| Wetland,as a unique ecosystem formed by the interaction between land and water on the earth,is an important land resource,which can regulate river runoff,resist floods,provide industrial,agricultural and domestic water,improve climate,multiply aquatic animals and plants,and communicate shipping.Improve regional ecological environment and maintain regional ecological balance and other functions.At present,international studies have shown that the supply of fresh water resources has led to a global water scarcity crisis due to global warming,rapid population growth,industrialization,urbanization,agricultural development and and lifestyle changes in water use.About 20% of the population currently lacks safe drinking water;Therefore,the study of wetland environmental factors is one of the hot spots that scholars at home and abroad pay attention to.In previous studies,the study of wetland environmental factors is mostly based on traditional hydrological monitoring,but its other constraints such as economic development level and climate are often concentrated in specific areas,while many areas are rarely distributed,and it is difficult to record data.Access to other factors greatly limits the need for research and management applications.In addition,the limited number of site observation data is difficult to effectively reflect long-term sequence changes in wetlands,making it more difficult to reflect spatial characteristics.Therefore,the study of wetland hydrological elements needs to combine multi-source remote sensing data and RS technology to further analyze the trends of various hydrological elements.The Lancang-Mekong River is an international river.The development and utilization of water resources affects the social and economic development of many countries and is a hot spot for international research.Therefore,this paper takes the largest freshwater lake in the Lancang-Mekong River basin,Tonle Sap Lake,as an example to study its hydrological elements.Based on MOD09A1 surface reflectance data,satellite altimetry data,Landsat-5 and Landsat-8 data,and TRMM 3B42V7 rainfall data,this paper uses single-band threshold method,MNDWI index and enhanced vegetation index(EVI),surface water index(LSWI)and the combination of EVI and LSWI difference(DWELL),satellite altimetry water level calculation and radiation transfer equation method,respectively obtained the water body area,water level and water surface temperature data of Tonle Sap Lake,Based on the measured hydrological data published by Mekong River Commission website,the variation trend and reasons of each factor are analyzed.Finally,the IPCC sensitivity study was used to analyze the response of the water area to the climate change of rainfall and temperature.The main contents and achievements of this paper include:(1)Monitoring the water body area of Tonle Sap LakeBased on MRT and ENVI software,using MOD09A1 surface reflectivity data as data source,single band threshold method,MNDWI and combined operation algorithm are used to monitor the water body area of Tonle Sap Lake.The results of three methods for extracting water area are numerically similar.The single-band threshold method and the combined algorithm are relatively close.The water area of MNDWI is generally lower than the other two methods.In space,the single-band threshold method and the MNDWI index cannot be extracted.The combination of small waters,and the combination algorithm can not only extract a small area of water,but also better identify the water with mixed pixels,which is the best in the three methods.The flood first spreads in the eastern part of the lake(along the Tonle Sap River)and its tributaries along the north-south flow,and then expands to the northwest;the minimum water area appears from April to May,and the maximum is from September to November.Between the years,the average annual water area is 7029.59km2,the average annual water area in the dry season is 5368.21km2,and the average annual water area in the rainy season is 9355.53km2.(2)Monitoring and verification of the water level in Tonle Sap LakeThe satellite altimetry data Jason-2 and Jason-3 have high water level monitoring accuracy,and the data periodicity has consistent and continuous characteristics.It is one of the important data sources for long-term sequence monitoring in surface waters.The water level monitoring results of Tonle Sap Lake show that the water level of Tonle Sap Lake is consistent with the water area in time.The lowest water level is from April to May,the highest water level is from September to November,and the average dry water level is 4.93 m.The average water level is 6.92 m and the average annual water level is 5.78 m.Combined with the measured data of Prek Kdam hydrologic station,the results show that there is a significant linear correlation between the two,and the correlation coefficient is 0.953,and it has passed the 98% significance test.(3)Inversion of surface temperature in Tonle Sap LakeLearn about the changing characteristics of the water temperature elements of Tonle Sap Lake.Based on the principle of surface temperature inversion,the surface temperature of Donglissa Lake is monitored by using the radiative transfer equation method.The results show that the lowest water temperature is between 26 ? and 27 ?(in January or December).The maximum water temperature is between 30 ? and 34 ?(in April or May),and the average temperature is between 26.5 ? and 31.6 ?.The results are basically consistent with the field survey measurements of Campbell and Burnett.(4)Analysis of the trend of environmental factors in Tonle Sap LakeUnderstand the change trend of each element of the lake and analyze the cause of the change.The linear tendency estimation,the moving average method and the Mann-Kendall trend analysis method were used to analyze the water area of the Tonle Sap Lake,the water level and the change of rainfall in the Tonle Sap Lake Basin.The results show that the water area,water level and rainfall all show a downward trend,but it is not significant.The change trend of water area,water level and rainfall is basically the same,but the annual extreme value appears,the water area and water level are about one month later than the rainfall.Combined with previous studies,it can be seen that the construction of the levee of the Tonle Sap Lake tributary,the control of the main stream of the Mekong River and the reduction of rainfall in the lower reaches of the Mekong River have led to a decrease in the amount of water from the Tonle Sap Lake,while the deforestation in the Tonle Sap area is severe and the soil erosion is aggravated.The sedimentation of lakes has increased.The above factors have led to a decrease in the water area of the Tonle Sap Lake and a decrease in the water level.In addition,the average temperature in Siem Reap from 2008 to 2018 is on the rise.For this reason,the water temperature has risen slightly.(5)Sensitivity of environmental factors in Tonle Sap Lake to climate changeThe environmental factors of wetland are complicated and varied,and the degree of response to climate change is also different.The sensitivity analysis method is used to analyze the response degree of the water body area of the Tonle Sap Lake under different temperature and rainfall scenarios.The results show that: The water area is positively correlated with rainfall and negatively correlated with temperature.Under the constant temperature,the water area change rate is about 15.2%.In the climate change scenario set in this paper,the water body area is more sensitive to temperature changes than the rainfall change.In the case of constant rainfall,the water area changes by 17.85%. |
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