4. DISCUSSIONS
To investigate the relationship between the potential maximum retention
and SY in the Chenyulan River catchment area in typhoon events during
1999–2009, the rating curves of the runoff and SY were first
established. Rating curve analysis results showed that the two
parameters were positively correlated (Y = 0.0002x² + 0.0046x), and the
correlation coefficient reached 0.95. Therefore, the rating curve is
applicable for estimating SY in rainfall events.
The relationship between the potential maximum retention and SY
indicated that first, the relationship of the two parameters conformed
to a power function, which was consistent with the aforementioned study
on rainfall events (Mishra, Tyagi, Singh, & Singh, 2006). Second, the
event-based cumulative rainfall was the primary cause of the difference
in functions, which was different from relevant studies, and thus this
study categorized cumulative rainfall into three scenarios. Type I
cumulative rainfall was less than 197.2 mm, which was smaller than the
invoked value of SY; Type II was 197.2–754.4 mm, and the corresponding
function was SY = 473.64S–0.866; and Type III was
754.4–1419.7 mm, and the corresponding function was SY =
13764S–1.133.
The difference in S–SY curves of Types II and III was primarily caused
by cumulative rainfall. Different cumulative rainfall levels affected
the relationship between the potential maximum retention and actual SY.
For Type II, the cumulative rainfall was slightly higher than the
invoked SY value; thus, the S–SY curve was relatively flat. For Type
III, because of higher cumulative rainfall level, according to the
concept of energy accumulation, the same condition for S would cause a
relatively large SY; therefore, the S–SY curve was relatively steep. In
addition, the difference in S–SY curves can also be explained by the
infiltration rate decreasing over time. When the duration of rainfall is
long, high cumulative rainfall would easily cause soil saturation,
thereby inducing more surface runoff and soil erosion.
To facilitate sediment management in the catchment area and effectively
determine key locations, this study took each subdivision as a unit. The
average potential maximum erosion and average slope of each subdivision
were considered to determine the subdivisions with top management
priority. If the average potential maximum erosion was larger and the
corresponding slope was steeper, sediment disasters would be more likely
to occur in such an area. Therefore, such regions should be prioritized
for management. The average potential maximum erosion of the
subdivisions could be estimated by the S–SY function (Figure 13(a)),
whereas the subdivision slope could be extracted using the DEM (Figure
13(b)). Through the normalization of the two parameters, the risk of
sediment disasters in each subdivision could be estimated (Figure
13(c)), and the higher the risk value, the higher the priority that
should be assigned to that subdivision.