"Fire flow estimates are not derived from a precise science but are based on historical observations on what works during various situations. When a fire protection engineer, inspector, Incident Commander, insurance representative or contractor discusses fire flow, six significant features must be considered. Taken together, these elements are evaluated to establish how much water is needed to control and suppress a fire."
Coffee Break Training - Fire Protection Series No. FP-2013-37
United States Fire Administration: http://www.usfa.fema.gov/