Abstract
Vegetation recovery monitoring is critical for assessing denudation areas after landslides have occurred. A long-term and broad area investigation using remote sensing techniques is an efficient and cost-effective approach incorporating the consideration of radiometric correction and seasonality variations across multi-date satellite images. This paper investigates long-term vegetation recovery using 14 SPOT satellite images spanning from 1999 to 2011 over the landslide area of Mt. Jou-Jou in central Taiwan, which was caused by the Chi-Chi earthquake in 1999. The vegetation status was evaluated by the Normalized Difference Vegetation Index (NDVI) with radiometric correction between multi-date images based on pseudoinvariant features, and subsequently a vegetation recovery rate (VRR) model was empirically established after seasonality adjustment was performed on the multi-date NDVI images. An increasing tendency of the vegetation recovery in the landslide area of Mt. Jou-Jou appeared based on the NDVI value rising to 0.367 in March 2011 from −0.044 right after the catastrophic earthquake. The vegetation recovery rate with seasonality adjustment approached 81.5% for the total area and 81.3% for the landslide area through 12 years succession. The seasonality adjustment also enhanced the VRR model with a determination coefficient that increased from 0.883 to 0.916 for the landslide area and from 0.584 to 0.915 for the total area, highlighting the necessity of seasonality adjustment in multi-date vegetation observations using satellite images. Furthermore, the association between precipitation and NDVI was discussed, and the inverse relationship with the reoccurrence of high-intensity short-duration rainfall and yearly heavy rainfall was observed, in agreement with the on-site investigation.
Highlights
Located in a sub-tropical and seismic area, Taiwan often suffers from the impacts of earthquakes, typhoons, and torrential rains, which induce a high probability occurrence of landslides.On 21 September 1999, the Chi-Chi earthquake with ML 7.3, which is the most serious natural catastrophe in Taiwan of the past century, shocked central Taiwan and significantly changed the geographical features of the area
Thousands of landslide spots induced by the Chi-Chi earthquake produced a large amount of soil and stones that could turn into debris flow and threaten the residents and agricultural activities [10,11,12,13]
Jou-Jou during the consecutive years after the earthquake, the Normalized Difference Vegetation Index (NDVI) was calculated to approximate the vegetation on the frames of SPOT images, because the spectral vegetation index is highly correlated with the green leaf biomass or projected green-leaf area
Summary
Located in a sub-tropical and seismic area, Taiwan often suffers from the impacts of earthquakes, typhoons, and torrential rains, which induce a high probability occurrence of landslides.On 21 September 1999, the Chi-Chi earthquake with ML 7.3, which is the most serious natural catastrophe in Taiwan of the past century, shocked central Taiwan and significantly changed the geographical features of the area. Located in a sub-tropical and seismic area, Taiwan often suffers from the impacts of earthquakes, typhoons, and torrential rains, which induce a high probability occurrence of landslides. Earthquake-induced landslides represent one of the most hazardous impacts after severe seismic events [1,2,3,4,5,6,7,8,9]. Thousands of landslide spots induced by the Chi-Chi earthquake produced a large amount of soil and stones that could turn into debris flow and threaten the residents and agricultural activities [10,11,12,13]. The subsequent intense rainfall following the Chi-Chi earthquake exacerbated the situation by causing an unexpected increase in the collapse of the terrain slopes because of the extent of the bare land area in the landslide area. Jou-Jou was one of Remote Sens. 2017, 9, 893; doi:10.3390/rs9090893 www.mdpi.com/journal/remotesensing
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