3.2. Statistical Analysis
Table 3 shows the results of correlation analysis among the six
variables in terms of the Pearson’s correlation coefficients and the
Spearman’s rank correlation coefficients. There are moderate
correlations among the duration and the total rainfall depths. The
maximum discharge is correlated with the total runoff volume and with
the bulk runoff coefficient, respectively. Correlation cannot be found
in the other combinations of two variables. The difference between the
two total rainfall depths in each event implicates spatial variability
of rainfall distribution within the catchment area. As the absolute
values of the Spearman’s rank correlation coefficients are small in
general, the six variables are considered independent and separately
fitted to probability distributions. The weak correlation between the
total rainfall depth at the observation system and the total runoff
volume also implies a significant limitation of the conventional SCS
runoff curve number method
where with the curve number represents the potential maximum soil
moisture retention, and the parameter determines the initial abstraction
(Zakaria et al. 2012). Figure 4 compares the observed total runoff depth
() for each flash flood event with the estimations by of different and .
The sum of the absolute errors, the sum of the square errors, and the
maximum absolute error are minimized with = , , and , respectively. With
the commonly used value of , those are minimized with = 76.8, 77.1, and
77.1, respectively. The incapability of the SCS runoff method to
reproduce the total runoff volumes from the total rainfall depths is
evident. To sum, we recommend direct monitoring of runoff discharges
rather than rainfall depths.
Table 3. Results of correlation analysis among the six variables