Abstract:

Systematic analysis of spatial change of polders is very important for revealing water storage change and flood occurrence in lake areas.This paper addressed spatial and temporal changes of the polders in the Dongting Lake in the period from 1921 to 1990s.Historical maps of the polders were digitalized for the five periods and change detection analysis were performed.The five periods include 1921-1930,1931-1949,1950-1963,1964-1980 and 1980s-1990s.Our main conclusions are as follows. (1) From an aspect of temporal change,the polders showed 〖JP2〗an increasing trend from 508419 km2 in 1921-1930 to 909813 km2 in〖JP〗 1980s-1990s.In spite of this,the polders changed with spatial and temporal differences.The ploders expanded at a rate of 3126%,3122%,1198% and 273%,corresponding to an annual rate of change of 313%,312%,1198% and 034%.In contrast,the polders diminished in some areas at a rate of 1268%,1370%,340% and 000%,corresponding to an annual rate of change of 127%,137%,024% and 000%.The largest change appeared in the shift from the first to the second period,during which land reclamation was excessive but effective management was lacking.Since then,the polder reclamation was replaced with maintenance and the polder areas remained stable with the policy of “returning farmland to lake”.(2) From an aspect of spatial change,the polders expanded generally and encroached on Dongting Lake.In 1921-1930,the polders were mainly distributed in the northern part of the lake area in a relatively sparse form.In 1931-1919,the polders appeared to intersperse with each other and the newly constructed polders were mainly concentrated in the central and eastern part of the Yuan River.In 1950-1963,the polders expanded in highelevation areas and the areas surrounding Datong Lake,the south and the west Dongting Lake.In 1964-1980,the polder expansion mainly appeared in the northwest corner of the lake area.In 1980s-1990s,the polders increased very slightly.Our analysis supports that excessive reclamation caused serious shrinkage of the lake water surface.Because Dongting Lake plays an important role in regulating the Yangtze River and flood prevention and has important ecological and environmental values,it is of priority to restrain lake reclamation and encourage the returning farmland to lake.In addition,our study should be valuable for understanding the water storage variation and floodings in the lake area,which needs further investigation