The CN-method implies a non-linear relation between rainfall and runoff: for a given CN-value, the runoff coefficient is not a constant but increases with increasing rainfall.
In addition to this, the CN-value itself is not a time-independent constant just depending on the local type of the soil and vegetation. Instead, it is affected by soil moisture resulting from previous rainfall. In addition, there is a strong seasonal effect.
The moisture status of the soil is called "Antecedent Moisture Condition" (AMC). Traditionally, 3 distinct AMCs are distinguished: AMC I (dry), AMC II (average) and AMC III (wet). The cumulative rainfall of the preceding 5 days determines, which AMC applies. these thresholds (of cumulative rainfall) are different for the "growing" and the "non-growing" season.
CN-values are mapped (i.e. geographically defined) for average moisture conditions (AMC II). For each day, the AMC has to be calculated and the AMC adjusted, if necessary.
Originally, this procedure had been done manually using tables. Besides the point, that this has been time-consuming, there are two big shortcomings:
The new model offers a solution for both shortcomings:
Both functions can be modified by the experienced user.