The study of lake environment by using remote sensing technique to take the Jianghan lakes as an example

The study of lake environment by using remote sensing technique to take the Jianghan lakes as an example

Zou Shang Hui
Department of Geography, Central China Normal university, Wuhan, Hubei, China

Abstract
The study results by using remote sensing have shown: that the area and the number of the Jianghan lakes have greatly decreased since 1950's the lake environment such as water quality is deteriorating. The speed of lake swamping is discussed, and it indicates that the halophyte will completely cover the Honghu lake in the next thirty years. With a land use program formulated by DTM, the lake environment will be well rebuilt.

Introduction
The Jainghan lake groups all over the Jianghan plain like stars in the sky. Since 1950's of the century, large-scale and unchecked activities of reclamation have resulted in a sharp decrease in the number and area of the Jianghan lake groups. The laudatory to the "Province of a thousand lakes" has been faced irrevocably lost. Even though lakes exist there, they all have been faced with a serious swamping crisis.

The action of reclamation and natural siltation of lakes which have resulted in the vicissitudes of lakes from water to land and the tendency of swamping are clearly reflected in the satellite remote sensing image. Thus we can make the historical lakes (palaeolakes), which sometimes could not be discovered in the field investigation reappear and forecast its future evolution tendency by using remote sensing.

Interpreting the Landsat MSS image and making The historical Jianghan lake groups reappear

The Synthetic Spectra of Lake Facise Sediments ad a Shallow Layer of water in Palaelakes have specific image properties.

In the places of palaeolakes, the sticky and blue-grey lake facise sediments (gley horizon) are deposited. Consequently the places of palaelakes are mostly single-cropping paddy field. During the imaging of the first tem days in June. There is only a shallow layer of water in the fields, without any seedings or sometimes or sometimes with seedings just planted in. the integral spectral of "water" and the lake facise sediments underneath obtained by the sensor differ sharply from the spectra of other water or crops (Fig.1). Therefore, there are specific image properties of these palaelakes.
The Swamping Vanguard Plants Wild Rice and Reed are the important Signs indicating that the Lakes Are becoming Marshes.

The vanguard plants which show signs of the swamping tendency and even of the transformation from lakes into land are usually wild rice and reed. So the communities of wild rice and reed is an important mark to locate the palaelakes d the bank belt of shallow lakes, in which the image features present dense grain, villose structure, and red or rose color, differing from farm crops.
Reappearing the Palaelakes with Image Feature of Embankments Encircling Low Paddy Fields.

For natural and human reasons, the Jainghan lakes have been spitted and dismembered showing a broken appearance. If we can delimit the extent of the initial embankments we will be able to retrieve the location of the palalakes. Although most embankments have already been destroyed today, the minor relief feature of embankments in different land use are still shown distinctly on the image, revealing three color belt structure.

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

He Water Quality

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
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.
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

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.

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.

Forecasting the future change tendency of jainghan lakes by using remote sensing.
All of the Jianghan lakes are faced with the serious swamping threat. Many lakes, Xiaujiahu lakes, Xingdouhu Lake etc., have only the wild rice and the reed communities left on their bottom, almost losing completely their function as lake.

The lake-swamping pace primarily depends on the amount of aquatic plants. The results of Image interpretations express that the stretching speed of the halophyte from the lake shore to the center is 7.6km2 per year in the Honghu lake, i.e the helopyte would completely cover the Honghu lake in the next thirty years.

To formulate a land use program for lake area with DTM

own a plan of returning farmland to lakes, in order to maintain the lake rate above certain level, so that the calamity of food and water logging will be lessened and the ecological environment will be improved.
Determine the four liens by DTM, these are store flood water line, aquatic product breed line, paddy field line and the dry land line. As a result, and ecological-economic system with high benefits and multiple functions could be established.
Divide the agriculture area into deep-water area, shallow water area and lake beach area, in order to develop a water-agriculture in a stereoscopic way.

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