Decadal Inundated Area Dynamics Monitoring And Analysis Of Three Typical Fresh Lakes Of China Based On Landsat Time-series Satellite Images

Water is an indispensable substance guaranteeing for the survival of Earth life,distribution of which has a significant influence on the regional social,economic and cultural development.China,with the world's maximal population,has a continuously increasing demand for freshwater resource,which is,however,challenged by its unsubstantial water resource per capita as well as uneven space distribution.Yangtze River Basin and its south region accounting for only 36.5%of Chinese territory area contain an amazing 81%water volume of total national freshwater resource.Meanwhile,these regions also contain three major freshwater lakes of China,which are known as Dongting.Poyang and Tai Lake,distributing from west to east,respectively.The three major lakes crucially supply for the human production and living of several major provinces along Yangtze Basin downstream.Considering anomalies have frequently happened to their water areas in specific years,a scientific,objective and accurate analysis for the reason of area variation is thus important for protecting national precious freshwater,which is also the foundation to design a reasonable and sustainable development plan for water resource in China.Based on Landsat satellite images obtained from 2000 to 2016,we firstly constructed a systematic water extraction methodology and then applied it to collect images so as to delineate water areas of three typical lakes in Yangtze Basin downstream from 2000 to 2016 year,including annual wet and dry season.Afterwards,correlation analyses between lake area and external factors were conducted to explore the practical reason for lake area variation in a long term series.The main contents of this study included:(1)We firstly selected effective water indexes(WIs)from common ones by means of comparison.Candidate effective WIs were subsequently combined with a modified fuzzy C clustering method(MFCM)to construct a novel water detection method,which would be testified in different heterogeneous water regions globally.(2)The constructed water extraction method was used to water area of three lakes in terms of annual wet and dry season by means of collected Landsat long-term time series satellite data.(3)External data such as annual runoff,precipitation,landuse data and sand mining record were then combined with annual lake areas and were applied to carry out correlation analysis.These data there were categorized into two classes,i.e.,nature and human factors.Area variation analysis in sixteen years was divided into different time ranges.For instance,we had compared different change rates of three freshwater lakes before and after the application of famous Three Geoges Dam in Yangtze River to further reveal the reasons for lake area variation.The results showed that:(1)According to the comparative results with regard to sixteen characteristic water sites and different seasons,we found MNDWI and AWEI manifested superior effect compared to several common WIs.Moreover,the method combined WI and MFCM showed more satisfactory and robust water extraction effect in different water environments with heterogeneous backgrounds compared with other methods.(2)The constructed water extraction method had been successfully implemented to delineate inundated area of three lakes in terms of annual wet and dry season.Relevant accuracy and error analyses showed the satisfactory results of the introduced method in target lake areas,and the overall accuracies for three lakes' extraction were 98.46%,98.77%and 99.17%,respectively.(3)By dynamic fitting analyses,we found the annual areas of Dongting and Poyang Lake exhibited obvious decreasing tendency with specific rates of-0.86%/yr and-0.74%/yr,but Tai yet showed subtle area variation in terms of whole time range,with a rate of 0.068%/yr.According to correlation analysis results,we discovered the area decrease for Dongting and Poyang Lake was induced by not only natural factor like annual runoff and precipitation,but also by anthropogenic influence such as city construction and sand mining.The area of Tai was hardly impacted by outside factors,which was benefited from its inherent bottom topography,space distribution of rive inflowing into lake and construction of artificial facilities around lakeshore.