5.3. The effect of soil moisture
The antecedent moisture condition (AMC) plays an important role in the
progress of runoff. The conventional SCS-CN method uses the 5-day
antecedent rainfall amount prior to a rainfall event to select three
levels of AMC. Measured values of CN are not limited to the three
discrete AMC level defined by the 5-day AMC which causes a sudden jump
in runoff prediction (Shi et al., 2017). Moreover, Koelliker (1987)
found that the probability of an AMC2 condition occurred
prior to a storm event was only 7%, rather than 50% (Hjelmfelt et al.,
1982), which indicated the AMC value was underestimated and thereby
resulting in the underestimation of runoff by the standard SCS method;
Huang et al.(2007) also indicated there was no evident relationship
between CN and the 5-day AMC, and that tabulated CN values are almost
always less than measured values, which confirmed the underestimation of
runoff. Therefore, the 5-day AMC used by the standard CN method is not a
reasonable.
Soil moisture is an alternative solution to characterize the AMC would
be to use direct measurements, or estimated values, of soil moisture
prior to a rainfall event, rather than using antecedent rainfall, which
is the most important factor defining the initial abstraction of the
SCS-CN method. Improved knowledge of antecedent moisture would greatly
enhance rainfall runoff prediction (Wood, 1976; Michele and Salvadori,
2002). The results of Huang et al.(2007) method preformed better than
Huang et al.(2006) and standard SCS-CN method (Fig. 2) which indicated
the equation of soil moisture (Eq.7) is more suitable for AMC than the
antecedent 5-day rainfall.
In the new CN value, three factors of slope, soil moisture and storm
event were introduced, which represented the characteristic of the land
condition, antecedent moisture condition and characteristic of the storm
event respectively, which all have great impact on the progress of
runoff. However, as compared with Huang et al.(2006) and Huang et
al.(2007) method, the performance of the Method 1 was significantly
improved after accounting for the storm duration factor (Table 3). From
this standpoint, it may be indicated the characteristic of the storm
event (storm duration) has greater impact on runoff prediction of the
SCS-CN method which has been frequently ignored in other modified SCS-CN
methods and followed by soil moisture and slope. Moreover, the λ = 0.001
is more suitable for the initial abstraction coefficient in the study
area according to comparison between Method 1 and 2, the same conclusion
was also obtained by Huang et al.(2007) for four plots in the Loess
Plateau. The slope parameters in Method 2 which is obtained from Huang
et al.(2006) with data from slopes ranging from 14 % to 140 % are also
more suitable for application as compared with Method 3. This inference
can be further confirmed by sensitivity analysis which also suggested
that the initial abstraction ratio λ and the slope parameters
(a1 and a2 ) are less
sensitive, thereby the Method 2 can be applied to similar semi-humid,
semiarid, and arid regions with the optimized parameters, but may need
adjustment for humid regions because the soil moisture condition and
rainfall characteristic may be different from those tested in this
study.