Farmland use changes at oasis areas in the middle and lower reaches of Shiyang River Basin, Gansu Province of China

Based on land use spatial data (1︰100000) of Landsat TM image in 1995 and 2000, by using GIS technology, the spatial distribution characteristics and change range of farmland use status quo, farmland expansion and losses of Wuwei oasis and Minqin oasis of Gangsu Province, China in the middle and lower reaches of Shiyang River Basin (short for SYR basin) from 1995 to 2000 were investigated. The results show: 1) farmland is the main type of land use in Wuwei oasis and Minqin oasis, the spatial distribution of possible transformation between farmland and other types of land use is broad; 2) farmland in middle and lower reaches of the SYR basin increased rapidly, the annual change ratio of farmland of Minqin oasis in the lower reaches of the SYR basin from 1995 to 2000 was 0.3282, the annual mean rate of increment reached 2.18%; 3) farmland expansion of Wuwei oasis in the middle reaches of the SYR basin originated from grassland and unused land, which accounted for 90.83% of the increased farmland areas, and farmland losses were caused by rural residential land expansion and farmland degradation; 4) farmland expansion of Minqin oasis in the lower reaches of the SYR basin typically resulted from grassland, which accounted for 54.58% of the total expanded areas, and farmland losses were mainly caused by degrading farmland to unused land and barren; 5) buffer analysis shows that there is a significant positive correlation between farmland expansion area in middle reaches of the SYR basin and farmland loss area in the lower reaches of the SYR basin, and the correlation coefficient is 0.8856. Increasing each 1 hm2 farmland of Wuwei oasis in the middle reaches of the SYR basin causes about 3 hm2 farmland loss of Minqin oasis in the lower reaches of the SYR basin.