Introduction to Rainwater Harvesting


A great introduction to rainwater harvesting for those eager to learn: advantages, methods, practical uses, and components.


Updated: November 13, 2023
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Also referred to as Rainwater Catchment or Roof Water collection, Rainwater Harvesting is used in both developed and developing countries and refers to the collection and storage of run-off rainwater for domestic use.

The most common residential setup uses the roof of a house as a catchment area and the rain gutters to channel water into a storage container.



Rainwater harvesting, while relatively straightforward in concept, can also be very elaborate—involving many components and precise measurements to allow for a sustainable water system. In this article, we'll cover the basic concepts of rainwater harvesting and how one can get started.


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




Why Collect Rainwater?



rainwater tutorial

Rainwater harvesting is the primary source of water for many in developing countries where the means to access groundwater and surface water sources aren't as readily available. In Western nations, it continues to serve as an alternative means of water collection for rural residents and businesses in countries such as Germany, Singapore, Australia, Japan, China, and the United States.

Why are more and more households and businesses relying on rainwater as an alternative water source?

  • In regions with adequate rainfall, rainwater harvesting creates water independence and self-sufficiency

  • Aside from the cost of parts, rainwater is essentially a free source of water

  • Rainwater is ideal for landscape gardens and plants

  • Rainwater harvesting also helps reduce runoff and solve drainage problems

  • It can act as an excellent backup source of water

  • Rainwater harvesting systems are fairly easy to maintain


How much rainwater can be collected from a rooftop?

Many factors, including the amount of rainfall in a given region, the size of the roof, and the size of the storage area, all determine how much rain can be collected and used. But generally speaking, the following formula can be used to calculate how much rain can potentially be gathered.

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

That is, about o.6 gallons will be collected for every inch of rain per square foot of roof area.












Advantages and Practical Uses of Rainwater Harvesting


roof rain / creative commons / pxhere.com

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


It is a Great Backup Water Source

One of the main benefits of having a rainwater harvesting system is that it acts as an excellent backup source of water. Even in regions where rainfall isn't heavy, the occasional wet season can provide just enough to supplement the main water source in cases of emergency or can help reduce water bills.


Water Conservation

The EPA states that the average American family uses about 300 gallons of water daily, 70 percent of which is used indoors. The extra 30 percent is used for outdoor chores such as gardening and washing cars.

Rainwater harvesting can significantly help reduce water use for outdoor activities, significantly reducing the use of potable water drawn from groundwater or surface water sources.


Low Initial Capital, Reduced Utility Costs, and Easy Maintenance

A simple rainwater harvesting system is relatively inexpensive to install. Because homes will already have a roof and gutter system installed for catchment and conveyance, it is typically just a rain barrel that is needed.

The extra water source provided by the rainwater harvesting system will reduce usage of the primary water source and ultimately bring down the water bill. With little maintenance costs, the entire rainwater harvesting application is cost-effective and can save money in the long term.


More Regular Watering of Crops

For homes with small gardens and vegetable crops, collecting rainwater may allow better distribution of water throughout the year, and can be particularly helpful during drier times of the year.


Helps in Reducing Flooding and Erosion

Rainwater harvesting systems can be strategically placed in areas known for flooding to prevent excess water flow to vulnerable areas.


Drinking and Cooking

With the proper purification system installed, rainwater harvesting can also be a water source for indoor use, such as drinking and cooking. Rainwater is generally clean before it reaches the roof's surface. However, because rooftops can harbor harmful bacteria, rainwater should not be consumed without being purified first.


Fire Protection

Finally, it can also be used to protect against fires. This is especially true in regions suffering from water scarcity or 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



green rain barrel

Dry System

Dry systems are the more common types of rainwater harvesting systems and are generally much simpler than wet systems. The components of a dry system typically consist of a rooftop, conveyance system, and rain barrel that is close enough to the rooftop to allow direct connection from the downspout to the barrel.

As the rainwater collects onto the roof, the gutters transport the water into the rain barrel. After the rain stops, the roof and gutters dry up, and the water remains in the barrel. The water in the rain barrel can then be accessed via a spigot.







rainwater harvesting diagram

Wet System

The main difference between a dry system and a wet system is that the storage area in a wet system is further away from the roof, so it can't connect directly to the downspout. This distance will generally require the installation of additional pipes to convey the water, which is usually stored underground. When stored underground, the water stays in the pipes even after rain has ended, hence the name "wet system."







