2. 2 The proposed Method
In the proposed method, slope gradient, soil moisture and storm duration are considered. An improved value of CN for a given storm event can be determined by multiplying the SCS handbook CN2, taken as a reference value, by a slope, soil moisture and storm duration function:
(5)
whereα is the slope gradient (m m-1), θ is soil moisture (cm3 cm-3) andt is storm duration (h). f(α), f(θ) and f(t) are the function of α, θ and t , respectively.
The slope equation proposed by Huang et al. (2006) to consider the effect of slope on CN2 value, who used SCS-CN method to evaluate an 11-yr runoff plot experiment with slopes ranging from 14 % to 140 % in Xifeng city located in the Loess Plateau of China, was adopted in this study:
(6)
An nonlinear equation between CN and soil moisture developed by Huang et al. (2007), who found that soil water storage in the top 15 cm soil depth could be used to represent the V0 value before a rainfall-runoff event, is also adopted in this study and can be express as follows :
(7)
For the storm duration function, we proposed a one-parameter (c ) equation for f (t ) after several trials:
(8)
where a1 , a2 ,b1 , b2 and c are the empirical coefficients.
Therefore, the Eqs.5-8 combing with Eqs.1-4 consists of the proposed method for runoff prediction.