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A Rainwater Harvesting Primer
By Neal Shapiro, CPSWQ, CSM, Secretary, ARCSA

This Menard, Texas grade school educational garden features two collection tanks, three rain gardens, a large variety of native and adapted plants, and 14 raised vegetable beds.

Since rainwater harvesting is a growing enterprise, LC/DBM asked the American Rainwater Catchment Systems Association (ARCSA) to share some concepts that can help landscape contractors expand into this sector.

Societies are facing numerous water supply challenges from a variety of areas, including shortages of freshwater, climatic oscillations, and degraded water quality. One presently underused water resource is precipitation, harvested from rainwater or stormwater for direct applications.

(Editor's note: In this article rainfall and precipitation are used interchangeably to describe the resource.)

Wherever it lands, rainfall can be harvested, given an approved design by the local jurisdiction and proper treatment that meets local health codes and water quality standards.

Benefits and Uses
The primary benefits of water catchment systems are non-potable and potable water supply augmentation and water quality improvement. Onsite rainfall harvesting:

  • Retains the maximum rainfall amount possible on an annual and sustainable basis.
  • Reduces the amount of stormwater runoff entering the public right-of-way, e.g. the municipal separate storm sewer system (MS4).
  • Delivers water with an acceptable water quality to an end at a minimal energy cost compared to pumped municipal water.
  • Is less polluted and cheaper to treat than stormwater. Less pollution enters the MS4 system, and ultimately water bodies, which helps municipalities more easily comply with the federal Clean Water Act and state water quality regulations.
Drip irrigation is most practical when using rainwater for landscape irrigation. It can often be applied by gravity pressure alone or used in combination with mechanical equipment. This water can also be used for spray irrigation if enough rain can be harvested.

Four Simple Design Steps for a Rainfall Harvesting System
Contractors' ability to install an effective rainfall harvesting system that requires minimal maintenance will be essential as the rainwater-stormwater harvesting phenomenon moves from simple rain barrels to large cisterns and municipal and commercial applications. No matter how big or small the system, four essential design steps will ensure a harvesting system is effective and sustainable, and requires little annual maintenance.

In the city of Santa Monica, a 12,000-gallon rainwater harvesting cistern was installed at Virginia Avenue Park; this collected water will be highly treated for indoor flushing.

Harvested rainfall in this 12,000-gallon cistern will be used for low-volume landscape irrigation at a multi-family residential facility in the city of Santa Monica.

A 1,000 gallon cistern supplies harvested rainfall for the drip irrigation system and a birdbath in a 600 square foot educational garden pavilion in Menard, Texas.

1. Rainfall that lands on a roof, parking lot, road or any other impermeable surface, or its runoff that flows through a storm drain system, must be properly pre-filtered to some level to remove debris, oil and grease, which collect on impermeable surfaces or in storm drains, before it enters a storage tank. The quality of the pre-filter will determine the amount of maintenance the rest of the harvesting system will need. Pre-filters can be self-cleaning, which requires minimal maintenance and provides highly oxygenated water. The correct pre-filter will significantly reduce the need to clean the storage tank.

2. The system should reduce any turbulence while the oxygenated water is introduced into the tank. Every rainfall storage tank will grow a bio-film that serves as an internal ecosystem, often assisting in cleaning the storage tank water. Disturbing this bio-film by simply "dumping" the water into the tank will not allow for this micro-ecosystem to flourish. Using the proper components will allow the water to gently enter the tank from the bottom and replenish the oxygen throughout the tank.

3. A properly designed rainfall harvesting system will extract the water from just below the surface of the water in the storage tank.

4. A storage tank needs an overflow, either to the landscape or storm drain system. An overflow is as simple as allowing the water to exit the tank once it becomes full. An overflow device that uses a skimmer removes floating matter such as pollen from the tank. The overflow should have some type of device to keep fauna from entering the tank.

Providing a high quality of water to the pump and any additional purification or filtration devices required for specific applications is essential for any rainwater harvesting system. Using this simple four step process will ensure your rainwater harvesting system is of the highest quality and will require minimal maintenance, regardless of the size of the system.

The use of rainfall for potable and non-potable applications offers a significant opportunity in water management to focus on sustainable local water resources, to maximize what we have locally to supplement or displace potable, leaving more freshwater in the environment.

Neal Shapiro, supervisor of the Watershed Section of the Office of Sustainability and the Environment, City of Santa Monica, is also the Secretary of the American Rainwater Catchment System Association.

Shapiro oversees water conservation & efficiency programs, and watershed management programs with a focus on rainwater/stormwater harvesting and use/reuse in association with post-construction structural Low Impact Development BMPs.

ARCSA is a national organization dedicated to promoting rainwater catchment systems in the United States. Its mission is to promote sustainable rainwater harvesting practices to help solve potable, non-potable, stormwater, water quality and energy challenges throughout the world. More information is available at

The opinions stated in this article are of the author only and do not necessarily reflect those of the City of Santa Monica or ARCSA.

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November 22, 2019, 12:53 pm PDT

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