Read our related article:
The Best Rain Barrels You Can Find






Components of a Rainwater Harvesting System




rainwater school building

Catchment Area

The home's roof will be the catchment area for most residential systems. But any flat surface with a downward angle can be the catchment area. Here are several factors to consider when building a rainwater harvesting system:

  • If you plan to build your system from scratch, a metal roof is the best material because water slides down immediately, avoiding the risk of bacterial festering. Wood surfaces tend to remain wet longer, increasing the risk for microorganisms.

  • Another factor is the roof's slope, which determines how quickly water drains gutters during rainfall. Steep roofs cause water to run off quickly, making cleaning the roof easier and preventing contamination. On the other hand, less-steep roofs reduce runoff, making it more possible for contamination to remain 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 larger the size, the greater the volume of water harvested.

Use this formula to determine the amount of water that can be collected: 1" of rain x 1 sq. foot = 0.623 gallons.






rain catchment




Conveyance

This refers to the components that transfer water from the catchment area to the collection area, typically gutters and drain pipes. 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. 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 of drain pipe. The same rule should be applied to circular PVC piping.

  • For effective draining, gutters should be sloped at 1/16” per foot of length.

  • Rounded-bottom gutters reduce the risk of buildup as it allows debris to flow more freely.

  • Consider putting gutter hangers every three feet to increase support. In areas with heavy snow, gutters should be placed after every foot.

  • Paint PVC pipes to minimize UV sunlight breakdown.





rain barrel gray

Storage

Storage for rainwater can come in varying shapes, sizes, and materials, but the 55-gallon rain barrel is the most common storage unit for rainwater collection and is seen in many gardens around the United States. Rain barrels typically have an opening at the top with a mesh screen where the downspout can easily be connected and a spigot at the bottom, making the water accessible via a hose.

Food-grade polyethylene plastic is the best material for rain barrels as it would reduce the risk of corrosion and mold as seen in wooden barrels.

Some other things to consider:

  • Choose dark, opaque-colored barrels because clear or translucent tanks encourage algae growth since sunlight can penetrate the material. 

  • It is also important to consider 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 tough to detect. Below-ground storage tanks are also costlier to install, remove and maintain. Aboveground storage tanks are subject to weather conditions, including sunlight, which may affect the water quality.





pouring a glass of water

Treatment

Various substances can contaminate rainwater after making contact with the roof. Microbiological organisms, such as viruses, bacteria, and parasites, can lurk in the water due to animal feces, and chemicals such as SOCs and VOCs are often present in rooftop materials.

Larger debris such as leaves and twigs can impede the flow of water but can be remedied with a downspout diverter.

If the rainwater is intended to be used for consumption, a water purification system will need to be installed in order to make it safe for drinking.






faucet and hand

Distribution

The final stage in a rainwater harvesting system is distribution. For rain barrels used for outdoor purposes such as gardening, the distribution system will end with a hose attached to the spigot.

Those intending to use the rainwater system for indoor purposes, such as cooking and drinking, will need a more elaborate distribution system. In this case, a rainwater pump and pressure tank will be needed to transport the water into the home's plumbing system and out of the faucets.





Conclusion


Rainwater is a source that, if harnessed correctly, can provide sustainable water to those where water isn't always readily available. It allows for full control of your 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.





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References

Conserve Energy Future. (2017, February 15). What is Rainwater Harvesting? Rainwater Harvesting Methods and Techniques. Retrieved from https://www.conserve-energy-future.com/methods-of-rainwater-harvesting.php

Greener Ideal. (2018, June 5). The Homeowner's Guide to Rainwater Harvesting. Retrieved from https://greenerideal.com/guides/rainwater-harvesting/

Maxwell-Gaines, C. (2018, May 5). Rainwater Harvesting 101. Retrieved from https://www.watercache.com/education/rainwater-harvesting-101

Poindexter, J. (2016, August 30). 23 Awesome DIY Rainwater Harvesting Systems You Can Build at Home. Retrieved from https://morningchores.com/rainwater-harvesting/

UNEP. (n.d.). Examples of Rainwater Harvesting and Utilisation Around the World. Retrieved from http://www.unep.or.jp/ietc/Publications/Urban/UrbanEnv-2/9.asp

United Nations University. (2012, September 10). Former World Leaders Call on UN Security Council to Recognize Water as a Top Concern. Retrieved from https://unu.edu/media-relations/releases/un-sc-called-on-to-make-water-a-top-concern.html