at least 5 inches wide.
  • Every 100 sq. feet of catchment area should have one inch drain pipe. The same rule should be applied to circular PVC piping.
  • It is important to properly install drain pipes and gutters for the system to function properly.
  • For effective draining, gutters should be sloped at 1/16” per foot of length.
  • Also, rounded-bottom gutters reduce the risk of debris buildup.
  • Also, consider putting gutter hangers after every 3 ft. In areas with heavy snow, gutters should be placed after every foot.
  • Paint PVC pipes to minimize UV sunlight breakdown.



This is usually the most visible component of a rainwater harvesting system. This is where rainwater harvested is channeled and stored for subsequent use. Its main use is the safety of water collected. Some things to consider include:

  • It is important to make sure that the observation port is inaccessible to anyone to prevent accidents and unfortunate incidents.
  • Also, it is important to carefully select the material for your storage tank. Some ideal storage tanks include concrete, enclosed metal, and monolithic-pour concrete tanks. Others include fiberglass, plastic (polypropylene and polyethylene) and wooden tanks.
  • Also, the color of the tank is very important as it plays a number of roles. First, it preserves the quality of the water. Clear (translucent) tanks encourage the growth of algae since sunlight can penetrate the material. It is therefore important for clear tanks to be painted. Also, tank color can affect the temperature of the water. During summer, water in tanks with lighter colors tend to be colder since the color reflects solar energy. Remember that the container still needs to be opaque. The best way to increase solar reflection and opaqueness is to paint a black storage container white.
  • It is also important to take into consideration the location of your tank – whether it will be underground or aboveground. While below ground tanks take up less space, are less visible and maintain a constant temperature all year round, they are prone to cracks due to soil shifting and settling. Also, they are constantly open to water pollution and cracks can be very difficult to detect. Below ground storage tanks are also costlier to install, remove and maintain. Aboveground storage tanks are subject to weather conditions including sunlight, and this may affect the water quality.



Treatment is a very important part of the system as it makes the water harvested safe for use. During harvesting, water can be contaminated by microbiological organisms, chemicals, and debris. Microbiological organisms include viruses, bacteria, and parasites. Chemicals include synthetic organic chemicals (SOCs), volatile organic chemicals (VOCs) and other metals.

  • To avoid chemical contamination, it is important to avoid roof materials and gutters that contain lead or copper.
  • To avoid contamination by debris, consider installing debris-screening filters. This should be installed in the gutter to filter out leaves and other debris from making their way into the system. Screening filters should also be placed in the drain pipe to filter out debris draining from the gutter. Finally, one should be placed at the inlet into the storage container.
  • Once the water has been stored, the treatment of the water will depend on its intended use. If the water is meant for irrigation, then the water doesn’t need any extra treatment. However, if the water is intended for indoor use, consider using different water filtration techniques to maximize effectiveness and eliminate chemical and microbiological contaminations. This can include one or all of the following techniques: chlorination, UV light, ozonation, disinfection, and absorption.



This involves all the devices used to move water out of the storage tank for use including pipes and pumps. If your RWH system is used for irrigation, all that is needed is a length of drip tubing. However, if the plan is to use the water for indoor purposes, then, distribution will involve a number of other components. It will be prudent to employ the services of a licensed plumber.

It is important to note that friction in the pipes will lead to a loss of pressure. This is especially true if you plan to move the water far from your storage tank. Consider calculating friction loss using the Hazel-Williams Friction Loss Equation.

Pressure tanks and Pumps are also two very important components of a distribution system. If your RWH system uses gravity flow, there is no need for a pump. However, it is important to note that selecting the perfect pump for your system will depend on your volume and pressure requirements. Pressure tanks make sure a certain volume of water is pressure-stored.


Rainwater is a source that if harnessed correctly, can provide a system of sustainable water to those where water isn't always readily available. It allows for full control of your own water supply for agricultural, home, and livestock purposes. The dry, wet, and barrel systems have different methods of capturing rain, but all include similar components: catchment, conveyance, storage, treatment, and distribution. Let's continue to educate each other on rainwater harvesting as a viable means of combating the global water crisis.

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I'm Jeremiah Castelo, the owner of World Water Reserve. I'm a writer and researcher with a particular interest in sustainability and rural living, water scarcity, and innovative water purification methods. I utilize my multimedia and communication experience in the NGO and humanitarian fields to bring light to important topics. My passion is to educate others on the reality of the global water crisis and on ways to sustain themselves and their families in the midst of it.

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