Keywords:
Remote Sensing, Land Use, PCA, TSA, NDVI
Abstract
This study reveals some of temporal-spatial dynamics of land-cover change in China from 1982 to 1999 through Principal Components Analysis (PCA) of Pathfinder AVHRR Land (PAL) normalized difference vegetation index (NDVI) data. A more overall and newer understanding of China's land-cover change dynamic can be achieved by a longer time series analysis (TSA). The results have proved that PCA/TSA is a very effective method to identify both macro and micro factors driving the change of NDVI. Especially the thesis to paves a way to detect the impacts of extreme physical accidents and human-induced activities upon the NDVI change. The analytical results are quite exciting and satisfactory.
Introduction
China is the world's third largest country by land area, the largest in population, and by some accounts, the third largest economy. In the past two decades, China's economy has experienced radical change, with double-digit growth rates in some years. As a result, the land use/cover change is also extensive. Rapid urbanization is leading to the loss of agricultural land. According to China's land resources management agency, the cultivatable acreage decreased by approximately 0.9% per year during the 1980s and early 1990s (Chen and Qiu, 1994). Moreover, desertification results in the gradual encroachment of originally vegetated areas. Meanwhile, some large catastrophic events, such as great floods and droughts, as well as increasing environmental pollution, may well have devastated vulnerable ecosystems, bringing about irreversible alteration of the vegetative cover. On the other hand, reforestation is, to some extent, potentially beginning to offset the negative impact of deforestation. It is apparent that the dynamic of land-cover change in China is quite complicated, because it is driven by biophysical, political, economic, and even cultural factors. China is, and will be, responsible for significant environmental changes at local to global scales (He, 1991; Young and Wang, 2000). Therefore, it is necessary to study China's land-cover change dynamic, and to find appropriate strategies to reduce the negative impacts of environmental changes.
This study reveals some of temporal-spatial dynamics of land-cover change in China from 1982 to 1999 through Principal Components Analysis (PCA) of Pathfinder AVHRR Land (PAL) normalized difference vegetation index (NDVI) data. It has been proved that unispectral time series remote sensing data can be used effectively to identify extensive and subtle changes of NDVI by means of PCA (Young, 1997). PCA is also suitable for removing noise within a multispectral image (Young, 1997). Analysis of unispectral PCA for China's vegetation change was performed by Young (1998), who averaged monthly composites to create eleven annual mean NDVI images for 1982 through 1992. He found some significant land-cover change patterns. Yet, a further study deserves undertaking, as a larger dataset with approximately 20-year coverage replaces the old dataset. A broader, updated understanding of China's land-cover change dynamic can be achieved by a longer time series analysis. Doing so, the following objectives can be expected. First, an overall spatial pattern and temporal trend of NDVI change for all of China can be gained by the yearly average NDVI value analysis and its standard variation analysis over the past two decades. Further, the difference of NDVI temporal-spatial patterns of different phenological regions can be examined. Second, using the yearly average NDVI value analysis within specific sub-regions, such as the Yangtze River Valley, the Yellow River Valley, the Pearl River Delta and Northeast China, some more subtle but sensitive changes in vegetative cover can be ascertained, such as the impact of natural disasters as well as man-made changes.
Metrology
Data
This study uses the National Oceanic and Atmospheric Administration (NOAA) / National Aeronautics and Space Administration (NASA)'s Pathfinder AVHRR Land data set with 8 km resolution (Agbu and James, 1994).
Analytical Technique
Principal Components Analysis (PCA) is an orthogonal transformation of n-dimensional image data that produces a new set of images. Known as components, these images are uncorrelated with one another and are ordered in terms of the amount of variance they explain in the original data set.
Framework
The purpose of this study, as addressed above, is intended to detect the temporal-spatial pattern of NDVI change in China and the possible natural and human agents responsible for these changes. Therefore, the author has designed a study framework following two main threads; that is, from macro to micro, as well as from the general to the specific, to detect these variations.
First, an overall analysis will be performed for all of China, the purpose of which is to investigate the general temporal trends and spatial distributions of NDVI within the entire country, and further to detect if there are possible regional differences in different sub-regions by means of the principal components analysis of the imagery.
