Water Clarity Monitoring Of Small River Based On Medium-resolution Remote Sensing Image

Water, as one of the core elements of ecology and environment, is the most important and the most dynamic factor in the earth’s environment. However, with the rapid development of economy in our country, the problem of the water pollution is more and more serious. Water quality monitoring is the basis of water environmental protection work, and the water clarity, an important water quality parameter, is an important reference index for water quality assessment and prevention and control of water pollution. Compared with conventional monitoring methods, remote sensing technology, which can detect the spatial-temporal change of water quality parameters and make for finding some migration characteristics of pollution sources and pollutants, is fast, macro, inexpensive and periodic, etc. However, for small or medium inland rivers, most pixels in medium-resolution remote sensing images blend various feature information. Therefore, it is necessary to avoid interference from water-shore mixed pixels for small and medium inland water body remote sensing monitoring.First, as an example, Xitiaoxi River is used to recognize and extract stable water pixels and develop a methodology to quantify the minimum river width for stable water pixels. Then, we use this method to assess the distribution of stable water pixels in Xitiaoxi River. A estimation model was established and validated between the spectral reflectance and the SDD based on stable water pixels in 30-m spatial resolution remote sensing image and field measured Secchi disk depth (SDD). Finally, this model is applied in different periods of medium-resolution remote sensing image to predict the Secchi disk depth (SDD). We analysis spatial-temporal variation characteristics of water clarity and explore the reason of the change in view of the land use/cover on both sides of the river, human activities, Socio-economic Policies and development.The results suggest that 1) For 30-m spatial resolution image, the minimum river width for water-quality remote sensing is 64.6-98.5 m, and in 59.6% of the sections of the Xitiaoxi River, at least one pixel with minimal disturbance from the shoreline could be obtained in a transect, in 9.35% of the sections, no pixels could be obtained due to the adjacency effect from the shoreline, whereas in the other 31.1% of the sections, there may or may not exist water pixels with minimal disturbance from riverbank, which was determined by the river direction and pixel position relative to the shoreline.2) The following model can well predict the clarity of Xitiaoxi River in winter:In (SDD)=4.980 b1/b3-28.436 b1-2.601 (R2= 0.94, RMSE= 0.35 m) And the minimum river width recommended in this study contribute to improve the model accuracy.3) Spatial-temporal variation characteristics of water clarity of Xitiaoxi River imply that the decisive factors which influence clarity are anthropogenic factors, such as mining, sand transport, living sewage, agricultural non-point source pollution and so on.