Study On The Flow Process And Carbon Flux From The Slope Of Longtan Karst Trough To A Small Watershed In Wuling Mountain

In response to global climate change,China has implemented the strategic goal of"carbon peaking and carbon neutrality".In October 2021,The State Council issued the Action Plan for Achieving the Carbon Peak before 2030.It pointed out that the integrated protection and restoration of mountains,rivers,forests,fields,lakes,grass and sand should be steadily promoted,especially the comprehensive control of karst rocky desertification and soil erosion,so as to improve the carbon sink capacity of karst ecosystem.The karst carbon sink and carbon cycle mechanism is one of the hot research directions of the academic circles today.The problem of soil erosion in Southwest karst trough valley basin is especially prominent,the problem of controlling the rocky desertification is severe,and there are many problems in monitoring and evaluating the effect of ecological protection and repairing the carbon sink.The hinterland of Wuling Mountain has special geographical conditions,forming karst trough landform with Longtan River as the erosion base.There are significant differences in water-rock-soil-gas-generation and other layers on the dip/anti-dip slope within the region.In the small watershed,karst landform is strongly developed,human activities are highly disturbed,and rock desertification is serious.In order to study the variation of hydrological resources and geochemical characteristics of groundwater and river water in Longtan Trough Valley,and to evaluate the carbon pooling amount in the region,this paper takes surface karst springs,typical underground rivers and different water bodies in the small basin of Longtan River in the hinterland of Wuling Mountain in Youyang,Chongqing as the research objects.Through five years of groundwater monitoring work from 2018 to2022,including routine sampling,online monitoring,rainstorm monitoring,day and night monitoring of underground river outlet,etc.Seasonal sampling and monitoring of the main stream and its tributaries in the Longtan River Basin in the past one hydrological year.The hydrological discharge data of surface karst spring,underground river and surface river were obtained,and the anions/cations and carbon isotope indexes of different water samples were determined.The hydrochemical runoff method,discharge attenuation method and mathematical statistics method were used to analyze the discharge process and ecohydrological characteristics from slope to small watershed scale.The carbon flux between the groundwater on the slope of Longtan Trough Valley and the small basin of Longtan River is preliminaries estimated.The results show that:（1）By comparing the flow attenuation process of the dip/anti-dip slope of Longtan Trough,it is found that the flow process of Laoquan in the anti-dip slope can be divided into two subdynamics,whose attenuation coefficients are 0.0892 and0.0196,respectively.The water storage ratio of aquifers medium corresponding to the first and second subdynamics is respectively 84%and 16%.Malutan is divided into two subdynamics,whose attenuation coefficients are 0.0858 and 0.0272 respectively.The water storage ratio of aquifers medium corresponding to the first and second subdynamics is 70%and 30%respectively.The difference of geological conditions,catchment area and aquifers medium of the longitudinal and the dip/anti-dip slope of Longtan Trough has great influence on the flow attenuation process.The karst spring has the characteristics of dual aquifer medium,the proportion of pipe and fissure is large,the aquifer medium has a strong water conduction effect,and the hydrological system has a weak capacity of regulating and storing precipitation infiltration water.In addition,it is found that the carbon flux of karst spring changes significantly with precipitation and discharge during Summer heavy precipitation.It is high in the discharge decay stage,and the carbon flux of Laoquan is 97.64 kg·km^-2·d^-1during this period.The carbon flux in the karst spring during the Summer drought period（from the end of July to the end of August）has the characteristics of diurnal variation,showing that it is low at night and high in the day.The carbon flux is low due to the decrease of the flow during the Summer drought period,which is 18.23 kg·km^-2·d^-1.The average annual inorganic carbon flux of the Laoquan is 14.71 t·km^-2·a^-1（CO₂）,and the variation of the carbon flux of the karst spring is closely related to the regional karst process,climate change and land use change.