Rainwater harvesting is a technique used for collecting, storing and using rainwater for landscape irrigation and other uses. The rainwater is collected from various hard surfaces such as rooftops and/or other manmade aboveground hard surfaces.
Harvesting rain is a practice that has been around for centuries. Cisterns and other rain harvesting systems are widely used in Europe, Australia, India, the Bahamas and countless remote countries - many who depend solely on rain for day to day life.
When water supply becomes limited, practical solutions can fill the gap. Rainwater harvesting systems provide distributed Stormwater runoff containment while simultaneously storing water which can be used for irrigation, flushing toilets, washing clothes, washing cars, pressure washing, or it can be purified for use as everyday drinking water. You can really take this as far as you want. But it all comes down to various filtration and processing systems.
Rainwater Harvesting Basic Components
Rainwater systems come in all shapes and sizes, from simple catchment system under a downspout to large above and/or underground cisterns with complex filtration systems that can store thousands of gallons of water. Most rainwater collection systems are comprised of the same basic components.
- Catchment surface - rooftop or another raised solid surface. The best catchment systems have hard, smooth surfaces such as metal roofs or concrete areas. The amount of water harvested depends on the quantity of rainfall, the size of the surface and the slope of the catchment area.
- Gutters and downspouts - also known as distribution systems. They channel water from the catchment area to a holding container such as a barrel, cistern or planted area
- Leaf screens - a screen that removes or catches debris.
- Roof washers - a device that diverts the "first flush" of rain before it enters the storage tank. Most rainwater suppliers recommend that the "first flush" of water is diverted to an outside area of the storage system since the catchment surface may accumulate bird droppings, debris and other pollution.
- Storage tanks - In general, the storage tank is the most expensive component of a rainwater harvesting system. There are numerous types and styles of storage tanks available. Storage can be aboveground or underground. Storage containers can be made from galvanized steel, wood, concrete, clay, plastic, fiberglass, polyethylene, masonry and more. Examples of aboveground storage include cisterns, barrels, tanks, garbage cans, aboveground swimming pools and more. Storage tank prices vary based on variables such as size, material and complexity. To inhibit the growth of algae, storage tanks should be opaque and preferably placed away from direct sunlight. The tanks should also be placed close to the area of use and supply line to reduce the distance over which the water is delivered. Also consider placing the storage on an elevated area to take advantage of gravity flow. The tank should always be placed on a stable and level area to prevent it from leaning and possibly collapsing.
- Delivery systems - gravity-fed or pumped to the landscape or other end-use areas.
- Purification/treatment system - needed for potable systems to make the water safe for human consumption. Please check with your local health department for information on filtration systems and certification requirements.
- Makes use of a natural resource and reduces flooding, Stormwater runoff, erosion and contamination of surface water with pesticides, sediment, metals and fertilizers.
- Reduces the need for imported water
- Excellent source of water for landscape irrigation, with no chemicals such as fluoride and chlorine, and no dissolved salts and minerals from the soil.
- Home systems can be relatively simple to install and operate May reduce your water bill.
- Promotes both water and energy conservation
- No filtration system required for landscape irrigation
- Limited and uncertain local rainfall
- Can be costly to install - rainwater storage and delivery systems can cost between $200 to $2,000+ depending on the size and sophistication of the system
- The payback period varies depending on the size of storage and complexity of the system
- Can take considerable amount of time to "pay for itself"
- Requires some technical skills to install and provide regular maintenance
- If not installed correctly, may attract mosquitoes (i.e.; West Nile Disease and other waterborne illnesses)
- Certain roof types may seep chemicals, pesticides, and other pollutants into the water that can harm the plants
- Rainwater collected during the first rain season is generally not needed by plants until the dry season. Once catchment is full, cannot take advantage of future rains
Safety Considerations: Parents/adults - Please make sure that children and pets do not climb on the storage systems and accidentally fall into the storage tank. Storage tanks should have locking lids and/or bars that keep the children and pets out!
Frequently Asked Questions
Why is rainwater harvesting important?
By harvesting rainwater, we can put it to good use. Captured rainwater can be used for crops, greenhouses, livestock and irrigating landscape. Or it can be used indoors for flushing toilets and washing laundry. Capturing rainwater can reduce the flow of Stormwater to urban streets, reducing flood risks. Also, capturing rainwater reduces the amount of pollutants which are picked up by Stormwater flows through streets and storm drains. Every gallon that is collected and utilized saves a gallon of precious drinking water.
Can I use the harvested rainwater indoors?
Yes, however, in order for collected rainwater to be used indoors, even for toilet flushing, the California Plumbing Code requires special filtering, plumbing, and permits.
What are the major components of a rainwater harvesting system?
The major components typically include the collection surface, gutters, downspouts, pre-filtration systems or first-flush devices, storage tanks and distribution systems — which can include sanitization (depending on what you will be using the rainwater for).
How much rainwater can I collect from my roof?
One inch of rain falling on 1,000 square feet of rooftop typically produces nearly 600 gallons of water. The following calculation can help determine the amount of rainwater that can be harvested from a given roof:
SUPPLY (gallons) = INCHES OF RAINFALL X 0.623 X CATCHMENT AREA (sq. ft of roof) X .90 (runoff coefficient for a metal or asphalt roof)
Can rainwater be harvested on a large scale?
Yes! Modern tanks used for residential and commercial applications are available in all sizes and range from the 50-gallon barrels that most people are familiar with to cisterns that hold up to one million gallons of water.
Can I leave my rain barrel valve closed during the winter?
Your rain barrel(s) will likely fill up and overflow in 1-2 precipitation events. Including rain gardens and vegetated swales in your landscape will allow the water to remain on-site, minimize runoff, and allow for water to slowly infiltrate the soil. Therefore, to protect our creeks as well as the storm drains, leave the valve open during the winter when you do not need the water. Near the end of March (or at the end of the rainy season), close the valve and use the last rainstorms to fill your barrels for storage.
What is the status of national codes that support rainwater harvesting?
Rainwater harvesting policies vary from state to state but there are no known local laws restricting rainwater harvesting at this time.
California State Assembly Bill 1750 (2012) enacted the Rainwater Capture Act of 2012. The bill authorizes residential, commercial and governmental landowners to install, maintain, and operate rain barrel systems and rainwater capture systems for specified purposes, provided that the systems comply with specified requirements. A landscape contractor working within the classification of his or her license would be authorized enter into a prime contract for the construction of a rainwater capture system if the system is used exclusively for landscape irrigation.
The American Rainwater Catchment Systems Association Design and Installation Standards were incorporated into the IAPMO Green Plumbing & Mechanical Code Supplement of the Uniform Plumbing Code (UPC), and the International Green Construction Code of the International Plumbing Code (IPC). The state of California incorporated these new standards into the California Plumbing Code and they became effective Jan. 1, 2014. The new code covers rainwater catchment systems intended to supply uses such as irrigation, water features (i.e., fountains, ponds, and waterfalls), toilet flushing, urinals, floor trap primers, industrial processes, and cooling tower makeup. No permit is required for exterior catchment systems used for subsurface or overhead irrigation up to 360 gallons.
Examples of Rainwater Harvesting Systems
Rain gutters and downspout with debris filter
Storage tank shown with an inlet pipe and overflow tube
Inlet pipe with debris filter
Serial connected rain barrels
Rain barrel overflow tube
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