The surface runoff RUNOFF is calculated using modified SCS equation:

where is the available storage in active soil zone (mm) calculated as:

where is the maximum available storage in active soil zone (mm) which is calculated with curve number for antecedent moisture condition I (CNI) as:

It’s assumed that soil is saturated (reaches to total porosity) when the amount of soil water equals to . CNI is calculated from CNII with

The effective soil storage is adjusted during winter conditions in the presence of a snowpack with a reduction factor SREDUCT. This factor simulates soil frozen conditions with an apparent reduction in available soil storage.