Study On The Impact From Water Level Dynamic On The Wetland Plant Around Poyang Lake With Remote Sensing

Wetland plant is an important component in wetland ecosystem. Water, sush as the depth of water, fluctuation, flooded time and flooded frequency, is the main factor influencing the growth of wetland vegetation. Hydrological processes is considered to be decided to wetland formation and maintained and the only important factor. The different variation period of water would affect the vegetation from different angles. Still or fluctuation water would affect the growth rate. Water depth would affect the respiration at root or rhizome, and then would affect the biomass and photosynthesis of the plants. Flooded time and flooded frequency would play a significant impact for the growth and survival of wetland plants. Water variation is the important factor which could decide the plant growth and community distribution.Poyang lake wetland is a kind of special natural wetland ecosystem, be river wetland when water reducing and lake wetland when water increases. Based on remote sensing, the grassland was extracted with the cloudless Landsat images in different time, was analyzed the relevance with the water level, was analyzed the season and elevation distribution characteristics. Besides, the essay analysis the relationship between time series (2002-2008) EVI (Enhanced vegetation index) from MODIS (the moderate-resolution imaging spectroradiometer) and the water level, and the distribution features in different elevation.(1) We establish the relationships between the grassland area and the water level, and explained that the second polynomial function gave a significant negative correlation between the area and water level. According the model, we got simulated area at average level of 52 years. We found the same season charateristics was showed by the simulated area and the mouthly area of images: the area reduced when the water rose and increased when it flowed back. The area decreased little by little in spring, until got the min in summer, then the area increase in autumn and got the max in winter.(2) The potential grassland map was made with the different images. The area of potential grassland reached 2441km2, was distributed at the lake rump and delta region with fan-shaped in Poyang Lake, more in west bank than east bank.(3) The time-space distribution of grassland EVI showed that, the changes were different in different elevation, and a unique seasonal feature was displayed. The mean values of EVI increased with the elevation increase above 11 meter; expect 16m, there were two growing seasones at other zonals.(4) We used time series EVI in combination with hydrology data to examine the relationships between hydrology factors and EVI for grasslands. There are no-significantly linear relation between EVI and water level, but significantly negative correlation parabolic relation. Also, we discussed contributors affecting the correlation between EVI and hydrology factors. The response of EVI to hydrology showed a time lag effect and different in each year. The time lag as to response of EVI to hydrology was 16 days in 2002-2003, 48 days in 2004, 64 days in 2007.