| Wetland is a particular kind of ecosystem between water and land,and the types of wetland are master of hydrological condition,which is often changed by the order of nature or the human activities.Especially the development of the agriculture drainage ditch had changed the hydrological condition of wetland immediately,and the wetland landscape changed greatly.The Bielahong River watershed,as a typical wetland watershed,in the heart of the Sanjiang Plain of northeast China,is covered by wetland landscape more than 95%,before the agricultural drainage ditch started.Along with the development of the agriculture ditch,the wetland had been lost cosmically.In this study,at the watershed scale,based on relief map and land-use map, according to the theory and methods of landscape ecology,using the RS and GIS techniques,the land use/wetland landscape maps in the year 1967(before the water-drainage works),1983(after the low-grade water-drainage works)and 2005(after the high-grade water-drainage works)are formed.And then analyze the intensity of water-drainage ditch in the different parts and different periods of the watershed and its influence to the changes of wetland landscape pattern.We calculated various landscape metrics of each zone at both the patch level and the landscape level using the powerful landscape structure analysis program,FRAGSTATS,including area, patch density and size,diversity,and so on.Finally we found that:(1)The special-temporal dynamic changes of agriculture water-drainage ditch: there was no ditch-building before 1967,and until 1983,the number of the ditch was 395,more then 1000 was increased in 2005;the total length and the density of the ditch respective were 880.28km and 0.2042 km/km~2,which had increased more then doubles until 2005;and the structure of the ditch was becoming more complicated. Besides,the scale and the structure of ditch were different spatially in watershed:the density of the ditch is maximal in the upriver reaches of river;the scale and total length of the ditch were uppermost in the middle reaches of river;in the lower reaches of river was the least.(2)The special-temporal dynamic changes of the wetland landscape structures: the wetland scope was being decreased extensively in the latest 40 years,the wetland area was 3927.03km~2 in 1967,until 1983,25.85%was lost,and the wetland accumulative lose ratio was 72.78%,and 1068.80km~2 were kept down.The more changes in the wetland landscape structure:The patch number increased from 435 in the year 1967 to 495 in 1983,until 2005 was 2475;the Landscape Diversity Index reduced from 0.8028 to 0.4047;the Landscape Separation Index increased 0.1679 to 2.6413.Besides the wetland landscape was different spatially in the watershed:the wetland coverage percentage in 1967,the upriver reaches of river 99.40%＞the lower reaches of river 96.67%＞the middle reaches of river 82.54%;until 2005,the wetland accumulative lose ratio,the upriver reaches of river 92.34%＞the middle reaches of river 77.07%＞the lower reaches of river 53.71%;wetland fragmentation index,the upriver reaches of river 7.1429＞the middle reaches of river 3.3750＞the lower reaches of river 1.3939.(3)The Influence of the agricultural ditch on the wetland landscape:The development of agricultural drainage ditch has significant effect on the wetland landscape changes.Firstly,the correlative index between the water-drainage ditch scale and wetland patch number was 0.8666;secondly,the ditch density had prominent influence on the wetland landscape structure and wetland scale,the correlative indexes of which were 0.7145 and-0.6256.Based on the analyzing,the effective measure of the wetland restoration should be the restoration of wetland hydrological condition,and then bring forward suggestion of wetland protection.Quantifying the Influence of the agricultural ditch on the wetland landscape should provide useful information for effectively monitoring, assessing,and managing watershed wetland. |
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