The name "CN-method" is derived from "Curve-number Method". It is an empirical, non-linear approach to calculate storm runoff from daily rainfall.

The Formula for runoff calculation is as follows:

It was developed in the 1960s by the U.S. Soil Conservation Service, it is thus also known as the SCS-method. The term "Curve Number" is used because originally is was applied by using nomograms, each CN-value was a curve within a nomogram.

The following table shows the relation between daily rainfall and runoff in mm for soil types found in Jordan, for AMC II. AMC stands for Antecedent Moisture Condition, represented by a value influenced by the rainfall of the previous days and the prevalent potential evaporation.

The values of the above table are displayed in the following chart: