Difference Between Rainwater Harvesting & Stormwater Management

Rainwater Harvesting (RWH)

Rainwater harvesting is rapidly becoming a standard process for many homeowners, institutions, and industrial plants to collect a natural resource and use it to reduce the amount and cost of purchased treated water. Rainwater reuse entails storing stormwater runoff and then using it as a source of irrigation water. There are many design considerations to examine to ensure success, as well as operation and maintenance considerations.

Stormwater Management

According to the United States Environmental Protection Agency (EPA), stormwater management is the effort to reduce runoff of rainwater or melted snow into streets, lawns, and other sites and improve water quality, according to the United States Environmental Protection Agency (EPA). Often rainwater harvesting for irrigation is not practical yet, and stormwater management is essential to reduce pollutants moving into groundwater. Stormwater often carries bacteria, chemicals, eroded soil, and other contaminants into rivers, streams, lakes, and oceans. It also puts a significant financial demand on cities to manage the stormwater. It is best to place the responsibility of managing stormwater on home and building owners. They can develop and implement practices specific to each property.

Proper design and appropriate installation of systems providing rainwater harvesting and stormwater management will save you money, time, and labor for years to come.  Here are some factors to consider when designing your systems.

Rain barrels, planter boxes, rain gardens, dry wells, and permeable pavement help keep stormwater on your property.

Stormwater Management

In each of these situations, water is directed into the ground. Every project in Los Angeles now has a stormwater component.

Gravity feed systems are preferred by builders now because they have no health department-required permit.  If your project has a pump on it, often the health department requires a permit.

Pumps help move the water greater distances so you can distribute the rainwater over the entire site instead of concentrating it on one location. This reduces the amount of water you put on the landscape in any one place.  Distributing water across an entire property often makes more sense than trying to disperse the water in a limited area.

For low-impact development, stormwater storage containers are often used and the most significant expense for the system. Tanks are not intended to store water for a month. The idea is to have the container empty for the next rain event.  So you need to design your system to move water every few days.

The first flush diverter moves pollutants away from your storage container when pollutants are the highest. This keeps the water in your tank cleaner, improves your pump’s life, and reduces tank maintenance.

There are sanitary issues with water stored for more than just a few days. You can use UV filters for your water or other ways to sanitize the water, but these get expensive.

Return on investment from the system can be in as few as seven years when you don’t use a UV filter.

When Rainwater Harvesting Does Not Work

In places like Southern California, rainfall occurs typically between late October and April, and no supplemental water is needed at this time. So when you have water, you don’t need it, and storing the water for months is not practical.

How To Make A Pump System Work

Using a three-way valve system is an excellent practice.  Install a  cistern with a submersible pump and a three-way valve from the residence. Here is a diagram.

Rainwater Harvesting Pumps

The three-way valve is an electric valve. When the system is told to run, the water is taken from the cistern first. When the water is low (during the summer), the system automatically switches back to the potable supply.   This is a simple system that works very well. Water travels to all irrigated areas on the property, providing greater water disbarment across the property. This is great to prevent slope damage or runoff.

Water tanks can be above ground or below grade.  Cisterns can be almost anything. In the middle east, they use concrete tanks below the ground, and they look to accumulate three days of water in a tank.

Be sure to match the need of your system to the output of your pump. If the pump delivers 10 gallons per minute, the system needs to be designed for that amount.

How Much Maintenance Do These Systems Need

The amount of maintenance depends on the cleanliness of the water.   The cistern may need cleaning every few years, or sometimes never if the water is clean. The pump needs to take water from above the bottom of the tank. The bottom of the tank is where the debris in the water settles. It would be best if you had filtration, more critical with this water. More common to have debris and organic products in the water.

Package systems are available and an excellent place to start. This eliminates redesigning the storage. 

To learn more about these systems, watch this training video featuring Lance Sweeney.


Lance is the President of Sweeney & Associates, Inc and has over 35 years of experience in design, installation, and maintenance. He is a strong proponent for water conservation and has been a certified irrigation auditor since 1991, a long time before it was cool to support water conservation. Lance has managed projects in the United States, China, Egypt, Saudi Arabia, and the United Arab Emirates. He is truly a person who takes pride in doing a job “right” and has a reputation of high quality in the industry. Sweeney & Associates, Inc. is a full-service consulting firm specializing in landscape and golf course irrigation design.


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