| In the context of global warming,the water temperature of many lakes has been rapidly increasing in the past 40 years,especially in high-altitude lakes which are more sensitive to climate change,such as lakes in the Qinghai-Tibet Plateau and Yunnan-Guizhou Plateau regions.Lakes are an important carrier of ecological resources and economic development,and their aquatic environment determines the quality of regional ecological environment.Therefore,lake water temperature is of great significance as a basis for exploring the physical,chemical and biological characteristics of lakes.This study takes Erhai Lake in China as the research area and conducts a comprehensive study on the trend and significance of Erhai Lake’s climate factors in the past 40 years.The change trend of Erhai Lake’s water temperature from1979 to 2019 and its correlation with climate factors were studied through climate data and measured water temperature simulation.The study found that the surface water temperature of Erhai Lake showed a significant upward trend in the past 40 years,with the most significant temperature changes occurring in spring and autumn.At the same time,the study also found that the change in water temperature of Erhai Lake is affected by various climate factors,such as temperature,precipitation,wind speed,and radiation.In addition,the study also found that there is a huge difference between the simulated and measured values of bottom water temperature.After introducing bottom geothermal heat,the simulated results are closer to the measured values.Therefore,this study verified that the bottom geothermal heat of Erhai Lake provides a heat source for the lake’s thermal mixing phenomenon.In conclusion,this study systematically studied the trend and influencing factors of Erhai Lake’s water temperature changes under climate change,providing an important basis for the ecological environment protection and water resources management of Erhai Lake.The conclusions of this study also have certain reference significance for the study of global lake water temperature changes.The main research results are as follows:(1)The study investigated the development trends of climate factors and surface water temperature in Lake Erhai by analyzing the Theil-Sen slope estimation and Mann-Kendall non-parametric test methods.The research time spanned from 1979 to 2019,and the research scale was at the annual and seasonal levels.The results showed that shortwave and longwave radiation had no significant impact on climate change in the region,while changes in near-surface air temperature and relative humidity were significant,indicating their significant influence on climate change.Changes in precipitation,snowfall,and wind speed did not significantly affect climate change in the region.From 1979 to 2019,the annual average surface water temperature of Lake Erhai showed a slight upward trend,but the trend was not significant.Over the past few decades,the average surface water temperature of Lake Erhai has remained around 16°C.At the seasonal level,near-surface air temperature and relative humidity showed significant trends in all four seasons.Near-surface air temperature increased significantly in spring and summer,while relative humidity decreased significantly.However,other meteorological factors did not show significant trends.In autumn,near-surface air temperature and relative humidity showed significant decreasing trends,while precipitation showed a significant decreasing trend,and other meteorological factors did not show significant trends.In winter,near-surface air temperature increased significantly,while relative humidity decreased significantly,and other climate factors did not show significant trends.It is worth noting that the trend of temperature increase and relative humidity decrease in autumn is the most prominent among all seasons,indicating a warming and drying trend in Lake Erhai,especially in autumn.Surface water temperature of Lake Erhai showed different trends in different seasons.The average surface water temperature in spring and autumn increased year by year,while the rate of increase in summer was relatively slow,and it decreased slightly in winter,but the decrease rate was relatively slow.Therefore,the surface water temperature of Lake Erhai showed different trends in different seasons,with the most significant changes in spring and autumn.(2)The correlation between climate factors of Erhai Lake and surface water temperature of the lake was analyzed and explored at the annual scale and seasonal scale.The results showed that short-wave radiation showed a certain degree of positive correlation with surface water temperature,while long-wave radiation showed a significant positive correlation with surface water temperature,relative humidity showed a significant positive correlation with surface water temperature,while wind speed showed a significant negative correlation with surface water temperature,precipitation showed a certain degree of positive correlation with water temperature,while the relationship between snowfall and surface water temperature was not obvious.On the seasonal scale,the correlations between surface water temperature and different climatic factors in the four seasons of the Erhai Lake differed.In general,long-wave radiation,relative humidity and wind speed were strongly correlated with surface water temperature in the Erhai Lake,with the most significant correlation for long-wave radiation and relatively small correlations for short-wave radiation,near-surface air temperature and other climatic factors with surface water temperature.These results provide a basis for a deeper understanding of the physical and chemical processes of the Erhai Lake ecosystem.(3)Based on measurements taken at four monitoring stations,the thermal structure of Erhai Lake was analyzed.The results showed that the surface and bottom temperatures of the lake were very close,with a temperature difference between 0.1°C and 0.5°C.Observations indicated that the lake underwent complete mixing,including in shallow and deep water areas.To explore the reasons for the thermal mixing phenomenon in Erhai Lake,we used the hydrodynamic model GLM for verification.The GLM model was calibrated using the measured water temperatures in Erhai Lake,and we determined the main parameters for model calibration,including the sensible heat transfer coefficient(CH),latent heat transfer coefficient(CE),proportion of solar radiation absorbed at the surface(asw),extinction coefficient(Kw),sediment temperature(TZmean),and bottom heat exchange coefficient(Ksoil).During the model calibration process,we conducted two simulations: the first using the annual average air temperature at the research site as the sediment temperature at the bottom of the lake,and the second taking into account the heat brought into the lake by geothermal energy at the lake bottom.The simulation results confirmed that geothermal energy did provide heat to Erhai Lake.Therefore,to further explore the actual contribution of geothermal energy to the lake’s heat during the entire year of thermal mixing in Erhai Lake,we used the lake heat budget equation,considering various heat sources including solar radiation,longwave atmospheric radiation,and geothermal energy,to reveal the heat conduction of geothermal energy at the bottom of the lake.According to our calculations,the annual average heat input from geothermal energy entering the lake was 12,387 MW(50 W/m2),which accounted for 10% of all heat sources in the lake.The geothermal heat released from the lake bed plays a crucial role in promoting the thermal mixing of the entire water column of Erhai Lake.Furthermore,during the experiment,we found that geothermal energy at the bottom of the lake had been increasing in recent years,which may be due to increased geothermal activity in the region and could have a significant impact on the ecosystem of the Erhai Lake area.In summary,this study emphasizes the importance of geothermal energy in shaping the thermal structure of Erhai Lake.The GLM simulation and heat budget calculation provide a comprehensive understanding of the lake’s thermal dynamics,which can provide information for the management and protection of this valuable resource. |
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