Rain barrels are low-cost, effective, and easily maintainable retention and detention devices that are applicable to residential, commercial and industrial sites to manage rooftop runoff. 

Typical rain barrel set-up for a residential property.
(Source: U.S. EPA Mid-Atlantic Region Green Landscaping)

For residential applications a typical rain barrel design will include a hole at the top to allow for flow from a downspout, a sealed lid, an overflow pipe and a spigot at or near the bottom of the barrel.  The spigot can be left partially open to detain water or closed to fill the barrel.  A screen is often included to control mosquitoes and other insects.  The water can then be used for lawn and garden watering or other uses such as supplemental domestic water supply.  Rain barrels can be connected to provide larger volumes of storage.  Larger systems for commercial or industrial use can include pumps and filtration devices. 

Simple rain barrel design.
(Source: Maryland DNR Green Building Program)

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Stormwater runoff cisterns are roof water management devices that provide retention storage volume in above or underground storage tanks.  They are typically used for water supply.  Cisterns are generally larger than rain barrels, with some underground cisterns having the capacity of 10,000 gallons.  On-lot storage with later reuse of stormwater also provides an opportunity for water conservation and the possibility of reducing water utility costs.

Diagram of a typical cistern.
(Source: Texas Guide to Rainwater Harvesting)

Above ground cistern that captures the
rainwater off the roof of a greenhouse.
(Source: Passive Solar Greenhouse)

Rain Water Harvesting for landscape use
(Source: The Texas Water Development Board)

Timber tanks for rain water harvesting
(Source: The Texas Water Development Board)  

Above ground cistern
(Source: American Rainwater Catchment Systems Association;
Photo Credit: Dr. Hari Krishna, ARCSA)