Journal of Earth Science, Vol. 21, No. 6, p. 846–855, December 2010 Printed in China DOI: 10.1007/s12583-010-0136-7
ISSN 1674-487X
Land Use Change and Causes in the Xiangxi Catchment, Three Gorges Area Derived from Multispectral Data Christoph Seeber* Center for International Development and Environmental Research (ZEU), Justus Liebig University Giessen, D-35390 Giessen, Germany Heike Hartmann Department of Geography, Geology and the Environment, Slippery Rock University, Slippery Rock, PA 16057, USA Xiang Wei (项伟) Three Gorges Research Center for Geo-hazard, Ministry of Education, China University of Geosciences, Wuhan 430074, China; Faculty of Engineering, China University of Geosciences, Wuhan 430074, China Lorenz King Center for International Development and Environmental Research (ZEU), Justus Liebig University Giessen, D-35390 Giessen, Germany ABSTRACT: The construction of the Three Gorges Dam on the Yangtze River has extensive impact on the ecosystems and the population of the Three Gorges Area (TGA). Inundation and resettlement have induced far-reaching land use and land cover change (LUCC). The areas that are affected by measures of resettlement are in a tense situation between the implementation of various governmental tasks addressing sustainable land use and water retention and the fulfilment of the population’s economic needs, which primarily depend on agricultural production. Destabilization of slopes and soil erosion are immediate hazards induced by the impoundment. Farming is a very important source of income and has to persist on the one hand to assure the income of the rural population. On the other hand, the environment has to be protected from runoff, soil erosion and instabilities connected to relief, geology and hydraulic influences. In this study, supervised classifications are performed using Landsat-TM (1987 and 2007) and ASTER (2007) images. LUCC is assessed by post-classification change analysis. On the catchment scale, arable land has decreased significantly, while garden land (citrus orchards) and woodland have increased. LUCC mainly affects the area surrounding the reservoir (“backwater”) of the Xiangxi ( 香 溪 ) River, driven by local This study was supported by the German Federal Ministry of
resettlement,
Education and Research (BMBF, No. 03 G 0669).
relocation of land cultivation, and conversion of
*Corresponding author:
[email protected]
arable land to garden land. In the hinterland,
© China University of Geosciences and Springer-Verlag Berlin
LUCC occurs in form of abandonment of land
Heidelberg 2010
cultivation as a consequence of the Grain-for-
newly
built
infrastructure,
Green programme. Manuscript received June 13, 2010.
KEY WORDS: land use and land cover change,
Manuscript accepted August 10, 2010.
Three Gorges Dam, Yangtze River, ecological
Land Use Change and Causes in the Xiangxi Catchment, Three Gorges Area Derived from Multispectral Data
847
impact, China, remote sensing.
INTRODUCTION The accomplishment of the Three Gorges Project (TGP) gives the inimitable opportunity to observe and assess the consequences of enormous land use changes and resettlement. The Three Gorges Dam is expected to threaten ecosystems due to population pressure, resettlement, increasing soil erosion, acceleration of mass movements, water pollution, decreasing river discharge and sediment transport. LUCC driven by inundation, resettlement and environmental policies stands in the focus of studies related to the TGP (Schönbrodt et al., Submitted; Liu et al., 2008; Ng et al., 2008; Heggelund, 2006; Jim and Yang, 2006; Long et al., 2006; Tan and Yao, 2006; King et al., 2004, 2002; Guo et al., 2003; He et al., 2003; Guo and Gan, 2002; Meng et al., 2001). The impoundment of the Three Gorges Reservoir leads to the loss of 24.500 ha of cultivated land (Tan and Yao, 2006). Accessory land loss due to reforestation programs increases the pressure on persisting farmlands and the stress on the rural population. According to Dai and Tan (1996), 33.000 km2 of farmlands in the TGA are heavily affected by soil erosion. This is a severe problem, as soil erosion endangers agricultural production and, therefore, the economic situation of the TGA. Farming is still an important source of income in the TGA, which belonged to the 17 poorest regions of China in the late 1990s (Ng et al., 2008). Besides LUCC, landslides and soil erosion are major research issues related to the Three Gorges Project (Ye et al., 2009; Liu et al., 2008; Fourniadis et al., 2006; Long et al., 2006; Tan and Yao, 2006; Guo et al., 2003). First and foremost, the geology is the primary determining factor contributing to the occurrence of landslides. Additionally, climate, river networks and land use are considered to be important factors that have to be included in landslide predictions (Kallen et al., 2006). In the TGA, an acceleration of the landslide frequency is presumed due to hydraulic flow in rock masses influenced by the reservoir and annual fluctuations of the water level (Fourniadis et al., 2006; Kallen et al., 2006). Ye et al. (2009) assessed the relationship between land use and water quality in the Xiangxi
