Sustainability Toolkit

Rain cisterns are storage tanks that can be used to collect rainwater runoff from rooftop downspouts or stormwater runoff from impervious surfaces such as a parking lot. The water collected may be used for any number of non-potable purposes such as landscape irrigation. In addition to water conservation, rain cisterns are also a Low Impact Development technique implemented to decrease the amount of runoff generated from impervious rooftop surfaces which may, in turn, decrease the magnitude of stormwater management systems required for the site.

Figure 1. Basic cistern design

Rain barrels are devices also used to collect stormwater runoff from rooftops but are considerably smaller than cisterns. Rain barrels are generally pre-manufactured containers placed above ground while cisterns may be either pre-manufactured or built on-site and installed above or below ground. Both devices typically have a screen or filter, faucet or hose for water removal, an overflow valve and a removable cover for maintenance. Additional delivery and treatment devices are available which broaden the possibilities for water reuse.

Selection of a rain barrel or cistern will rely on the area of impervious rooftop, anticipated rainfall and the desired capabilities of the cistern or barrel. The formula below, developed by the Texas Water Development Board, provides guidelines for the sizing of a rain barrel or cistern¹. Please note this formula is generic and location specific factors will affect the results.

V = A² * R * 0.90 * 7.5 gals/ft³

Where:

Figure 2. Rain Barrel

V = volume of rain barrel or cistern (gallons)

A² = area of impermeable roof surface (square feet)

R = rainfall (feet)

0.90 = system loss coefficient (no units)

7.5 = conversion factor (gallons per cubic foot)

The cost of implementing a rain cistern will vary depending on:

storage volume capacity
materials used for construction
construction method (i.e. above vs. below ground)

UFC 3-210-10, Low Impact Development provides additional planning, programming and design information.

Cistern water collection and storage is a potential strategy to achieve the intent of LEED SS Credit 6.1, SS Credit 6.2, WE Credit 1.1, and WE Credit 1.2. Refer to the LEED Guidance section for specific requirements.

References

¹Texas Water Development Board, The. “Rainwater Harvesting,”

http://www.twdb.state.tx.us/iwt/Rainwater.asp.