2.3 Methodology
The research method comprised five parts: (1) the natural resources conservation service -CN (NCRS-CN) method, (2) land use and soil, (3) SY model, (4) typhoon event analysis, and (5) management practices. Details of these parts are introduced in the following sections.
2.3.1 NCRS-CN Method
The NCRS-CN method is widely used to determine the relationship between rainfall and runoff and can be applied to study areas with insufficient data. This is because the primary input parameters of this method are land use and soil type, which are usually available and readily acquired. Furthermore, land use and soil type data can be collected from public survey data provided by government agencies. The NCRS-CN method was developed by the Natural Resources Conservation Service in the US. It was established based on a water balance equation and two fundamental hypotheses. The first assumption is that the ratio of direct surface runoff and potential maximum surface runoff equals the ratio of infiltration and potential maximum retentions. The second assumption is that the initial abstraction is part of the potential maximum retention. The formulas expressing the relationship between rainfall and runoff in the NCRS-CN method are as follows:
Q = 0, P < 0.2S… (1)
Q = (P – 0.2S)2/(P + 0.8S), P > 0.2S… (2)
where Q is the direct runoff depth (mm); P is the rainfall depth (mm); and S represents the potential maximum retention (mm). The relationship between S and CN can be expressed as
S = (25400/CN) – 254… (3)
The formula for estimating the average CN of a catchment area is
CNav = ∑ (Ai × CNi)/∑Ai… (4)
where CNav is the average CN of the catchment area; Ai represents the grid area (m²); and CNi is the CN of the grid.
2.3.2 Land Use and Soil
Land use is an essential input factor for the NCRS-CN method. Survey data from 2008 provided by the National Land Surveying and Mapping Center were used as a reference for land use classification (Figure 3). The percentages of land use area in the Chenyulan River catchment area are as follows: buildings (1.1%), coniferous forest (14.0%), water body (0.4%), upland field (3.0%), wasteland (9.7%), broadleaf forest (13.9%), orchard (5.6%), and other forest (52.2%). In addition, the hydrologic soil groups could be classified according to soil data (Figure 4); the classification results indicated that the catchment area was primarily composed of B and D Groups.