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.