catchment. They concluded that high concentrations of pollutants resulting from runoff occur in the agricultural areas around the reservoir and along the main rivers; good water quality in forested regions beyond the backwater area was detected. In any way, the mountainous ecosystems of the upper reach of the Yangtze River are fragile due to inappropriate use of land and soil erosion (Long et al., 2006). Since the 1980s, measures addressing erosion control have been enforced, as conventional slope farming is considered to be not sustainable in terms of soil erosion and nutrient loss; in the TGA, 74% of farmlands are situated at slopes reaching from 7° to 25°, and 12% of slope farmlands exceed 25°. Most serious soil erosion is reported on sloping farmlands reaching from 10° to 25° (Long et al., 2006). Slope farmlands tend to be overused, as farming is an important source of employment and income (Ng et al., 2008). Fertile level areas have become inundated, forcing agricultural production to be relocated to slopes. Additionally, the protection of woodlands has gained emphasis in terms of soil conservation and water retention. Until 1998, it was barely acknowledged officially that deforestation is one important cause of serious floods in the upper reach of the Yangtze River. After the 1998 flood of the Yangtze River valley, runoff and soil erosion have become of greater concern. The Chinese government initiated the Grain-for-Green programme in order to stop land reclamation and to enforce the conversion from farmland to woodland or grassland on slopes exceeding 25° (Tan and Yao, 2006; Guo and Gan, 2002). Concerning the impact of LUCC on soil erosion, there is a clear agreement that the lowest soil erosion rates occur under woodland. Studies on the influence of cropping types on soil erosion come to dissenting conclusions. Hill and Peart (1998) compiled results of research activities on soil erosion in southern China and concluded that there is a clear relation between cultivation types and soil erosion rates. Meng et al. (2001), however, considered citrus orchards, even in case they are not terraced, to be the most sustainable land use form on steep slopes in terms of soil erosion and nutrient loss. Ng et al. (2008) did not emphasize
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the type of agricultural production, but recommend contour hedgerows as a low-cost and effective strategy to diminish soil erosion and nutrient loss on steep slope farmlands. They lament, however, the poor acceptance within farming communities, which is pretentiously founded on the alleged competition between crops and hedgerow shrubs. He et al. (2003) investigated the land use development in the Xiangxi catchment from 1949 to 1999, recognizing a general decline of land cultivation during this period, but prevailing dependence of the population on the agricultural sector. The aim of this article is to analyze multitemporal land use changes in the Xiangxi catchment and to identify their causes. This research is carried out in the framework of the Sino-German Yangtze Project, which is applied to joint research on land use,
geomorphology, geotechnical engineering and hydrology, in order to assess environmental hazards in the TGA. Doing so will enable us to provide concepts for a low-risk and sustainable development of the TGA. To do so, the land use and land cover assessment exposed in this study is indispensable. MATERIAL AND METHODS Study Area Large areas in the TGA are subject to enormous land use changes and environmental hazards. As an example, the Xiangxi catchment in western Hubei Province is taken in this study. The Xiangxi catchment is located 40 km upstream of the Three Gorges Dam and drains into the Yangtze River from the north (Fig. 1).
Figure 1. Xiangxi catchment and observed subunits. Lower left corner: the Xiangxi catchment is situated in the western part of Hubei Province, 40 km upstream of the Three Gorges Dam.
Land Use Change and Causes in the Xiangxi Catchment, Three Gorges Area Derived from Multispectral Data
The Xiangxi catchment extends to approximately 3.200 km2, covering Xingshan County and parts of Shennongjia County and Zigui County. The topography of the Xiangxi catchment is scattered and mountainous, not allowing for land cultivation and settlement in large parts. Human activity is mainly restricted to level areas and smoother slopes. The lower section of the Xiangxi River, extending to about 30 km, is a part of the Three Gorges Reservoir. This section of the Xiangxi River is the backwater of the Xiangxi River. According to this, the southern part of the catchment is named the backwater area. Hence, the backwater area and the Xiangjiaba and Quyuan subcatchments stand in the focus of the observation, as LUCC largely takes place in this area due to inundation and local resettlement. Data and Methods The most suitable approach for large-scale land use and land cover assessment is provided by remote sensing techniques. In this research, Landsat-TM and ASTER images are used to deliver information on land use and land cover in two classification levels (L1 and L2, listed in Table 1). L1 is applied to the catchment scale (Landsat-TM, resolution: 30 m× 30 m) and L2 is applied to the backwater area (ASTER, resolution: 15 m×15 m). Landsat-TM is furthermore applied to assess the LUCC that has taken place between 1987 and 2007 on the catchment scale. On the catchment scale, land use is classified according to the Chinese National Standard of Land Use Classification (Chen and Zhou, 2007) to ensure data consistency and comparability with previous studies in the TGA (Long et al., 2007, 2006; He et al., 2003). Initially, the multispectral data were co-registered to an ortho-rectified panchromatic SPOT image which was used as master dataset. A common problem using remotely sensed data is the occurrence of errors due to topographic and atmospheric effects. A standard image routine used in this study to diminish the topographic effect is the calculation of band ratios. Preliminary, it was necessary to adjust the bands by atmospheric correction, which is achieved by dark pixel subtraction (Crippen, 1988). In preparation for the classification, the ISODATA (Iterative Self-Organizing Data Analysis;