Second, if regional differences in NDVI change patterns can be found, some further analyses for characteristic sub-regions will be undertaken. These sub-regions will be chosen for their physical phenological conditions, such as climate and hydrological conditions, as well as for their significance to China with respect to their agricultural importance and population characteristics.
Finally, more detailed analyses will be undertaken to further identify the impacts of natural catastrophes and human interventions upon the change of vegetation land-cover within the sub-regions through specific PCA. The study will especially focus on flood hazards and urbanization phenomena.
Study Areas
China Nation-Wide
As the third largest country in the world, China is so large that its territory spans multiple climatic zones, from the tropical and subtropical, to the temperate, to frigid zones, to the specific high-frigid climate style on high elevation plateaus. Taken as a whole, the vegetation types in China are very rich and versatile, with a nation-wide distribution geographically. In addition, with extensive human development over millennia, land use and land cover have experienced profound changes.
The study considers the entire area of China as well as its three sub-regions as the study areas, that is, Northeast China, the Yellow River Valley, and the Yangtze River Valley.
Northeast China
The northeastern area is the furthest northern part as well as the largest natural forest zone in China, where the climate is frigid and semi-humid. Natural vegetation there has been well maintained due to relatively less human development compared to the southern part of China. Biophysically, this area is an independent geographic division in terms of its climate and hydrological conditions.
Yellow River Valley
The Yellow River is the cradle of Chinese civilization. Human development, as well as the conversion of natural conditions over the past five millennia, has deeply transformed the landscape of this area. As a result, the natural vegetation coverage is much less than in ancient times, and much of the land has been converted to agricultural use. Monthly PCA/TSA was applied to this area for detecting the present-day pattern of NDVI change.
Yangtze River Valley
The Yangtze River is the longest river in China, beginning in Qinghai province located on the world's highest plateau, and crossing over almost the entire continent easterly to the sea. The Yangtze River Valley is the most vigorous and important economic zone of China. Here, the LiangHu plain of the Hubei and Hunan provinces is well known as the famous "Cereal Depository", and the city of Shanghai at the river's estuary is China's economic and financial center, playing the leading role in a new round of Chinese economic reform and development. However, the fertile Yangtze River Valley also suffers frequent natural hazards. Further, overdevelopment (its population is about 40% of the entire nation (Huang, 1994)) and unsustainable practices such as agricultural expansion by means of constructing closed levees directly cause increased soil erosion and sediment accumulation in river courses, aggravating the recurrence and magnitude of hazards (Cai, 1992). The following section of the thesis will analyze and explain the impacts of natural disasters and human-induced activities upon regional vegetation cover change through NDVI analysis.
Lake Dongting
Lake Dongting is famous for its cereal and fish production in China. Although its cropland area is just over 2% of the nation, its cereal production is 6% of the nation's total output (Huang, 1994). However, this area is also a hazard zone with very frequent floods as well as droughts. In the past 50 years this area has been subjected to 38 flood disasters, which brought huge economic losses and deaths (Li and Duan, 1997). Besides regional physical and hydrological conditions that cause flood hazards, (e.g., its lower elevations and its four big branches at the confluence of the Yangtze River), human factors have increasingly been contributing to the formation of the disasters. The most significantly negative human development is the so-called "create croplands by encroaching the lake area" phenomenon (Xiang, 1999). Due to the creation of new croplands, the lake area has dramatically shrunk 1, 659 km2 (Wang, 1999), in turn further triggering large amounts of sediment accumulation and river course blockage, and aggravating the threat of flood disasters.
For natural hazard analysis, I concentrate on the Lake Dongting region. In the 1990s, except 1990, 1992, and 1997, floods occurred every other year. In particular, the floods of 1996 and 1998 created new historical records (Xiang, 1999). For this, the author selected the NDVI data of this area ranging from 1995 to 1999, and attempted to identify the flood accidents as well as the possible submerged extent using PCA.
Shanghai Area
Shanghai is one of China's most important economic and financial centers. Since the early 1990s, along with the opening and extensive urban construction of Pudong district, Shanghai has become the locomotive of the new Chinese economy.
Pearl River Delta
The Pearl River Delta was the first area to be developed in China upon her economic reform and opening. Because of its very tight trade and financial connections with Hong Kong, the delta has become extremely important for China. Population here is highly dense, and development activity has been very extensive. The pattern of land-cover change of this area has its own specific characteristics.
