Spatial-temporal characteristics of cultivated land in Tibet in recent 30 years

Abstract: Cultivated land plays a vital role in ensuring food safety and promoting social and economic development. At the same time, ecological environment in Tibet is fragile. Therefore, an in-depth understanding of the changing process of cultivated land will provide reference information for agricultural development and ecological security in Tibet. In this paper, based on remote sensing images of four periods and other related data, the temporal-spatial characteristics of cultivated land area in Tibet from 1980 to 2010 was analyzed. By using ERDAS IMAGE9.3 software, the remote sensing images were registered and geometrical corrected based on topographic maps of 1:100000 scale. According to the remote sensing image shape, texture and color, combined with land use status data, remote sensing interpreting marks were established. The supervised classification of automatic interpretation method was used to gain interpretation result, from which data of cultivated land were extracted. After correcting the interpretation cultivated land results combined with field investigation data and visual interpretation of remote sensing images, cultivated land area's identifiable information of four periods were gotten. In order to preferably understand the temporal and spatial changes of cultivated land, the study period was evenly divided into three time periods: the first stage was 1980-1990, the second stage was 1990-2000, and the third stage was 2000-2010; and the units including administrative region, interval of altitude as well as latitude were divided. By using dynamic degree of land use, location index of land use, the temporal-spatial characteristics of cultivated land area were analyzed. Analysis of different conversion types showed that the increase of cultivated land was mainly from grassland and forest land, while the decreased cultivated land mainly was converted to grassland, forest land and construction land. From the climatic conditions, the period of 1990-2010 was beneficial to expand the cultivated land, and cultivated land in Tibet also showed a faster growth rate. The implementation of the western development strategy sped up the development of social economy in Tibet, also brought the building occupancy and ecological restoration, and therefore the cultivated land has decreased. Results showed that: 1) Between 1980 and 2010 cultivated land increased by 451 km2, from 3 252 km2 to 3 703 km2, and its proportion to the total area of the Tibet increased from 0.27% to 0.31%, with an annual change rate of 0.46% and an annual variation of 15.37 km2. 2) Cultivated land in Tibet was mainly distributed in four regions, i.e., Shigatse, Lhasa, Chamdo and Lhoka, and distributed in the altitude range of 3 200-4 200 m and the latitude range of 26°-33°N, of which, Lhasa, 3 800-4 000 m altitude and zone of 29°-30°N were the regions that cultivated land centered most. 3) Cultivated land expanded to higher altitude and lower altitude areas. Chamdo, the altitudes over 4 200 m and lower than 3 200 m and the zone of 30°-33°N increased most rapidly in area of cultivated land. It was the comprehensive effect of natural conditions and human activities, and produced the result of temporal and spatial changes characteristics of cultivated land in Tibet in recent 30 years. The special geographical environment and ecological function of Tibet had decided that in the conditions of climate warming, the impact of human activities on cultivated land must be fully understood, the consciousness of cultivated land protection should be raised; and scientific planning and reasonable use of cultivated land should be achieved so as to improve the land use efficiency and supply capacity.