Table 1
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Classification levels applied to the Xiangxi
catchment (L1) and the backwater area (L2) L1/Landsat-TM (Xiangxi catchment) Arable land
L2/ASTER (Backwater area) Paddy field Dry field Steep garden plots
Garden land
Dense orchard Sparse orchard
Woodland Grassland
Sparse shrubland, rock and grassland
Bare rock, gravel, river bed Built-up land
Urban built-up land Rural built-up land
Water/reservoir
Venkateswarlu and Raju, 1992) algorithm was applied in order to gain information on the occurrence of spectrally homogeneous areas. ISODATA clustering does not provide information on types of land use and land cover yet. Ground truth data were collected in the field. According to spectrally homogeneous areas provided by clustering, training areas were deducted from point information on land use and land cover. Field data were collected in September 2008 and May 2009. The supervised land use and land cover classifications were performed using the Maximum Likelihood algorithm (Thomson et al., 1998). The land use and land cover classification performed on the Landsat-TM scene from 1987 was performed using reference data derived from a land use map of Xingshan County from 1990. The training areas derived from the land use map were selected corresponding to the spectrally homogeneous classes of the Landsat-TM scene from 1987, which were derived from ISODATA clustering (Schönbrodt et al., Submitted). The result of the L1 classification for each year of origin is a land use and land cover dataset in a spatial resolution of 30 m×30 m, representing 7 L1 land use and land cover classes. The result of the L2 classification is a land use and land cover dataset of the backwater area in a resolution of 15 m×15 m, representing 11 land use and land cover classes. Accuracy assessment on the land use classifications was carried out using the overall class performance and the Khat
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statistics (k) (Sun et al., 2009; Couto, 2003; Congalton, 1991). A raster dataset providing qualitative information on LUCC on the catchment scale was generated by post-classification change analysis (Fan et al., 2008; Zhang and Xu, 2008). The result of the change analysis was re-classified to a binary dataset that differentiates changed (pixel value=1) and unchanged (pixel value=0) raster cells. A gradient resulting from the density of changed raster cells is taken as a measure of the intensity of LUCC. This gradient is achieved by a quadratic neighbourhood function summing up the pixel values for each new 150 m×150 m raster cell that includes 5×5 raster cells of the former binary dataset. The values of the new raster grid range from 0 to 25. Multiplied by 4, we achieve a change gradient expressed as a percentage. Hence, the change gradient expresses the percentage of the area within each 150 m×150 m raster cell that has been subject to LUCC between 1987 and 2007 (0%=no change, 100% complete change within the area represented by the raster
cell). This facilitates to delineate spatial concentrations of LUCC on the macro scale. Field observations play an important role in this research in terms of verification of remote sensing results and interpretation of findings. RESULTS AND INTERPRETATION Results In the L1 classification from 1987, woodland covers 83.9% of the Xiangxi catchment, followed by arable land (13.2%), garden land (1.7%), grassland (0.6%), bare rock (0.3%) and built-up land (0.2%). According to the L1 classification from 2007, the Xiangxi catchment is largely covered by woodland (87%). Cultivated land consists of arable land (9.5%) and garden land (2.2%). Marginal parts of the catchment are covered by grassland (0.4%), bare rock (0.4%), reservoir (0.3%) and built-up land (0.2%), whereas rural built-up land is hardly detectable in the L1 classification (Fig. 2).
Figure 2. L1 classification of land use and land cover in the Xiangxi catchment in 1987 (left) and 2007 (right) derived from Landsat-TM. The loss of suitable land for settlement and agriculture from 1987 to 2007 induced by the inundation is marginal on the catchment scale, making up 0.3% of the entire catchment area. Nevertheless, 9.4 km2 of land suitable for farming and settlement along the lower Xiangxi River has become inundated. Generally, LUCC between 1987 and 2007 is characterized by the increase of woodland (83.9% to 87%) and decrease of arable land (13.2% to 9.5%).
Garden land increased from 1.7% to 2.2% (Table 2). The expansion of garden land mainly takes place in the backwater area, most considerably within the Xiangjiaba subcatchment. The classification accuracy was assessed using the overall class performance (L1/1987: 89.3%, L1/2007: 92.8%), and the Khat statistic (kL1/1987=0.783 and kL1/2007=0.913), indicating strong agreement for k>0.80 and moderate agreement for k60%, dense understory) and sparse orchard (newly established; canopy cover
