Figure 4. Hydrologic soil groups of the study area.
2.3.3 Sediment Yield Model
Numerous studies have used the NCRS-CN method to estimate SY (Mishra,
Tyagi, Singh, & Singh, 2006; Gajbhiye, Mishra, & Pandey, 2014). It can
effectively improve estimation efficiency and overcome the problem of
inadequate data in some areas. This method’s formula is as follows:
C = Q/P = F/S = P/(P + S) = (SY)/A… (5)
where C is the runoff coefficient; Q is the direct runoff depth (mm); P
is the rainfall depth (mm); F is cumulative infiltration (mm); S is the
potential maximum retention (mm); SY is the actual sediment yield (mm);
and A is the potential maximum erosion
(mm).
The characteristics of the catchment area, including A, P, and S, would
affect the SY. Increasing the runoff would increase the volume of
sediment erosion and transportation, and may result in an increase in
SY. The formula is as follows:
SY = AP/(P + S) … (6)
In Eq. (6), S and SY are in inverse proportion. If S approaches 0, SY
would approach A. Sediment erosion is affected by runoff; thus, SY and S
are correlated. The formula is as follows:
SY = a(S)-b… (7)
where a is a coefficient and b is the exponent.
2.3.4 Typhoon Event Analysis
To investigate the relationship between rainfall and SY, this study
explored the process of rainfall events. The concepts of runoff analysis
and SY estimation can be explained by taking the rainfall events between
August 16 and September 13, 2007 as examples (Figure 5). The details are
as follows: (1) For the rainfall analysis, four rainfall stations were
present in the catchment area; thus, the Thiessen polygons method was
used for estimation. (2) For the runoff analysis, because this study
investigated the runoff produced by rainfall events, the straight line
analysis was adopted to deduct the base flow. (3) A rating curve was
used to estimate SY. Rainfall events (Figure 5) indicated that the
values of cumulative rainfall, cumulative runoff, and cumulative SY were
684.1, 487.6, and 9.6 mm, respectively. In addition, the potential
maximum retention could be estimated using the cumulative rainfall and
cumulative runoff in rainfall events, and the value was 205.0 mm.