What is Composite Runoff Coefficient or C-Factor?
C-Factor is the runoff coefficient. It is the ratio of the amount of excess rainfall or runoff to the amount of precipitation for a given time over a given area. Without getting into time and area, a very basic example might be: the C-Factor for pavement is 0.87; this means that for every 100 cups of water you pour on pavement (concrete or asphalt), 87 cups of water will run off of it. The C-factor for grass is 0.07. Again, for every 100 cups of water poured on it, 7 cups of water will run off of it. The Composite Runoff Coefficient or Composite C-Factor then is a weighted average of all areas and their corresponding C-factors that make up a parcel.
