Geoscience Reference
In-Depth Information
samples selected with the NDVI values were used to estimate and simulate NDVI dis-
tributions using kriging and SGS. The simulated NDVI images were compared with
actual NDVI images induced by the disturbances.
MATERIALS AND METHODS
Study Area and Remote Sensing Data
The Chenyulan watershed, located in central Taiwan, is a classical intermountain
watershed, and has an average altitude of 1,540 m and an area of 449 km
2
(Figure
1). The Chenyulan stream, which coincides with the Chenyulan fault, flows from
south to north and elongates the watershed in the same direction. Differences in
uplifting along the fault generated abundant fractures over the watershed and re-
sulted in an average slope of 62.5% and relief of 585 m/km
2
. Moreover, the main
course of the Chenyulan stream had a gradient of 6.1%, and more than 60% of its
tributaries had gradients exceeding 20%. The special geological and geographical
characteristics of the watershed result in frequent landsides and debris flows (Lin
et al., 2003). The September 21, 1999, Chi-Chi earthquake occurred at 1:47 am
local time (17:47:18 GMT the previous day) at an epicentral location of 23.85_N
and 120.78_E and at a depth 6.99 km (Figure 1). It was caused by a rupture in the
Chelungpu Fault. The magnitude of the earthquake was estimated to be ML = 7.3
(ML: Local Magnitude or Richter Magnitude), and the rupture zone, defined by the
aftershocks, measured about 80 km north-south by 25-30 km downdip (Lin et al.,
2006a; Roger and Yu, 2000). Iso-contour maps of the earthquake's magnitude were
reproduced from the Central Weather Bureau (Figure 1) (Central Weather Bureau,
1999). After the earthquake, from October 31, 2000 to November 1,2000, the center
of typhoon Xiangsane moved from south to north through eastern Taiwan (Central
Weather Bureau, 2000), with a maximum wind speed of 138.9 km/hr and a radius
of 250 km (Figure 1). The maximum daily rainfall was 550 mm/day. On July 30,
2001, the Toraji typhoon swept across central Taiwan from east to west (Central
Weather Bureau, 2001), with a maximum wind speed of 138.9 km/hr and a radius
of 180 km (Figure 1). The typhoon brought extremely heavy rainfall, from 230
to 650 mm/day, and triggered more than 6,000 landslides in Taiwan. After cross-
ing Taiwan, typhoon Toraji became a tropical storm; however it brought 339-757
mm of total accumulated rainfall in the watershed (Central Weather Bureau, 2001)
(Figure 1). After typhoon Toraji, typhoons Dujuan with a maximum wind speed
of 165.0 km/hr, a radius of 200 km and maximum rainfall 200 mm/hr (August 31,
2003-September 2, 2003) and Mindulle with maximum wind speed of 200.0 km/
hr, a radius of 200 km and maximum rainfall 166 mm/hr (June 29, 2004-July 2,
2004) chronologically produced heavy rainfall that fell across the eastern and cen-
tral parts of Taiwan on September 2003 and June 2004 (Central Weather Bureau,
2008) (Figure 1). The two study area with dimensions of 50 × 50 km
2
(250 × 250
pixels) was selected from the upstream of the large debris flood announced in the
watershed, as shown in Figure 1.