（2）During the rainstorm monitoring period,there is a good correlation between the water levels of Longdongtang Underground River Outlet Channel,Flood Channel and Reshui River（[Flood Channel water level]=1.402[Channel water level]+0.005,R²=0.54）and（[Reshui River water level]=0.764[Flood Channel water level]+0.535,R²=0.74）.During rainstorm,the discharge of Longdongtang Channel,Flood Channel and Reshui River is 19×10⁴m³,55×10⁴m³and 231×10⁴m³,respectively.The discharge of Longdongtang（Channel and Flood Channel）accounts for about 32%of the total discharge of Reshui River,which is one of the important confluence water sources of Reshui River.The annual average discharge of Longdongtang Underground River outlet shows an obvious downward trend in the interannual variation.On the seasonal scale,the peak flow rate appeared in Spring,early Summer and Autumn,and the low flow rate often appeared in Summer drought period.Longdongtang is divided into three hydrological periods,with September to November and March to June as wet periods.The period from December to February of the next year was normal.The dry season is from July to August.In addition,it is found that the inorganic carbon flux in Longdongtang showed a gradual decline on the interannual scale,and the average annual carbon flux of 545.83 t·km^-2·a^-1（CO₂）.The decrease of the annual runoff of underground rivers directly resulted in the decrease of the annual carbon flux in Longdongtang.At the seasonal scale,the contribution ratio of inorganic carbon flux in Spring,Summer,Autumn and Winter was 26.70%,19.34%,30.03%and23.94%,respectively.On the diurnal scale,it is estimated that the inorganic carbon flux in the underground river domain during the period of no rain is about 68.16kg·km^-2·d^-1,which is about 6.6 kg·km^-2·d^-1higher than that in the daytime.It is accounting for about 9.7%of the total which is the loss of total DIC produced by carbonate weathering in Longdongtang Basin（the inorganic carbon dissolved in water fixed by photosynthesis of submerged plants during the day）.Plants play an important role in the carbon sink process.（3）The hydrochemistry of Longtan River Basin in Longtan Trough Valley has significant temporal and spatial characteristics.In terms of seasonal variation,Summer precipitation and discharge had obvious dilution effects on ions and elements in water bodies,whereas in early Spring and Autumn they were opposite.In terms of spatial variation,the HCO₃^-content in the main stream of Longtan River（the north of Longtan Trough）is generally higher than that in the main tributary of Rongxi River（the south of Longtan Trough）,while the Ca²⁺,Mn²⁺and electrical conductivity of the Rongxi River are generally higher than that of the Longtan River.The difference of geological conditions and human disturbance（Manganese mining）have important influence on the difference of hydrogeochemical characteristics of rivers.The discharge characteristics of the Longtan River Basin are generally high in early Summer（June）,low in Autumn（September）and flat in early Spring（March）.The main tributary,Rongxi River,which is located in the south of the trough Valley,is the main supplementary discharge in rainy season,accounting for 55.6%of Longtan River Basin.It is one of the most important flood peaks of Longtan River into tributaries in wet season（June）.In addition,there is a positive correlation between HCO₃^-concentration andδ¹³C_DICvalue in Longtan River in different seasons.The HCO₃^-concentration is low in March and June,and theδ¹³C_DICvalue is negative.And vice versa in September.In general,theδ¹³C_DICvalue of the upper reaches of Longtan River main stream is relatively positive,while theδ¹³C_DICvalue of the middle and lower reaches is relatively negative.The discharge of sewage in the middle and lower reaches of urban areas may cause the overallδ¹³C_DICvalue of the region to be negative.The carbon flux in the Longtan River is estimated to be 135235.64 t·a^-1（CO₂）.The annual inorganic carbon flux of Reshui River,a northern tributary,accounting for13.72%of the total contribution to Longtan River.The annual carbon flux of Wangjia River,a western tributary,accounting for its 24.58%.The annual carbon flux of Rongxi River,the largest tributary in the south,accounting for its 37.90%.