A Study Of The Development History For The Ecological Environment From The Region Of Dali Lake At The Margin Of The East Asian Summer Monsoon,Based On The Analysis Of The Precipitation Process

Inner Mongolia's geographic location is crucial.It is affected by the East Asian summer monsoon and the western circulation in the northern hemisphere.It reacts positively to climate change.Not only is it an important ecological barrier in northern China,but it is also an ideal area for studying climate and environmental changes.In this article,therefore,Dali Lake(also known as"Dalinor Lake")on the northern edge of the East Asian summer monsoon is selected as the target area for research and atmospheric precipitation,water from the lake,lake sediments and other samples are selected as research objects.Based on the modern precipitation process in Inner Mongolia and the hydrological process of Dali Lake,through a comprehensive analysis of the stable isotopes of hydrogen and oxygen(?D and?¹⁸O),the content of alkanes of different lengths and other indicators Indirect environment,the vegetation types,atmospheric precipitation and hydrological cycle in the Dali Lake area in the marginal area of the East Asian summer monsoon are analyzed in the past 2400 years.The history of change has been reconstructed.The main results are as follows:(1)The changing properties of?D and?¹⁸O in modern atmospheric precipitation in Inner Mongolia show that the process of precipitation is significantly affected by precipitation and humidity.The composition of?D and?¹⁸O of atmospheric precipitation in western Inner Mongolia(at the northwestern edge of the East Asian summer monsoon)is influenced by moisture and precipitation and shows a negative correlation.The influence of temperature is relatively weak;correspondingly,the composition of the atmospheric precipitation?D and?¹⁸O in the southeast region of the edge of the East Asian summer monsoon is influenced by the temperature,the influence of moisture and precipitation is positively correlated with temperature and negatively correlated with moisture and precipitation.In general,locally evaporated water vapor is an important source of water vapor in Inner Mongolia.(2)In the Dali Lake region,the source of water vapor in the process of atmospheric precipitation is more complicated.The source of water vapor in the typical atmospheric precipitation process of June is primarily the mixture of water vapor provided by western circulation and local evaporation of water vapor.The source of water vapor in July is summer monsoon water vapor in East Asia.In August,it is mainly a local evaporation of water vapor.(3)The range of?D in chain length n-alkanes with C₃₁ in Dali Lake sediments ranges from-240.37 to-196.84‰,with the most negative?D record appearing at about 1730-1660cal.a BP,and the most positive?D record appears in modern times.,The mean value is-207.59‰.Specifically,around 2350-1790 cal.a BP,the?D value is progressively positive,indicating that the climate is changing from humid to dry;about 1790-1450 cal.a BP,the?D value changes drastically and at the same time is negative,indicating the process of atmospheric precipitation at this stage.The fluctuation is clear;about 1450-480 cal.a BP,?D value is relatively stable(of which about 1450-990 cal.a BP,?D is continuously enriched,indicating that the climate is changing from humidity to drought;while about 990-480 cal.a BP,the climate changes from dryness to wet);around after 480 cal.a BP,?D has been continuously enriched,indicating that regional atmospheric precipitation has decreased.Overall,the?D value has shown an upward trend over the past 2,400 years,indicating a gradual decline in regional atmospheric precipitation.(4)The process of evolution of the ecological environment of Dali Lake can be roughly divided into 6 stages:about 2350-2180 cal.a BP,about 1550-990 cal.a BP and about 600-0cal.a BP,the area of Dali Lake decreases,The effective humidity is lowered and the grass grows,indicating that the area is relatively cold and dry;about 2180-1550 cal.a BP and 990-600 cal.In BP,regional atmospheric precipitation changes little,the effective humidity has increased,and the climate is relatively hot and humid.Currently,regional atmospheric precipitation decreases,effective humidity decreases,grass grows,and the region is relatively hot and dry,in the modern hot period.In general,the evolution of the ecological environment in Dali Lake region is related to solar activity,the intensity of the summer monsoon in East Asia,the oscillation of the decade of the North Atlantic and the Pacific Decade Oscillation.The change in regional vegetation responds naturally to the change in regional precipitation.(5)Over the past 2,400 years,the area of Dali Lake has shrunk and the lake's water storage capacity has decreased.In particular,Dali Lake maintains a stable state of the total water balance at around 1730,1530,1330,630,430,180 cal.a BP and modern stages.Over the past 2,400 years,the amount of groundwater replenishment in the Dali Lake area has decreased,but its share in the total Dali Lake replenishment has not changed significantly.The average is 73%,the highest is 77%and the lowest is 71%.This is the main source of supply for Dali Lake.This result is comparable to the structure of modern Dali Lake,the groundwater replenishment of which can reach more than 70%.Due to the relatively large change in the ratio between replenishment of atmospheric precipitation,the highest ratio between replenishment of atmospheric precipitation and replenishment of groundwater is 42%and the lowest is 31%.