Study On Thermodynamics And Stable Isotopes Of Water Body In The Lugu Lake

Lugu Lake is located in the southeastern margin of the Tibetan Plateau,southwest of Sichuan Province and northwest of Yunnan Province at the junction of the two provinces.Lugu Lake is in the southwest monsoon climate zone,belonging to warm temperate mountainous monsoon climate.Its climatic characteristics have many aspects,for instance,rainy season and dry season is clearly demarcated,the rainy season's precipitation is abundant and accounts for about 87%of the annual precipitation,which is mainly affected by the southwest monsoon,the dry season is affected by the continental dry air mass,which is dry and less rain and evaporation is large.Lugu Lake recharges mainly rely on precipitation and surface runoff,and consumes through the Caohai leakage and lake evaporation,and the water budgets keep basically balance.Lugu Lake is a freshwater lake belonging to deep water type,its water is not frozen throughout the year,the impact on the lake heat budget of inflow to and outflow from the lake is minimal,at the same time,the redidence time of lake water is long,and less affected by human activities.Therefore,it is the best choice for studying thermodynamics,isotopic characteristics of lake water and hydrological cycle.However,thermodynamics and hydrogen and oxygen stable isotopes of large deep lakes in the Yunnan-Guizhou Plateau are basically blank at present.With the development of testing technology and further research of the stable isotope,the oxygen and hydrogen stable isotopes of water body have been widely used as a mature and accurate tracer method in the field of hydrology.Therefore,based on this situation,this study takes Lugu Lake as the research object,and makes exploratory study on thermodynamics,lake water isotopic compositions(?²H and?¹⁸O)and hydrological cycle of deep lake located in the Yunnan-Guizhou Plateau.In this study,firstly,the thermal stratification and heat budget characteristics of Lugu Lake were analyzed based on the water temperature.Secondly,the?²H and?¹⁸O in various water bodies were analyzed,and the mutual recharge relationship between different water bodies was obtained,and its hydrological cycle process was preliminary understood,and the oxygen isotopic model of semi-closed lakes in southwest China was established to estimate evaporation of the Lugu Lake.Finally,the isotopic characteristics of the different vertical sections were analyzed and the influence mechanism was discussed preliminarily.The conclusions of this study are mainly reflected in the following aspects:1.The water temperature of the Lugu Lake was present in both the whole stratosphere and the positive temperature layer.The three-layer structure was formed in the vertical direction,such as epilimnion,thermocline and hypolimnion.Thermal stratification of the Lugu Lake was formed between March and June,developed stably from July to September,began to enter the extinction period in October,and thermocline completely disappeared in February.Through the calculation of the heat budget of the Lugu Lake,the maximum solar radiation absorbed by the lake water was from March to May,the effective radiation was larger in dry season,and sensible heat was minimum among all of the thermal components.2.The spatial distribution of?¹⁸O in the Lugu Lake was characterized that?¹⁸O of surface lake water in the south was lower than that in the north,mainly due to the result of the dual influence of evaporation effect and river water recharge in the southern lake water,revealing that the inner circulation process of lake water.According to the water balance and isotope mass balance equations of lake,the oxygen isotope model of semi-closed lakes in southwest China was established.Meanwhile,combined with relevent meteorological data,lake evaporation?E/I?was preliminarily estimated in Lugu Lake.3.The surface water line equation was established:?²H=5.34?¹⁸O-30.43?R=0.99,P<0.01?,which indicated the surface water experienced a dramatic evaporation process after atmosphere precipitation supply,and the initial isotopic value of the precipitation of the recharge source of surface water was calculated.By analyzing the isotopic compositions of precipitation,river water and lake water,it was concluded that the isotopic values of lake water were obviously positive,which illustrated that the lake water was strongly evaporated and the river water had experienced a certain degree of evaporation before supplying the Lugu Lake.4.The variation of isotopes in the Lugu Lake was small in vertical direction,indicating that the lake water isotope was influenced by the many years cumulative evaporation effect.The stratified phenomena of the water isotope with the depth change were basically consistent with the thermocline position.The isotopic fluctuation of the upper lake water was great,which was influenced by the external factors such as temperature,precipitation,relative humidity.However,the isotopic fluctuation of the lower lake water was very small,mainly related to the water residence time and the slowly mixing.Furthermore,the seasonal variation of isotopes in the Lugu Lake was very obvious in vertical direction that dry season was larger than rainy season.5.According to the characteristics of the isotopic compositions in the lake water,the lake water evaporation line equations were established in the dry and rainy seasons,respectively.They were as follows:?²H=5.65?¹⁸O-28.86?R=0.86,P<0.01?and?²H=7.43?¹⁸O-18.67?R=0.84,P<0.01?.The slope of the Lake water evaporation line in the dry season was smaller than that of the global meteoric water line?GMWL?and Lijiang local meteoric water line?LJLMWL?,which reflected the evaporation effect of the precipitation in the warm temperate mountainous monsoon climate zone before supplying with lake.The slope of the lake water evaporation line in the rainy season was close to the slope of the GMWL,but was slightly larger than the slope of the LJLMWL,indicating that the lake was caused by a mixture supply of precipitation in different periods or different water bodies.6.Based on the preliminary study on the influent mechanism of lake water isotopes,we found that there was positive correlation between?¹⁸O and water temperature(?¹⁸O=0.035T-5.53,R=0.49,P<0.01).However,Rayleigh fractionation theory points out that the equilibriun fractionation coefficient is inversely proportional to temperature,revealing that the heavy isotopes are more enriched in water body under the lower temperature.Our result was in contraction with Rayleigh fractionation theory,indicating that water temperature was not main controlling influence factors impacting on?¹⁸O in lake water.In addition,we also analyzed other factors that could affect isotopic compositions in lake water,such as dissolved oxygen?DO?,electrical conductivity?EC?and pH.After analysis,?¹⁸O in lake water was very weakly positive correlation with these water quality parameters,illustrating that the influence factors of isotopic compositions in lake water were very complex.By comparing the influence of relative humidity and water temperature with?¹⁸O of the lake water,it was found that the relative humidity was significant,indicating that the relative humidity was the main control factor influencing the isotopic composition of the lake water.