The Auxiliary Water Supply System (AWSS, though often referred to on manhole covers and hydrants as HPFS for High Pressure Fire System) is a high pressure water supply network built for the city of San Francisco in response to the failure of the existing emergency water system during the 1906 earthquake. The majority of damages to the city from the earthquake were caused not by the seismic event itself, but from the fires that ensued, resulting in the destruction of 80% of the city's property value at the time. The improved water system was originally proposed by San Francisco Fire Department chief engineer Dennis T. Sullivan in 1903, with construction beginning in 1909 and finishing in 1913. The system is made up of a collection of water reservoirs, pump stations, cisterns, suction connections and fireboats. While the system can use both fresh or salt water, it is preferential to not use salt water, as it commonly causes galvanic corrosion in fire equipment. The large, white oversized hydrants that are supplied by the AWSS/HPFS, of which there are 1889, are visible throughout the city. The hydrants have painted tops that are color-coded as to zone:

Upper zone

Reservoirs

Twin Peaks Reservoir

The Twin Peaks Reservoir acts as the backbone of the AWSS system and is located in San Francisco's Twin Peaks hilltop. It is made up of a 10.5 e6U.S.gal storage reservoir made out of 6 in reinforced-concrete slabs. Fresh water is delivered from the city's domestic water system by two 750 USgal/min centrifugal pumps. For safety, the reservoir is broken up into two tanks, and each tank can be emptied separately so that in case of a pipe breakage only half of the reservoir is lost. The tank is set at 758 ft.

Ashbury tank

The Ashbury tank has a direct connection to the Twin Peaks reservoir and has a total capacity of 500000 USgal. The tank is set at 494 ft and, when combined with the Jones street tank, can provide hydrants with 214-psi pressures. It is located at 1234 Clayton Street, in the city's Ashbury Heights neighborhood.

Jones Street tank

The Jones Street tank has a direct connection to the Ashbury tank and has a total capacity of 750000 USgal. The tank is set at 369 ft, providing hydrants with 160-psi pressures. It is located at 1239 Jones Street in the city's Nob Hill neighborhood.

Lower zone

Pump stations

There are two emergency pumping stations present within the AWSS. Both stations are capable of pumping 10000 USgal/min of salt water at a pressure of 300 psi with on-site generators. Pumping Station No. 2 is listed on the National Register of Historic Places.

Fireboats

To supplement any failure of the pumping stations or reservoirs, the city’s two fireboats can be utilized to deliver salt water into the system. They can also be used to deliver water by hose line to areas close to the water.

Cisterns

As a final measure to counter a failure of the AWSS piping, the city maintains network of 177 independent underground water cisterns. Sizes vary from 75000 USgal to over 200000 USgal depending on location with a total storage capacity of over 11 e6U.S.gal of water. These cisterns are easily spotted at street level with manholes labeled CISTERN S.F.F.D surrounded by red brick circles or rectangles.