Rainwater Harvesting
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Stewardship is the path to abundance. A rainwater harvesting property hydrates itself by productively planting the rain with the on-site harvest and infiltration of rainwater and runoff.

The Idea

Rainwater harvesting is an innovative alternative water supply approach anyone can use. Rainwater harvesting captures, diverts, and stores rainwater for later use.

Implementing rainwater harvesting is beneficial because it reduces demand on existing water supply, and reduces run-off, erosion, and contamination of surface water.

Rainwater can be used for nearly any purpose that requires water. These include landscape use, stormwater control, wildlife and livestock watering, in-home use, and fire protection.

A rainwater harvesting system can range in size and complexity. All systems have basic components, which include a catchment surface, conveyance system, storage, distribution, and treatment.


The storage container (cistern or tank) is often the most visible or recognizable component of a RWH system. It is where the captured rainwater is diverted and stored for later use. The main goal of the storage tank is safety. It should store water that is safe to use, and it should be secure so that children or animals cannot access the tank.

Safety: The main goal of safety for the user is good water quality. For a storage tank to maintain good quality water, it must not allow for any light penetration. Sunlight entering the tank causes algae growth. Also, the tank must be properly screened and secured so that no insects or animals can enter. Unintended access to a storage tank is a safety risk that must be prevented. The first step is making sure that the observation port (the large opening at the top) cannot be accessed by anyone. This may mean either having a tall tank or having a lock on the lid.

System Components

Catchment Area

The catchment area is the first point of contact for rainfall. For the vast majority of tank-based rainwater harvesting systems, the catchment area is the roof surface. There are some important factors about the roof to consider when planning for a RWH system:

Roof Material – The material of the roof is not as important as contaminants that may be on the roof. For landscape purposes, the common asphalt shingle will work fine. If you are starting from scratch, we recommend a metal roof because they easily shed contaminants. In all cases, it’s important to avoid wood shingles or metal flashing that contains lead.

Slope – The slope of the roof affects how quickly water will runoff during a rain event. A steep roof will shed runoff quickly and more easily clean the roof of contamination. A less-steep, flatter roof will cause the water to move more slowly, raising the potential for contamination to remain on the catchment surface.

Sizing a Catchment Area – The size of the catchment area or roof will determine how much rainwater that you can harvest. The area is based on the “footprint” of the roof.


The conveyance system usually consists of gutters and downspouts - the network of pipes that move the water from the roof surface to the storage containers. When selecting gutters and downspouts, it’s important to consider three factors: sizing, proper installation, and aesthetics.


The gutters should be sized so that they adequately move rainwater runoff from a 100-year storm event. A 100-year storm event has a 1% chance of happening every year and produces rainfall with great intensities. Places that have intense storm events, such as Corpus Christi, would need wider gutters than places with less intense rain events, like Seattle. As a general rule, gutters should be at least 5 inches wide.

Provide one square inch of downspout area for every 100 square feet of roof area. For example, a 2″ x 3″ downspout (6 square inches) can accommodate runoff from a 600 square foot roof. A 3″ x 4″ downspout (12 square inches) can accommodate runoff from a 1,200 square foot roof. The same rule can be used for circular PVC piping. In either case, a slight increase in pipe or gutter size can drastically increase the price of a system, so it’s important to size a system accurately.


For both gutters and downspouts, proper installation is critical so that they function properly and will not be a safety concern.


Gutters are a common sight and there are countless colors and designs to match the appearance of a building. Because of this, the aesthetics of gutters usually are not a concern. However, in RWH the use of PVC piping is most recommended to convey water and can be (in some opinions) be unsightly. To improve the appearance, you can paint the PVC to match the color of the building. On metal structures, you can also place the old gutter over the PVC pipe to hide it.


Treatment of rainwater ensures that the water will be safe to use by insuring the quality of the initial water captured, and both pre-storage and after-storage treatment.

Roofwater Quality Rainwater is evaporated water that condenses as part of the water cycle. Because of this, rainwater itself is generally considered clean. It is when that rainwater comes into contact with the catchment area (roof) do we have the potential for contamination. Water is the universal solvent so will pick up many contaminants that may cause harm to human health or cause damage to components of the system. The following contaminants can be treated so that they are not a threat to water quality.

Pre-Storage Treatment The goal of pre-storage treatment is to clean the rainwater runoff as much as possible before it enters the storage tank. This helps to reduce organic matter from collecting in the tank, and it reduces the amount of treatment needed after storage. Pre-storage treatment targets “large” debris that can easily be seen. It is done by two different methods: diversion or screening.

After-Storage Treatment Treatment of the water after the storage and before use is critical for both health of the users and maintenance of the system. The level of treatment will depend of the intended use of the water. For example, water used for landscape drip irrigation would not need the same level of treatment as water used for potable indoor purposes. More than one method of treatment may be used to maximize effectiveness. A common treatment-train for potable water is a set of cartridge filters followed by an ultraviolet light.

Filtration Filtration is similar to screening but on a smaller scale. There are various levels of filtration, and they are measured on a “micron” level. This measures the diameter of a particle that would be blocked by the filter. For example, a 5-micron filter would block particles in the water that are 5 microns or larger. (a micron is one-millionth of a meter). Filters can remove microorganisms, sediment, metals, and other organic matter. If larger sized filters are used, small microorganisms, such as bacteria can pass through, so a disinfection method is needed (explained next). It is important that they are checked and changed on a regular basis so that they maintain their effectiveness.

Disinfection The goal of disinfection is to destroy the microbiological organisms that have the potential to cause illness or harm. Some of the smaller microorganisms, such as bacteria, may pass through large cartridge filters so they must be targeted with disinfection. Three common disinfection methods are chlorination, ultraviolet light (UV), chlorination, and ozonation.


The distribution components include all of the piping, pumps, and other devices that move water from the storage and treatment to the point-of-use. If you are using a system for drip irrigation by gravity flow, the distribution system may simply be a length of drip tubing. However, if you plan to use the water indoor under pressure, there will be a few more key components. (Piping water into a home involves complex water pressure and should be done by a licensed plumber. If you plan to connect the rainwater to a plumbing system where municipal water already exists, you must check with local laws/ordinances to determine what type of backflow prevention devices are necessary.) One issue to keep in mind is a loss in pressure due to friction in the pipes. If you plan on moving water a good distance from your storage, it may be useful to calculate friction loss to make sure the end-use has enough pressure. To do this you need to know the length of pipe, diameter of pipe, fittings included, and flow of water leaving the storage (or pump).

Pumps and Pressure tanks are two important parts to ad distribution system. If you are using gravity flow from a rain barrel or tank, you won’t need to worry about a pump. Pumps come in several types and sizes. Choosing the right one for your situation will depend on the pressure and volume requirements that you need. For example, an indoor shower may only require 30 psi (pressure) and 2.5 gallons per minute (volume). You need to add up the volumes needs for all of your intended uses. The pressure requirement depends on the appliance/irrigation device requirements. Go to a pump supplier pressure and volume requirements to pick the ideal pump for your situation. In addition to the pump, a pressure tank may be useful, especially when the rainwater is used for indoor purposes. Indoor use often involves several small uses throughout the day (i.e. turning on the sink, flushing the toilet, doing dishes). Having water demand several times a day requires the pump to turn off and on repeatedly, which is bad for the pump. The pressure tank keeps a certain amount of water stored under pressure and only forces the pump to turn on periodically to fill the tank back up.

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