The symbol used by hydrologists to represent flow. It may also mean the design flow for a particular project. With a subscript number, Q refers to a flow recurrence interval; Q10 is the flow that occurs, on the average, only once in 10 years (also known as the 10% frequency, or recurrence, event), Q25 is the flow which occurs only every 25 years (4% event), and Q100 occurs once in 100 years (1% event). The recurrence interval is a statistical average, not a fixed number of years. Q10, Q25, and Q100 are common design standards for flood control projects.