2.1 The Original SCS-CN Method
The SCS-CN method (SCS, 1972) was originally developed based on two fundamental hypotheses and one simple water balance equation, which can be expressed as follows:
(1)
(2)
(3)
where P and Q are the depth of observed rainfall and direct runoff, respectively (mm); Ia and λare the initial abstraction (mm) and coefficient of initial abstraction (dimensionless), respectively; F is the cumulative infiltration (mm) and S is the potential maximum retention (mm), which can be calculated by:
(4)
where CN is a non-dimensional parameter varying from 0 and 100. This CN value is determined by CN value (CN2) of average moisture condition (AMC2), which depends on land cover, soil group, and hydrologic condition using a table from the SCS handbook (SCS, 1972), and then converted to AMC 1 or AMC 3 based on 5-day prior rainfall depth.