Sustainable Urban Design: Educator’s Toolkit for Project-Based Learning, page 16 of 20

A rainwater harvesting set-up at W.B. Saul High School in Philadelphia.
Setting up rainwater harvesting tanks at W.B. Saul High School in Philadelphia. © Andrew Kahl/TNC

Green Stormwater Infrastructure: Rainwater Harvesting

Harvesting, or collecting, rainwater is a great way to supplement municipal or wellwater for a variety of uses. One of the most obvious uses is for garden irrigation, but because rainwater is collected before it can pick up ground pollutants, it can also be used for washing, providing water for animals, and in some cases even drinking and cooking (we recommend consultation with an expert if you plan to consume rainwater in any way)! It can also come in handy in an emergency, when drought or some other circumstance makes other water scarce. However you hope to use harvested rainwater, you can save both water and energy by harvesting it—think about a well pump or municipal sewer systems—and you can reduce the load on natural systems.

Harvesting systems can include simply collecting the rain in an open container, though without any covers or screens this water will almost certainly not be potable, as bacteria and algae may collect in it and it may host mosquito larva. It may also be cumbersome to distribute the water. A rain barrel solves these problems with a cover, screens to keep out insects, and a spout to which you can attach a hose. You may want to have students study the rainfall patterns in your area to determine what system will work best and how you will use the stored water. Students can also construct a rain barrel themselves.

A rainwater harvesting system and rain garden at W.B. Saul High School
A rainwater harvesting system and rain garden at W.B. Saul High School. © Andrew Kahl/TNC

Get Started

  1. Check to see if your state or municipality has regulations about harvesting rainwater.
  2. Calculate the average rainfall for your area to get a sense of how much you can expect to collect.
    • Rainfall Harvest Calculator (Gardeners)
    • Alternatively, have students determine the roof catchment area and amount of water that can be collected from a rooftop by using the equations found on the Ecology Action Center's Rain Barrel page.
  3. Select a location. Confirm that the roof does not have asbestos shingles. Determine how you will use the water before designing your system.
  4. Identify what size and type of barrel will work for your design. You'll also need to keep your barrel covered to protect it from mosquitoes. Consider using a dark barrel to inhibit algal growth.
  5. Identify a downspout where you would like to set-up the barrel(s). Note that the barrel must be placed on a level surface and it's recommended to place the barrel on a platform at least 2 feet off the ground. The higher the barrel, the better the water pressure from the outflow. Some rain barrel set-ups involve using a pump, but this is often not necessary.
  6. Be sure to investigate the safety needs of your site with respect to the barrel placement. Full water barrels are heavy (50 gallons weighs 400 pounds) and can tip over if not placed on a flat surface. If the barrels are on school grounds, it's imperative to make sure there is no chance of tipping.
  7. Rain barrels should be placed on top of surfaces like gravel that will allow water to drain. Additionally, you'll need to plan for where barrel overflow will be directed. It is recommended that overflow be directed to a pervious area like a lawn or garden area a minimum of 4 feet away from the building's foundation. Check if there are additional siting requirements for your location.
  8. Consider having students keep track of daily rainfall amounts by taking measurements using a rain gauge or by going online. Students can use their knowledge of the roof catchment area to determine how much rainfall they have harvested over the course of the school year. They can share this information with the community through presentations or infographics and explain how they have put the harvested water to good use! For example, they can describe how many gallons of water have been redirected from city storm drains to gardens and other areas.

Common Challenges

  • States and municipalities may have regulations about rainwater harvesting.
  • You may not be able to depend on having a set amount of water available at a given time, since amounts of rainwater will vary.
  • You will want to make sure to have a method for keeping mosquitoes from breeding in the water.

Maintenance Schedule and Other Notes

  • If you are using the water on an edible plant garden, consider releasing some of the water from the bottom of the barrel into an adjacent lawn or non-edible garden to "flush" the system before using it on your edible plants.
  • Make sure to keep the downspout feeding the rain barrel free from debris.
  • If you live in an area where temperatures are below freezing in the winter, the system will need to be emptied and retired for the season.
  • If you are cooking with or drinking the water, you will need to have it tested on a regular basis. We recommend you consult an expert if you want the water to be potable.

Related Lessons and Classroom Integration

Additional Project Resources