Extending flood-frequency graphs by comparison with rainfall

Flood discharge is the consequence of many contributing hydrologic events which may be presumed to occur fortuitously and independently, such that the probability of a given flood is the product of the probability of each independent contributing event.  Of the many factors that lead to a flood, the two most prominent are (1) storm rainfall and (2) the "antecedent conditions" (3.g., conditions of the soil encountered by the rainstorm).