# Runoff estimation by Soil Conservation Service Curve Number

The Soil Conservation Service Curve Number (SCS-CN) approach is widely used as a simple method for predicting direct runoff volume for a given rainfall event. In SCS-CN, the derivation of the equation starts with an assumption:

Runoff/Maximum possible runoff = Storage in catchment/Maximum possible storage in catchment.

I have not come across any book explaining the logic behind this primary assumption. I appriciate if any one can explain me this logic.