| Wetland is a very important component in the ecosystem.We used Poyang Lake as the object of study.In order to get the flooding frequency chart of Poyang Lake from 2001 to 2015,the paper utilized a different period of Landsat images and the250 m resolution Modis data preprocessed between 2010 and 2015.Then,combined with the data of water level,we analyzed the temporal and spatial characteristics of flooding range of Poyang Lake during this period.After that,based on the data of water level observation,we analyzed the variation of water surface slope in the flood season and the Poyang Lake,and discussed the difference and the causes of the flooding characteristics of Poyang Lake.On the basis of these,We measured the test results according to the time-series MODIS-NDVI data and field plant quadratic biomass of Poyang Lake.More over,we discussed the relationship between the temporal and spatial characteristics of vegetation growth and the flooding characteristics of Poyang Lake.The following conclusions:(1)The spatial and temporal distribution of flooding in Poyang Lake is significant.In the case of the main lake,it was almost flooded throughout the year.Its southern basin was broad and flat,and it had a higher terrain.In addition,its flooding frequency was lower than that of the lake center.The main lake presented river facies when the Xingzi water level was lower than 5.7m and a large number of sub lakes were formed due to the separation between the retained water surface and the main lake.When the water level was higher than 9.6m,the main lake and the sub lakes gradually connected into one piece and it presented as lacustrine facies.When the water level was higher than 15.5m,the surface of the lake basically reached the maximum state,the flooded area of Poyang Lake stabilized between 3000-3200 km2.(2)Though a significant linear relationship was found for the water level and inundation area for the Poyang Lake,the water surface area shows uncertainty.Water level in Poyang Lake is higher in the south and lower in the north when lake water level is low,and the spatial heterogeneity of water level was decreasing with lake level increasing.Water level slope in Poyang Lake in the time period of water withdrawal was greater than that in flood period.Affected by fishery practice by levees of dish-shaped sublakes,the water surface area of the sublakes in waterwithdrawal period is greater than that in flood period.Sand mining also changed the water extent around the sandpits when in low lake level.(3)According to the spatial distribution of vegetation,it can be found that the vegetation index was in the area which covered by water obviously lower,such as the main lake and the part of the sub-lake.The interannual difference of vegetation distribution was related to the variation of annual ample water and low water.The water flooded time was prolonged in the year which has more annual precipitation and instead the vegetation coverage was decreased,Conversely increase;(4)The areas of the wetland vegetation where showed a positive growth trend of Poyang Lake was mainly distributed in the western,southwestern,southeastern,the main lake and partial sub-lake.On the contrary,the negative growth trend was mostly distributed in the southwestern sub-lake.(5)The biomass of wetland plants of Poyang Lake was significantly affected by flooding process,while the flooding frequency rose,the value of NDVI declined. |
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