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Land Use Dynamics from Multi-temporal Remotely Sensed Data: A Case Study Northern Thailand
3. Change detection and discussion
The land use pattern along several periods has had the same characteristics. Dense forest is the major land use, usually along steep slopes in inner highlands in the eastern parts of the watershed. Agriculture is the second largest land use type after dense forest. The major crops identified by area were paddy, field crop and orchard, respectively. Paddy is located in rainfed and irrigated lowland at altitude less than 220 MSL. Orchards or mixed fruit trees are normally situated near to the lowland villages. Field crop comprises of two major cash crops: maize and cotton. These crops are scattered along the upland and highland valleys at altitude more than 220 meters. Disturbed forest is the third major land use, located near to the cultivated fields in uplands and highlands, along stream valleys where logging concessions were allowed (1973-1989). After the construction of Song reservoir, water body increases respectively since 1995. It is difficult to use the results of image classification to investigate land use changes because of the class confusion and uncertainty of some land use types among the three dates. As the aerial photographs have more advantage on resolution, they are selected to compare land use changes. During the two periods (1989-1991 and 1991-1996), the land use dynamics has an identical trend (figure 4 and table 1). Dense forest was changed to disturbed forest and field crops and, some disturbed forests were altered to field crops but at a decreasing rate. It is remarkable that the increase was most profound in the first two years of the seven-year period. The borderzone in-between the uplands and lowland were the most affected. These land use types have common physical and geographical inter-connections, as an area increase in one type of land use category will be associated with a area decrease in another land use category. The changes happened in the areas with soil fertile enough to grow crops (normally along highland valleys and low and high terraces), less steep slopes, and close to former field crop areas. The diminishing rate of decreased dense forest and the increased disturbed forest and field crop could possibly indicate that land occupation of arable areas has reached its limit in the watershed.  Figure 4 Land use changes from 1989-1996 Table 1 Land use types and changes from 1989-1996 (area unit: km2)
The satellite data covers larger temporal space than the aerial photographs (1977-2000). During the period of logging concession from 1973-1989, there are traces of logging areas in upland and highland from Landsat MSS 1977, appearing in bluish color at the western part of the study area. From 1980 – 1990, land became more shortage for the poor in lowland, they have to expand their cultivated areas towards forestlands along small valleys following logging trails after tree cutting. Landsat TM 1989 depicts well this situation. Expanded fields appear as pale cyan narrow strips along stream valleys image in the eastern part. The lost of forest areas has continued until 2000. Despite the haze problem, Landsat ETM+ indicates more encroachment trend into former forestlands in inner upland and highland. At the same time, the advantage of Song reservoir, finished since 1993, also increases more irrigated areas in lowland. |
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