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This page describes some of the basics behind designing Rainwater Catchment Systems. It is a collection of resources pulled from similar projects that are documented throughout Appropedia, and contains helpful resources for those interested in learning more. Please feel free to add even more helpful content!

Rainwater Catchment[edit | edit source]

A rainwater catchment system is a system that collects, filters, and stores rainwater for community or household use.[1]

Rainwater catchment offers significant advantages over other sources of water (e.g. surface water and groundwater). Water that has reached the ground has had an opportunity to entrain or dissolve constituents from soils, rocks, or other surfaces, which can contribute qualities that are undesirable. The most undesirable qualities would include pathogens, carcinogens, or other contaminants. Rainwater, by contrast, is subjected only to contaminants that it reaches as it descends through the atmosphere and as it flows on the collection surface. As opposed to other water treatment methods, hydrologic, solar-driven evaporation requires no funding, but rainwater supply is susceptible to seasonal and climactic variations in quantity and frequency.[2]

In most urban areas, population is increasing rapidly and the issue of supplying adequate water to meet societal needs and to ensure equity in access to water is one of the most urgent and significant challenges faced by decision-makers.[3]

Rainfall[edit | edit source]

One of the first steps to designing a Rainwater Catchment system is to determine the amount of rainfall that will fall in the desired location. A good practice is to determine the average precipitation depth in inches for each month. This data can typically be found from sources such as the PF Data Server from NOAHH.

Catchment Potential[edit | edit source]

Besides the amount of rainfall that is available to be collected, the size of the collection surface will determine how much of the available rainfall will be collected for usage. Sites such as the Rainwater Collection Calculator and Basic Rainwater Collection Calculations can aide the design process.

Roofing[edit | edit source]

The type of construction material is important for a rainwater catchment system. Depending on what material is used for the roof, the collected water can be contaminated by carcinogens.[1] The roofing materials used in a rainwater catchment system should be chemically inert to keep contaminants out of the system. In addition to the roof material itself, the climate can influence the contaminate runoff depending on the material. While some roofing materials may be more efficient than others, none can allow for the collected rainwater to be consumed by humans without a filtration system.

Types of Roofing Material[edit | edit source]

The following are brief descriptions of a few common roofing materials and their benefits and drawbacks when being considered for Rainwater Catchment Projects.

Thatch: This type of roofing material generally will yield some of the lowest volumes of rainfall when compared to other roofing materials due to infiltration. Thatch can also add organic materials and other contaminants to the captured rainfall, making the water quality less than ideal for human consumption without additional water treatment.

Wood: Wood roofing captures more rainfall runoff than the thatch roofing, however there can still be minor abstractions due to infiltration into the wood grain. Wood roofing also may contain less contaminants that could be picked up in the runoff.

Green: Green roofs are increasingly being installed for their storm water retention characteristics, and both green and cool roofs (made of reflective roofing material) are being installed for their energy conservation potential. However, neither has been highly considered for rainwater catchment as the rainfall in part is utilized by the roofing and the optimal rainfall volume cannot be captured in full.[4]

Concrete: Concrete roofs have the potential to catch larger amounts of rainfall than the aforementioned roofing materials. However, the rainfall runoff can contain contaminants that originate from the concrete.

Metal: Metal roofs are commonly recommended for rainwater harvesting applications, because rainwater harvested from metal roofs tends to have lower concentrations of fecal indicator bacteria as compared to other roofing materials.[4] Roofs made with metal also tend to have little to no losses or abstractions and as a result can catch more rainfall than other materials such as thatching or wood.

Water Quality[edit | edit source]

There are three criteria to having acceptable quality drinking water.

  • No fecal bacteria
  • No harmful chemicals
  • No bad taste or smell

Rainwater catchment systems have commonly found contaminants due to the location and isolation of rooftops.

Common contaminants of rainwater catchment systems[5][6]
Contaminant Source Risk of entering Rain Tank
Dust and Ash Surrounding dirt and vegetation
Volcanic activity
Moderate:Can be minimized by regular roof and gutter maintenance and use of a first-flush device
Pathogenic Bacteria Bird and other animal droppings on roof, attached to dust Moderate:Bacteria may be attached to dust or in animal droppings falling on the roof. Can be minimized by use of a first-flush device and good roof and tank maintenance.
Heavy metals Dust, particularly in urban and industrialized areas, roof materials Low:Unless downwind of industrial activity such as a metal smelter and/or rainfall is very acidic (this may occur in volcanic islands)
Other inorganic
Contaminants(e.g. salt from seaspray)
Seaspray, certain industrial discharges to air, use of unsuitable tank and/or roof materials Low:Unless very close to the ocean or downwind of large-scale industrial activity
Mosquito Larvae Mosquitos laying eggs in guttering and/or tank Moderate:If tank inlet is screened and there are no gaps, risks can be minimized.

