Study on Mutual Transformation Characteristics between Paddy Field and Dry Land in Sanjiang Plain from 1986 to 2005

As grain production base and freshwater wetland concentrated distribution area of China,Sanjiang Plain plays a unique part in guaranteeing nation's grain safety and regional ecological security.Many studies about wetland landscape change and land use and land cover change in Sanjiang Plain have been conducted,but the knowledge about mutual transformation between paddy field and dry land is still deficient.With the support of RS and GIS,this paper used four-period(1986,1995,2000,2005)land use data to analyze from three respects(total trend,mutual transformation difference,spatial pattern)the feature of mutual transformation between paddy field and dry land in Sanjiang Plain during the twenty years.The result showed that:1)From 1986 to 2005,both paddy field and dry land increased significantly and the increasing extent of paddy field was greater than that of dry land.The area of paddy field was 57.7×104hm2 in 1986 and increased to 150.9×104hm2 in 2005 with the dynamic degree of 5.08%,which indicated that the development speed of paddy field was faster than that of dry land;2)The total quantities of paddy field and dry land had significant differences in various stages.From 1986 to 1995,the development speed of dry land was fastest among the three periods.The exploitation scale of dry land enlarged obviously while paddy field was occupied by other types of land use continuously.The speed at which paddy field was occupied was faster than the increasing speed of dry land.From 1995 to 2000,the area of paddy field increased remarkably with the dynamic degree of 36.49%;3)In different periods,the feature of mutual transformation between paddy field and dry land was quite distinguishing.The most significant period during which paddy field transformed to dry land was from 1986 to 1995 and the reversed transformation from dry land to paddy field was between 1995 and 2000.The transformed areas were 33.1×104hm2 and 63.5×104hm2 respectively;4)The mutual transformation between paddy field and dry land had significant regional characteristics.Generally,the center of gravity for paddy field and dry land moved towards northeast and the total offset of paddy field's center of gravity was more than that of dry land.The spatial distribution of dry land transformed to paddy field determined the deviation direction of paddy field's center of gravity,but the deviation direction of dry land's center of gravity didn't depend on the spatial distribution of paddy field transformed to dry land.