Using the method of composing digital images of two periods to research the vicissitudes of lakes.
It is difficult to distinguish the logged water of the palaelaked from the existing lake water in the image during rainy season, but it is easy to do when palaelakes become dried and only existing lakes have water in the drought and whe, we compose the image of these two period into one dgital image. Thus, the difference the two period lakes areas can be shown sharply in the composite image, yellow referring to the palalakes, while a bright red, the existing lakes (fig.2).

The lake vicissitude map of the Sihu area in Hubei Province has been completed with the analysis results above. It shows that the total number and area have decreased greatly since 1950's

Remote Sensing investigation of lake environment

1. He Water Quality
   
   
   1. There are two features of the spectral curve of water polluted by both living sewage and industry sewage (Fig.3), i.e. (a) the reflectance of all MSS electromagnetic bands in low; (b) The reflectance decreases with the increase of wavelength. The sewage water presents black in the MSS FCC images such as the Shahu lake and the Moshiuihu lake surrounding wuhan
      
      
   2. The principal sign of water nutritiousness is the high density of some kinds of algae whose main criterion to identify is the chlorophyll density. We can use the chlorophyll density to evaluate the nutritious conditions. The higher the chlorophyll density is, the darker the blue color will be in the FCC image. Once the algae highly concentrated on the surface layer of the water, it will show silken pattern in the image as seen in that of west part of the Donghu Lake.
      
      
   3. As Fig3 shows, the high suspending-silt content of water leads to the high reflectance, especially in the red band, therefore, the regression equation of image density corresponding to the silt content can be established. The water of high content of suspending silt appears green in image.
      
      
   
   Fig. 2 The two-period composite image of the Honghu area, yellow referring to the palaelakes, while a bright red, the existing lakes
   
   
   Fig 3 The spectral curves of varieties of the water (Accordign to the CCT tape on June 28,1980)
   
   

   a.	lush aquatic plants
   b.	the changjing river
   c.	the breeding farm with less aquatic plants
   d.	a little bit aquatic plants
   e.	the polluted water, it is similar to the water of lush bottom plants

   
   
2. The Aquatic Plants.
   
   They hydrophytes, the float-leaf plants and the pleustons may be directly interpreted on the image because they are exposed above the water surface. As to the bottom plants, we can only use indirect methods. As Fig 3 shows the reflectance of the bottom plant in Honghu lake is low in all MSS bands and its image appears dark such as those of the Changhu lake, Wuhu lake etc.
   
   The low reflectance of water is due tot eh high transparency and the lack of the reflectance energy of suspending particles. The reasons are as the following ; (a) The aquatic plants block the windwave and current to stir the bottom mud up; (b) They also limit the growth of plaktons, (c) They absorb the pollutants. Table 1 reveals the relationship between the water color an biomass in image has been established. Take 1. The relationship between the biomass and the water transparency
   
   
   Take 1. The relationship between the biomass and the water transparency
   

   	Honghu Lake	Wuhu Lake	Liangzihu lake	Taibaihu lake
   Biomass(g/m3)	4423	3800	2123	1699
   Transparency(m)	1.9	1.52	1.19	0.3-0.6

   
   The spectral information of aquatic plants is weak because of covering water and its influence. After digital enhancement, the information will be enhanced (fig.4), float-leaf plants ad bottom plants also could be distinguished.
   
   
3. An analysis of Dynamic changes of lake environment



1. As the analysis report shows (linkaiyu, 1984) in the middle and late 1950's there were 609 lakes with the area of more then 0.5km2, but in the early 1980's only 309 lakes were left. The number of lakes decreased by 49%




Fig 4 After digital enhancement of the Changhu lake images in which, some kinds of aquatic plants could be distinguished


2. The open lakes linked by river-lakes in 1950's such as the Honghu lake, now have become partly closed on the image.
   
   
3. The water quality of lakes become worse and worse because of the waste water pollution. For example, the water in the donghu lake has started its nutritiousness stage since 1980's
   
   
4. The Biota has become more unitary due the influence of human activities.

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