The ancient practice of rainwater harvesting is widely used throughout the world and is gaining popularity in commercial and residential applications in the United States. Rainwater harvesting is the collection and storage of rainwater from impervious surfaces, typically a roof area, for use at a later time. Rainwater harvesting systems can also provide stormwater, erosion and flood control benefits.
Capturing and storing rainwater is a practical water conservation practice due to the sheer volume of water that flows off of roof surfaces during a rain event. For every one-inch of rain and every one-square foot of roof area, the potential exists to capture 0.623 of a gallon of water (Mechell 2010). To put this in perspective, for a one-inch rainfall, 1000 square feet of roof area can capture over 600 gallons of water. This harvested water can be used in non-potable ways including irrigation of landscaped beds, butterfly gardens, container plants, and vegetable and fruit gardens. With additional design considerations, water purification features and cost, a rainwater harvesting system can provide water for flushing toilets, taking showers and even for drinking. The recommendations in this factsheet focus on the more commonly found, non-potable rainwater harvesting systems.
In South Carolina, there is an increased interest in the use of harvested rainwater to irrigate fruits, vegetables, and other edibles. In 2014, as part of the Ashley Cooper Stormwater Education Consortium’s rainwater harvesting program evaluation, participants in the Charleston, South Carolina Tri-county area, were asked to indicate the primary use of their rainwater harvesting system. Of the 67 respondents, 43% indicated that watering their vegetable garden and/or edibles was their primary use of harvested rainwater (Wooten et al, 2014 unpublished data).
To ensure human health and safety, additional design, maintenance and application strategies should be employed when utilizing non-potable rainwater harvesting systems to irrigate fruits, vegetables, and other edibles.
Pollutants, including heavy metals, bacteria, pathogens, herbicides, and pesticides, can accumulate on rooftops and can potentially be transported to the rain barrel or cistern following storm events. The sources of these materials are numerous and include atmospheric deposition, animal waste, roof materials, shingle treatment, and others.
Rainwater Harvesting System Design & Maintenance for Optimal Use on Edibles
All rainwater harvesting systems are comprised of the following:
- A catchment area (roof) where rainfall is collected;
- A conveyance system (gutters, downspout, rain chain, or sheet flow) which helps to transport water;
- A storage system (rain barrel or cistern), which contains the water for later use.
Though rain barrels and cisterns differ in size and shape, both are rainwater-harvesting systems and the main components remain consistent.
Rain barrels are commonly used to harvest rainwater as they are typically easy to find and relatively inexpensive. Barrels are used for small-scale rainwater harvesting, and sized to hold less than 100 gallons. Rainwater is directed to the barrel using a downspout, rain chain, or simple sheet flow from the roof. The point where water enters the rain barrel, typically the top, must be screened to prevent mosquito breeding and to restrict debris such as leaves and twigs, and small animals from entering the barrel.
Rainwater can be harvested on a larger scale through the use of cisterns. A cistern is a storage tank that has more than 100-gallon storage capacity. Cisterns come in all shapes and sizes, can be made of a diverse array of materials, and may be installed either above or below ground.
Cisterns tend to be more complex than rain barrels as they typically employ some type of filtration system to remove pollutants before they reach the tank. This pre-filtration is done using a number of methods including downspout, basket, and vortex filters. In addition, a first flush diverter may be used to direct the initial rainfall coming off of a roof, away from the tank; this is the dirtiest water known as the “first flush.” The first flush diverter uses conveyance piping designed to divert one gallon of water per one hundred square feet of roof area and can be used to remove small contaminants such as pollen and bird feces. Maintaining the first flush diverter and downspout, basket and vortex pre-filtration devices is critical to assuring the integrity of the tank, as cisterns cannot be cleaned as easily as rain barrels.
Regardless of size or system design, if a rainwater harvesting system is poorly maintained, water quality will degrade. In any system that is being used to irrigate edibles, maintenance and use is integral to ensuring optimal water quality. See Table I for system design and maintenance recommendations.
A Note on Harvested Condensate Water
Capturing the condensate from a cooling coil, such as those found on air conditioning units, refrigerators or ice machines, is another form of harvesting water. Although this water can be safely utilized on bog gardens, wetland gardens, and other moisture loving areas; the use of harvested condensate water on edibles is not recommended. The cooling coils can be breeding grounds for bacteria, such as Legionella; therefore, it is best to avoid this potential risk and only use harvested condensate in non-edible applications where human contact is limited.
Recommended Practices for Application of Harvested Rainwater on Edibles
To safely utilize harvested rainwater on edibles, a few simple considerations and application practices are recommended. See Table II.
Harvesting rainwater is a fun way to conserve water and help protect downstream water quality. Although this water is not treated, and therefore not drinkable, it may still be safely applied to edibles. By following the recommended application, design, and maintenance practices, home gardeners can experience the satisfaction of irrigating home vegetable gardens with harvested rainwater. To learn more, visit clemson.edu/carolinaclear.
Table I: Recommended Practices: Design and Maintenance for Rainwater Harvesting Systems Used to Irrigate Edibles
|Cistern or Rain Barrel
Design & Maintenance
|Prevent exposure to potentially harmful chemicals.
|When building your own rain barrel, assure the barrel is food grade and never transported chemicals.
|If concerns exist over the quality of water in a rain barrel, disinfect using chlorine application recommendations from Rutgers University. Note: chlorine will kill beneficial microbes in the soil and should be used sparingly and only when necessary.
|Before irrigating a vegetable garden, add unscented bleach with a 5% to 6% chlorine solution, at a rate of 1/8 teaspoon per gallon to the harvested water. After treatment, let water stand for 24 hours before applying (Bakacs et al. 2013).
|Prevent accidental ingestion of untreated water.
|Label spigot with “Do Not Drink” or other non-potable water message. Use hang tag or sticker. Make sure language is culturally appropriate.
|Prevent insects and small animals from entering the tank.
|Assure screening, point of entry and top are secured.
|Remove leaves, twigs, sediment and other forms of debris that may contaminant water.
|Clear debris from gutters and downspouts (at least twice a year).
|Clear debris from screened inlet (at least four times a year).
|Clear debris from downspout filter, basket filter, vortex filter, and/or first flush diverter (at least four times a year).
|Healthy soils have greater ability to offset risk.
|Maintain healthy soils by adding compost and other amendments rich in organic content.
|Regular use of harvested rainwater will prevent stagnation and associated water quality degradation.
|Use the water!
Table II: Recommended Practices: Application of Harvested Rainwater on Edibles
|Allow time for solar treatment
|Apply water in the morning.
|Do not apply water the day of harvest.
|Decrease direct contact of potential pollutants in harvested rainwater with plants and fruit.
|Do not apply water directly to plant (foliar application), instead apply to soil around the plant base.
|Apply with drip irrigation or another method that applies water to soil (ex. watering can).
|Prevent ingestion of harmful materials such as bacteria.
|After harvest, wash fruits, vegetables and herbs thoroughly with cool, potable/treated water. Store at proper temperature.
|After hands or other body parts have had direct contact with harvested rainwater, wash thoroughly with soap and potable/treated water.
Originally published 09/15