| After Processing and interpreting the remote sensing data of 1964, 1973, 1990, 2000 Based on the technology of the RS, GIS and GPS. We analyzed the changes of landscape patterns of last 40 years in YanQi oasis in class and landscape level by Fragstats with four landscape maps .then we ananyzed the ecological security of Yanqi Artificial Oasis on the water security, soil security and Artificial Oasis Defend System in past fifty years.We reached a conclusion of the Analysis of the the changes of landscape patterns :Landscape class level analysis shows: from 1973- 2000 oasis cultivated area expanded rapidly and gradually link each other together; its area had became the largest patch among the oasis landscape type; the patches of natural vegetation Increased , but area reduced, Aggregation degree was lower than before Obviously .Landscape level analysis shows : from 1973 to 1990 and 1990 to 2000,the number of patches increased and patches density gradually increased but the mean area of patches gradually decreased; the SHDI index have a downward trend ;the size difference between landscape patches had increased.Transfer matrix calculations indicate that :in the period of 1973 to 1990,1990 to 2000 .the increased farmland converted mainly by natural vegetation, and urban and rural settlements in the new area mainly from farmland, grasslands, and the transformation from Gobi. Reed marshes between 1973 to 1990, had decreased of 17.52%. This period ,the marsh reeds mainly transform into farmland, natural vegetation and salinization land.The area of Salinization land had increased of 24.24% from 1973 to 1990,this part of area converted mainly from natural vegetation and marsh reeds.Ecological security analysis shows : between 1950s to the mid-1960s, Yanqi oasis fanning areas was irrigated excessively, facility of farmland drainage was poor, leaded to the water rise, internal oasis secondary salinization of soil is serious. Meanwhile, the shelter system of forest has not been established ,as a result , internal oasis without proper ecological protection functions, artificial oasis internal ecological safety is low.The mid-1960s to mid-1970s, Yanqi oasis farmland increased in a lower level and drainage measures gradually improved so that agricultural land area of heavily salinity reduction. farming areas is in a period of desalination; for Bositeng lake, because the quantity of water flow into Lake decline, the quantity of farmland drainage water increased, mineralization degrees increased from about 1g/L to 1.44g/L. water level of Bositeng Lake declined, river wetlands shrinked, reed marsh area significantly reduced. In the absence the advanced irrigation means, gross irrigation rate was still high, secondary salinization of soil farming areas was also high. During this period, the ecological safety of inner artificial oasis was lower.1976～1987 period, water level dropped gradually, water area shrinked, the Lake reached the minimum water level, from 1975 to 1987, mineralization degrees raised from 1.44 g/L(1975) to 1.87g/L(1987). 1970s to the mid-1980s ,Bositeng Lake had the largest water mineralization degrees. In 1982, West pumping station began to operate, improved water cycle, the salt quantity flow out of Bositeng Lake increased, slow down the quality deteriorating of Big Lake water. Since the 1990s, by the effects of climate change, water quantity increased, water level of Bositeng Lake began to rise, Lake water quality continued to improve. From the late 1970's of the 20th century to 2000, most of natural vegetation had been displaced by the farmland .Yanqi oasis has been a artificial oasis; the system of irrigation and protection forest has tended to perfection.artificial oasis of ecological security was at a relatively good position.
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