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.