Filtration[edit | edit source]

Filtration is necessary not only for the quality of the water but also for the longevity of the rainwater catchment system. The first instance of necessary filtration occurs right after the rainfall makes contact with the piping that leads it away to the storage. Detritus such as leaves, twigs, and other large objects need to be removed in order to prevent clogging within the system. Common methods for the removal of such objects are grit screens that can be manually cleaned with ease. Depending on the desired uses of the water, additional filtration may be necessary.

When considering water usage for human consumption, the following types of filtration must be considered in order to remove smaller particles and pathogens. [6]

Carbon/Activated Carbon: "Activated carbon chemically bonds with and removes some contaminants in water filtered through it. Carbon filters vary greatly in effectiveness: Some just remove chlorine and improve taste and odor, while others remove a wide range of contaminants including asbestos, lead, mercury and volatile organic compounds (VOCs). However, activated carbon cannot effectively remove common “inorganic” pollutants such as arsenic, fluoride, hexavalent chromium, nitrate and perchlorate. Generally, carbon filters come in two forms, carbon block and granulated activated carbon."[7]

Carbon Block: "Carbon block filters contain pulverized activated carbon that is shaped into blocks under high pressure. They are typically more effective than granulated activated carbon filters because they have more surface area. Their effectiveness depends in part on how quickly water flows through."[7]

Granulated Activated Carbon: "These filters contain fine grains of activated carbon. They are typically less effective than carbon block filters because they have a smaller surface area of activated carbon. Their effectiveness also depends on how quickly water flows through."[7]

Ceramic: "Ceramic filters have very small holes throughout the material that block solid contaminants such as cysts and sediments. They do not remove chemical contaminants."[7]

Deionization: "These filters use an ion exchange process that removes mineral salts and other electrically charged molecules (ions) from water. The process cannot remove non-ionic contaminants (including trihalomethanes and other common volatile organic compounds) or microorganisms. EWG’s water filter guide does not include any filters based on this technology."[7]

Distillation: "This technology heats water enough to vaporize it and then condenses the steam back into water. The process removes minerals, many bacteria and viruses and chemicals that have a higher boiling point than water. It cannot remove chlorine, trihalomethanes or volatile organic chemicals (VOCs). EWG’s water filter guide does not include any filters based on this technology."[7]

Ion Exchange: "This technology passes water over a resin that replaces undesirable ions with others that are more desirable. One common application is water softening, which replaces calcium and magnesium with sodium. The resin must be periodically “recharged” with replacement ions."[7]

Mechanical Filters: "Like ceramic filters, these filters are riddled with small holes that remove contaminants such as cysts and sediments. They are often used in conjunction with other kinds of technologies, but sometimes are used alone. They cannot remove chemical contaminants."[7]

Ozone: "Ozone kills bacteria and other microorganisms and is often used in conjunction with other filtering technologies. It is not effective in removing chemical contaminants. EWG’s water filter guide does not include any filters based on this technology."[7]

Reverse Osmosis: "This process pushes water through a semi-permeable membrane that blocks particles larger than water molecules. Reverse osmosis can remove many contaminants not removed by activated carbon, including arsenic, fluoride, hexavalent chromium, nitrates and perchlorate. However, reverse osmosis does not remove chlorine, trihalomethanes or volatile organic chemicals (VOCs). Many reverse osmosis systems include an activated carbon component than can remove these other contaminants. Quality can vary tremendously in both the membrane system and the carbon filter typically used with it. Consumers should also be aware that reverse osmosis filters use 3-to-20 times more water than they produce. Because they waste quite a bit of water, they are best used for drinking and cooking water only."[7]

UV (ultraviolet): "These systems use ultraviolet light to kill bacteria and other microorganisms. They cannot remove chemical contaminants. EWG’s water filter guide does not include any filters based on this technology." [7]

Gutters, Pipes, and Downspouts[edit | edit source]

The gutter is used to transport water from the catchment surface to the first flush. The efficiency with which the gutter collects runoff water from the roof is dependent on the placement of the gutter. If the slope of the gutter is too steep, some of the runoff water will flow over the gutter. If the slope of the gutter is leveled, not all of the runoff water will flow out of the gutter into primary screening. Water left to set in the gutter will form stagnate water that will pose a risk of producing mosquito larva and algae growth. It is a useful precaution to place an end cap on the gutter so that rodents and other small animals are unable to enter the system.[8]

First Flush[edit | edit source]

Rainwater catchment systems often include first flush systems. A first flush system is a component of a rainwater catchment system that preserves water quality at the expense of water quantity. These systems work by wasting or diverting the first volume of collected rainwater because of the elevated amounts of solids or contaminants entrained in the collected rainwater. When the first wash of the collection surface by rainfall is diverted or wasted with a first flush system, the collected rainwater contains less solids or other contaminants. Usually, rainwater catchment systems equipped with first flush systems provided safe drinking water as well (Zhu et al., 2004).[9][10]

