Application of NRCS-CN method for estimation of watershed runoff and disaster risk

Figures & data

Figure 1. Watershed with locations of rainfall stations and gauge sites. Source: Author

Table 1. Study materials.

Map/information	Year	Resolution	Source	Application
Satellite imagery (Landsat)	1990 ∼ 2010	30m*30m	U. S. geological survey	Land cover types extraction to estimate CN
Aerial photography	2010	1/5000	Google Earth	Land cover verification
DEM	2004	5m*5m	Department of land administration	Terrain analysis
Soil	2005	1/5000	Soil and water conservation bureau	To calculate CN
Rainfall	2005 ∼ 2009	Daily data	Water resources agency	Estimation of direct runoff
Runoff	2005 ∼ 2009	Daily data	Water resources agency	Model verification

Figure 2. Using statistical methods to sort land use on the basis of satellite images. Source: Author

Table 2. Results of accuracy assessment.

Observed Estimated	Non-vegetation	Forest	Agricultural	Grass	Water	Total
Non-vegetation	33	0	0	0	0	33
Forest	8	315	4	6	1	334
Agricultural	1	3	5	0	0	9
Grass	0	12	0	7	0	19
Water	0	0	0	0	5	5
Total	42	330	9	13	6	400

Overall accuracy = 92%, Kappa = 0.71.

Figure 3. Rainfall and runoff for HAITANG typhoon event. Source: Author

Figure 4. Comparison of land cover types obtained using satellite-image-based classification with aerial photographs. Source: Author

Figure 5. Spatial distributions of land cover types, hydrologic soil group, and CN values of the Chenyoulan watershed in 2008. Source: Author

Table 3. Typhoon events and related data.

Typhoon event	Date	Rainfall depth (mm)	Observed runoff depth (mm)	Flood return period (year)
HAITANG	16/07/2005	594.7	528.3	3
LONGWANG	30/09/2005	104.3	20.4	1
BILIS	12/07/2006	408.9	251.9	2
SEPAT	16/08/2007	684.1	487.6	2
KROSA	05/10/2007	503.8	140.3	1
KALMAEGI	16/07/2008	463.2	50.7	1
FUNG-WONG	26/07/2008	308.8	77.7	1
SINLAKU	11/09/2008	824.6	433.8	2
MORAKOT	05/08/2009	1419.7	1152.9	82
PARMA	03/10/2009	40.8	8.8	1

Figure 6. Relationship between observed and estimated runoff depths. Source: Author

Figure 7. Percent deviation between observed and estimated runoff values. Souce: Author

Figure 8. Relationship between grid-based estimated values and observed values. Source: Author

Figure 9. Spatial distribution of flooding risks during Typhoon Morakot. (a) Spatial distribution of the potential maximum retention in 2008 (b) Spatial distribution of the runoff during Typhoon Morakot (c) Spatial distribution of the runoff after model revision (d) Spatial distribution of the topographic wetness indices (e) Spatial distribution of Typhoon Morakot-induced flood risks (f) Spatial distribution of the disaster locations. Source: Author

Figure 10. Flooding risk grades of disaster sites during Typhoon Morakot. Source: Author

Figure 11. Relationship between flooding risk grades and number of corresponding disaster sites during Typhoon Morakot. Source: Author

Figure 12. Hotspots based on analysis of Typhoon Morakot. Source: Author

Figure 13. Scouring caused by Typhoon Morakot (Source: Yang et al. 2009).

Figure 14. Scouring of riverbanks. Source: Author

Table 4. Notable disaster sites in the Chenyoulan watershed.

Site	X(m)	Y(m)	Description
A	240775	2608037	Scouring at the toe of the slope along the Habilan rock shed caused extensive slope collapse
B	238250	2609078	Scouring at the toe of the slope below the road caused extensive slope collapse, cutting off the road
C	237370	2607916	Long Hua elementary school was destroyed by a mud slide, forcing the school to be temporarily closed
D	235183	2602756	Collapses and mud slides occurred along several tributaries in the watershed. The mud and rocks that ran down the slopes, which amounted to tens of thousands of cubic meters, destroyed the bridge spanning the Aiyuzi River, cutting access to the outside world
E	2403368	2603974	Scouring of the roadbed along the road temporarily disrupted traffic

Source: Yang et al. 2009.

Yang MD, Lin JY, Lin WJ, Huang KS, Wu DY. 2009. Disaster investigation in Chenyulan river basin during Typhoon Morakot. J Chin Soil Water Conserv. 40(4):345–358.

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Data availability statement

Not applicable.