The amount of actual runoff from a watershed is dependent upon the amount of precipitation that occurs, the initial amount of precipitation that is intercepted, infiltrates, or is stored before runoff begins, the actual retention that occurs after rainfall begins, and the potential maximum retention that can occur after rainfall begins.

The SCS method for estimating the volume of direct runoff from storm rainfall relates the initial abstractions, and retention parameters to watershed properties as described by the curve number (CN).

The potential maximum retention after runoff occurs is related to the CN as follows:

The initial abstraction is related to the potential maximum retention as follows:

The runoff volume is related to the precipitation and the potential maximum runoff as follows:

For complex watersheds that consist of several subareas each having a distinct CN, the total actual runoff volume can be computed in two different ways.

• The cumulative volume method computes the actual runoff occurring from each subarea individually (using the individual CNs and areas), and then sums these runoff volumes to determine the total for the watershed.
• The composite volume method computes the actual runoff using a composite CN and the total watershed area.