First flush systems are comprised of a storage volume, a connection to the rest of the rainwater catchment system, and a way to drain the water to be wasted (e.g. a hole in the bottom of the storage volume). More advanced systems contain an apparatus that reduces mixing of the wasted water with the water to be filtered (e.g. a floating ball valve). As rain hits the collection surface, accumulated particles (contaminants) are dissolved or otherwise entrained in the flow of collected rainwater. These are then transported to the first flush storage volume, which fills with the first collected rainwater. As more rain hits the collection surface, less particles are available to be transported by the flow, which means the water will contain less contaminants.After the first flush volume is filled, the subsequent rainwater will bypass these systems and can be collected for use. [10]

Storage[edit | edit source]

Figure 1: A corrugated iron rainwater tank in Uganda

The water storage tank usually represents the biggest capital investment element of a rainwater catchment system. It therefore usually requires careful design – to provide optimal storage capacity while keeping the cost as low as possible.[11]

The choice of storage system will depend on a number of technical and economic considerations listed below.

  • Space availability
  • Options available locally
  • Local traditions for water storage
  • Cost – of purchasing new tank
  • Cost – of materials and labour for construction
  • Materials and skills available locally
  • Ground conditions
  • Desired quality and usage of water
  • Style of rainwater catchment – whether the system will provide total or partial water supply


Storage systems can either be underground or above ground, and can be anything from a cistern, to a tank or an old oil drum. Container materials may include (but are not limited to) clay, fiberglass, concrete, plastic, or galvanized steel. To avoid algae growth, the tank should be stored away from direct sunlight and should not be transparent. Placement of tank is also important, in order to take advantage of gravitational flow, it should be placed in an elevated area. In order to reduce the distance traveled by the captured water, which helps avoid contamination, tanks should be placed close to the area of use. The storage systems interior should also be lined in order to make sure that the storage tank is watertight. (City of San Diego, 2007)[12][13]

References[edit | edit source]

  1. 1.0 1.1 Palau Catchment Manual http://ag.arizona.edu/region9wq/pdf/Palau_catchmentmanual.pdf
  2. Shrader, J. A. (2012). La Yuca Rainwater Catchment [PDF]. Cal Poly Humboldt. Page 4 - Media:Dominicana2012N (reduced size).pdf
  3. Rainwater Harvesting and Utilization Resource
  4. 4.0 4.1 Mendez, C.B, Klenzendor J.B, Afshar B.R, Simmons M.T, Barretta, M.E, Kinneya K.A, Kirisits M.J.(15 December, 2010). The Effect of Roofing Material on the Quality of Harvested Rainwater. Department of Civil, Architectural, and Environmental Engineering, The University of Texas at Austin, TX.
  5. WATER QUALITY OF RAINWATER HARVESTING SYSTEMS http://www.pacificwater.org/userfiles/file/MR0579.pdf
  6. 6.0 6.1 https://www.appropedia.org/Practivistas_rainwater_2014/Literature_Review#Roofing_Basics
  7. 7.00 7.01 7.02 7.03 7.04 7.05 7.06 7.07 7.08 7.09 7.10 THE NITTY GRITTY OF FILTER TYPES AND TECHNOLOGIES. (2013, February 27). . Retrieved June 8, 2014, from http://web.archive.org/web/20150120211400/http://www.ewg.org:80/report/ewgs-water-filter-buying-guide/filter-technology
  8. https://www.appropedia.org/La_Yuca_rainwater_catchment_2012#Literature_Review
  9. Kun Zhu, Linus Zhang, William Hart, Mancang Liu, and Hui Chen. Quality issues in harvested rainwater in arid and semi-arid Loess Plateau of northern China. Journal of Arid Environments, 57(4):487 { 505, 2004. ISSN 0140-1963. URL http://www.sciencedirect.com/science/article/pii/S0140196303001186
  10. 10.0 10.1 Shrader, J. A. (2012). La Yuca Rainwater Catchment [PDF]. Cal Poly Humboldt. Page 5
  11. 11.0 11.1 https://www.appropedia.org/Original:Rainwater_harvesting
  12. City of San Diego (2007). Water: A Branch of Public Utilities. Retrieved from http://web.archive.org/web/20180903164253/https://www.sandiego.gov/water/conservation/rainwater.shtml
  13. Governor, Bob T., and Nick J. Baird, MD. "Plans for Developing a Rainwater Cistern or Hauled Water Supply." A Markov-Weibull Rain-sum Model for Designing Rain Water Catchment Systems 10.2 (1996): 147-62. Springer Link. Web. 08 June 2014.
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