Process of Wetland Reclamation in the Bielahong River Basin of the Sanjiang Plain

Since the 1950s several reclamations of wetlands have been carried out in the Bielahong river basin of the north of the Sanjiang plain.Farmland instead of wetland has been the main landscape through the large-scale reclamation.The wetland ecosystem was badly disturbed and destroyed while continuing cultivation to the Bielahong river basin.Using theory and methods of landscape ecology,land use and land cover changes in 1954,1976,1986,2000 and 2005 were analyzed based on RS and GIS.The data sources mainly came from the Landsat TM digital images,topographic maps and statistical data.The landscapes changes,especially analyzing the process of land reclamation and wetland shrinkage was focused on in the paper.As a result,Large area of wetland patches were replaced by fragmental farm patches that were then combined into large patches gradually.This resulted in the increase of patch shape index and the fluctuant change of landscape evenness index.From 1954 to 1986,the farm area grew rapidly,especially in the early stage of reform and opening policy from the end of 1970s to the beginning of 1980s when the walue of landscape dynamical index reached as high as 0.201.The analysis results on the ecological landscape patterns of land use indicated that farmland was the main landscape types which occupied 63.69% of the total study area,comparatively wetland occupying nearly 19.44% in 2005.The study results on centroids of landscapes showed that the shift degree of farmland centroid was much higher than that of wetland centroid in recent 50 years,that the centroid of farmland moved from west to east with a distance of 44.7 km while wetland centroid moved 20.5 km to the northeast.Based on the analysis of the spatial and time variations of wetlands,the causes of wetland shrinkage were further analyzed that the agricultural cultivation with the population increasing was the primary factor,while the mean annual temperature increases and the annual precipitation decreased obviously in the study area in recent 50 years.