Land cover in the Horqin Grasslands, North China. Detecting changes between 1975 and 1990 by means of remote sensing

Brogaard, S. & Prieler, S. ORCID: https://orcid.org/0000-0002-7024-1075 (1998). Land cover in the Horqin Grasslands, North China. Detecting changes between 1975 and 1990 by means of remote sensing. IIASA Interim Report. IIASA, Laxenburg, Austria: IR-98-044

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Abstract

This paper aims to identify land cover changes of the western part of the Horqin steppe by means of Landsat MSS satellite data. The Horqin steppe is located in the temperate semi-arid zone of North China, Inner Mongolia Autonomous Region. The steppe is considered to be one of the most drastic examples of land degradation in China's semi-arid region.

Two Landsat MSS scenes, one for the year 1975, the other for 1989, with an overlapping area of around 20000 km2 were used for the land cover change detection. After performing geometric and radiometric corrections, an unsupervised cluster classification revealed six major land cover classes including water. Then a supervised Maximum Likelihood classification was applied to both scenes using the results from the unsupervised classification as well as the Horqin desertification/land use map (Lanzhou Institute of Desert Research, 1991) and filed data, which have been collected in September 1996. 5-10 training areas for each class where identified for the five land cover classes: (1) Moving sand with less than 10% vegetation cover, (2) farmland, (3) grassland with a vegetation cover of 10-60%, (4) grassland with a vegetation cover of 60-100% and (5) scrub forest. Further a post classification filtering process was applied to produce more homogenous areas. Finally by means of visual interpretation and a digital elevation model obvious misclassifications, such as overrepresentation of cropland at the expense of scrub forest or grassland with vegetation cover >60%, were changed.

A comparison of the 1989 image classification with the collected field data gave poor results. Most likely this is due to the time difference between image acquisition and field data but also the result of low precision when georeferencing the image. Another source of error could be the actual sampling method, which has occurred close to river systems, where land use and land cover may undergo rapid changes.

Comparing the 1975 and 1989 final classifications three main types of changes may be observed. First, larger areas of farmland are found in the 1989 image compared to 1975, mainly distributed around the Xiliao River system. Second a significant area of the class `grassland with 10-60% vegetation cover' in 1975 has become converted to the class `grassland 60-100% vegetation cover' by 1989. Third, sandy surfaces with the lowest vegetation cover, <10% have also decreased. Therefore this study does not confirm any overall degradation of the area as far as concerns vegetation cover. On the contrary, the study implies a possible improvement of grassland, especially in the loess area south of the Xilao River. Precipitation data seem not to explain these changes. However, additional data including monthly precipitation data should be analyzed.

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