Program Specialist, Value Added Agriculture
Iowa State University Extension & Outreach
Harvesting rainwater has gained attention in recent years, especially after the Midwest experienced a couple seasons of wet springs followed by several dry weeks during the growing season. Rainwater harvesting systems conserve water and help with storm water management. Use of collected rainwater reduces the use of potable water for outdoor water uses, such as watering landscape plants and washing vehicles. By collecting rainwater, homeowners divert the water away from their home’s foundation and reduce the amount of water that runs off the roofs, over the land and into creeks and storm drain systems that ultimately discharged into our rivers and lakes. Some communities, such as Ames, Iowa, offer rebates to their utility customers who install rain catchment systems on their homes.
Rainwater catchment systems are not difficult or expensive to install on a home or other buildings. A system can be retro-fitted to existing buildings or integrated into new building design. The system consists of two basic components: the collection and the storage. Parts for both can be purchased from many sources, such as farm and building supply stores, as well as online.
- Collection. The gutters on building collect and move rainwater from the roofs, through a downspout to the rain barrel. A diverter can be added to the downspout to direct the water flow into the rain barrel or away from the building in the normal discharge.
- Storage. The average residential rain barrel holds 50 gallons of water. They are typically a food grade barrel made of high density polyethylene. Homeowners sometimes find used barrels of various sizes for sale from businesses or use new heavy duty garbage cans. Always use caution and know the history of the contents of the used barrels. Do not use barrels that contained anything other than food material or water; never use a barrel that contained industrial chemicals, petroleum products or pesticides. All used barrels should be scrubbed with soap and water or power washed and triple rinsed. To collect as much of the rainwater as possible, install a larger tank or connect several rain barrels together so that the overflow from a full barrel can be routed into empty barrels. An overflow outlet at the top of the barrel can be designed to channel the excess water when the barrel is full. A hose can be attached to it to direct the excess water back into the downspout system and out 10 feet from the building foundation. A spigot near the bottom of the side of the barrel fitted with hose will enable easier access. A cover on the rain barrel will reduce mosquitos and contaminants from entering the water. Set the water collection tank on a solid level surface. It is a good idea to elevate the tank a few feet off the ground so that a watering can or bucket can fit under the spigot. The extra elevation will improve water flow from the bottom spigot. For information and instructions on building a rain barrel, refer to Rutgers’ Factsheet E329, Rain Barrels Part I: How to Build a Rain Barrel.
How much water can you capture from a roof?
First you need to calculate the amount of space from which you will be collecting rainwater, sometimes referred to as the “footprint” of your roof, like a “bird’s eye”, straight overhead view. You can use the dimensions of your house, adding a bit for overhang. You will also need to estimate the runoff coefficient. The runoff coefficient accounts for the water loss resulting from evaporation and minor infiltration; most impervious surfaces, such as roofs or nonporous pavement can lose 5% to 20% of the rain falling on them. For general purposes, it is estimated that the runoff coefficient for asphalt roofs is .90, meaning it will lose about 10%.
When you calculate the amount of rainfall from one rain event, put the amount in feet, not inches. For example a half an inch of rain is .04 ft.
The equation to calculate the approximate amount of rainfall from your roof is:
| _____ft2 × ___ft. × 7.48 gal/ft3 × .9 = ___ gallons
(catchment area) × (rainfall) × 7.48 gal/ft3 × (runoff coefficient) = gallons.
For example, you are putting up a rainwater collect and harvesting system on detached double garage (22 x 24) - 528 ft2. You need to add approximately 92 ft2 for overhang, so the foot print is approximately 620 ft2. If it rains ½ inch, you will collect approximately 167 gallons of water from the roof of your garage.
620 ft2 × .04 ft × 7.48 gal/ft3 × .9 = 167 gallons
Harvested rainwater can be safely used for non-potable activities, such as yard and landscape irrigation, watering potted plants and washing vehicles. Collected rainwater should NOT be used for drinking or other potable purposes if it is not filtered and disinfected before use. Gardeners often collect water in a rain barrel with little to no protection from the roof’s “first flush” of runoff. The first flush water is the initial rainwater that drains off an impervious surface, such as a driveway, parking lot or roof and has shown to have the highest levels of contaminants. Primary substances of concern in roof runoff include heavy metals, polycyclic aromatic hydrocarbons (PAH’s) microbes, pathogens and pesticides. Birds, insects, and small mammals deposit fecal matter on rooftops and in gutters, contributing to bacteria and pathogens in the runoff water. On metal roofs, water can react with the roof surface and absorb metals, such as zinc, copper, and aluminum. Roofs with wooden or asphalt shingles can increase concentrations of chemicals used for waterproofing/weathering treatments. The question is whether these levels are high enough to be of concern for a gardener who uses a rain barrel to water a vegetable garden. Results of a 2011 study conducted in New Jersey showed that overall the water quality of the rain barrel water was very good with regards to heavy metals and posed minimal risk for irrigating a vegetable garden. A low percentage of samples in the study exceeded the irrigation limits for E. coli. However, a certain amount of caution should be taken when using harvested water to water a vegetable or herb garden to reduce the risk of exposure to a harmful contaminants, like E. coli. The best practices when using rainwater for food crops are:
- Use of a drip irrigation is the best way to utilized harvested rainwater in a food garden to avoid getting water on the plant itself.
- Always wash produce under cool running water before consuming.
- Treat the tank monthly to reduce the risks caused by pathogens. (Refer to Rutgers’ Factsheet FS1218, Rain Barrels Part IV: Testing and Applying Harvested Water to Irrigate a Vegetable Garden, for more information on treating collected rainwater.)
References and resources
Bakacs, Michele, Mike Haberland, and Amy Boyajian. 2010. Rain Barrels Part I: How to Build a Rain Barrel. Factsheet E329. Rutgers.
Bakacs, Michele, Mike Haberland, and Steve Yergeau. 2013. Rain Barrels Part IV: Testing and Applying Harvested Water to Irrigate a Vegetable Garden. Factsheet FS1218. Rutgers.
Water Quality of Rooftop Runoff: Implications for Residential Water Harvesting Systems. North Carolina Cooperative Extension.