Another type of runoff model commonly used is the API model.
Antecedent Precipitation Index (API) based procedures have been used for many years by National Weather Service (NWS) River Forecast Centers (RFCs) for producing flood forecasts.
The API procedures developed by the RFCs are applied on a storm basis.
The API value at the beginning of the storm is typically related to time of the year, storm duration, and storm rainfall to compute storm runoff.
A description of the process follows.

The Continuous API Model consists of 4 quadrants, equations to compute baseflow runoff, and a few additional features including an option to account for the effect of frozen ground on runoff.
Baseflow runoff is computed based on the total water in groundwater storage and the amount that has entered the storage in the recent past.
The model also allows for impervious area runoff and riparian vegetation losses.