Hillsborough County FL - Backflow Prevention Ordinance
Auxiliary Water Supply System: A pressurized or pumping-ready water supply system other than a Public Potable Water System which is located on or available to the Customer's property whether or not connected to a distribution system within the property. Such auxiliary systems include but are not limited to Reclaimed Water systems and private wells. As further defined in AWWA M-14 Second Edition (Chapter 5) 5.2.
Backflow: The undesirable reversal of flow of water or mixtures of waters and other liquids, gases or other substances into the distribution pipes of the potable supply of water from any source(s).

American Water Works Association Manual, potential hazards for residential water services:
"pets, livestock, fish, chemicals, pools, fountains, irrigation, dialysis equipment, developing equipment, gray water, reclaimed water, an auxiliary water supply, heating & cooling equipment and other equipment or operations that use water."

Hillsborough County has established a policy that mandates residential water customers to install expensive backflow protection devices if they have an auxiliary water supply on their property. Even if no cross connection exists and the auxiliary water is used soley for irrigation.
Rain Barrel = $15 Backflow Device = $510 Hillsborough Co = Priceless

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http://www.ncriverwatch.org/wordpress/2008/06/26/water-catchment-article-and-discussion/
As we (SCWC tech session) were told by the Santa Rosa staff folks, a 55 gallon drum under your downspout with a garden hose on it and no connection to the municipal plumbing is still considered an “auxiliary” water supply, like an old well, and thus necessitates the purchase and installation of a several hundred dollar double back flow prevention device at the street, annual inspection of it for $60 to $100 and a $60 permit fee. Talk about lack of incentives!!