Evolution Process Of Chagannur Lake In The Past 46 Years Under Changing Environment And Its Water Source Analysis

The environmental impacts of global climate change has been one of the important study area for the researchers at home and abroad.The impact of global temperature increase on lake water area is obviously increasing,and the lake water area can be a responding indicator of climate change within the region.The remote sensing technology can be used to monitor the lake water area on a large scale and long time scale.The remote sensing technology is used to deduce the variation of the lake water area of the Chagannur Lake.The variation trend of the lake water area is obtained by interpreting remote sensing images.The satellite images of the lake water area from 1973 to 2018,i.e.about 46 years,are used for the deduction.The satellite images include RGB=431 color synthesis from Landsat 1-4 series images,RGB=742 color synthesis from Landsat 5-7 series images,and RGB=753 color synthesis from Landsat 8 series images.The lake water area of the lake is calculated based on the maximization of the abstraction of the lake water body.It is found that the lake water area of Chagannur has been decreased dramatically in the last 46 years although casually some expanding happens,with an average annual variation of-2.37 km2/a.The Lake water area is decreased from 100.09 km2 in 1973 to 30.16 km2 in 2018.The total water area of the lake was reduced sharply from 2000 to 2002,and only the small part of the Chagannur Lake remained in 2002,i.e.east lake,with a sharp decrease of 59.92%in 2001.The precipitation,evaporation and temperature,etc.of 3 weather stations from 1960 to 2017 are used to overall consider the climate factors to impact the lake water area combined with the characteristics of the variation trend of the lake water area obtained.The analysis results show that there is an overall slight decreasing trend of precipitation in the watershed of the Chagannur Lake,and the annual average precipitation is 277.91mm.The evaporation is increased slightly with an annual average 2020.43mm,while,the annual temperature is increased significantly with annual average temperature 1.73?.The results show that the lake water area is of a significant correlation with evaporation,temperature and precipitation.While the correlation between the lake water area and precipitation is less because the variation of the precipitation is less comparing with that of evaporation with the climate warming and drying increasing.The results of mutation point test and wavelet analysis also confirm that the warming and drying trends of Chagannur Lake Basin are obviously strengthened in the early 21st century,which is also consistent with the warming trend in the most parts of the world.The main affecting factors of the water area shrinking of the Chagannur lake are evaporation and temperature,then the secondary affecting factor is precipitation.In order to find out the water recharge source of the Chagannur Lake,the characteristics of hydrogen and oxygen isotopes in different water bodies in the basin were analyzed.It is found that the hydrogen and oxygen isotope values of lake water are the largest and are more abundant than those of well water are.This indicates that there may be secondary evaporation of lake water after groundwater recharge,which result in excessive isotope fractionation,and cause the large concentration of delta ?D and delta?18O of lake water.The Delta ?D and delta ?18O in the well water are below the left of the local precipitation line and are on the global precipitation line,which indicates that the groundwater mainly comes from the meteoric water.The deuterium surplus value of well water is the largest,while that of lake water is-19.84%,with the greatest negative degree.This shows that the evaporation of lake water is the strongest,which is related to the geographical location,climatic characteristics,low water level and open water area of Chagannur Lake.It is found that the hydrogen,oxygen isotopes and TDS in Chagannur Lake Basin have a consistent variation trend in different water sources,and do not change with season.All the above results indicate that the Chagannur Lake is recharged by both groundwater and river water.