The Gas Exchange Coefficient?k? Of CO₂ And Its Influencing Factors Across Water-Air Interface In A Typical Karst Groundwater-Fed Stream

The gas exchange coefficient?k?is a key parameter for quantifying and predicting the CO₂ gas exchange process and flux across water-air interface in water body.Commonly,the partial pressure of CO₂?pCO₂?and the concentration of dissolved inorganic carbon?DIC?are higher in karst groundwater because they are affected by soil CO₂ content and carbonate dissolution.When karst groundwater discharge into surface streams,it will form large pCO₂ gradient between water and air,and produce CO₂ outgassing strongly.Gas exchange rates are frequently used to estimate metabolism of aquatic systems,emission of greenhouse gases like CO₂ to the atmosphere and the role of karst waters in global carbon cycling.CO₂ exchange flux for whole stream can be calculated by coupling the gas exchange coefficient?k?into a thin boundary layer model?TBL?.This will be beneficial to the accurate evaluation of CO₂ exchange flux across water-air interface in karst water and to provide a basis for evaluating the contribution rate of karst water in regional carbon budget.At present,the research on k in a stream is mostly located in temperate or tropical regions.Some studies also established empirical formulas of k based on hydrodynamic parameters of streams.The physical and chemical properties of karst water are significantly different from other research water.In addition,environmental characteristics of different waters are unique.Therefore,some factors influencing the gas exchange process across water-air interface are different,which particularly necessary to conduct research in situ.Especially,there are few studies on the CO₂ exchange coefficient and its influencing factors in karst water through directly field measure.Hence,to solve the scientific problem of the k characteristics across water-air interface and its influencing factors in a karst stream with higher CO₂ concentration contrast between water and air,this paper chose a groundwater-fed karst stream?Guancun surface stream?as research area in subtropical China.This study revealed the variation law of k and main influencing factors under different condition of hydrodynamic and hydrochemistry in different river reach,compared differences of k obtained through field experiment and empirical formulas and discussed spatio-temporal variations of CO₂exchange flux across water-air interface calculated by TBL model and the floating chamber methods.This study selected underground river outlet?G1?and other four points along the flow stream direction?G2,G3,G4 and G5?for monitoring,sampling and gas tracing experiments at quarterly interval.This study also analyzed the spatio-temporal variation of hydrochemistry in Guancun karst surface stream,and seasonal variation and main influencing factors of the k in reaches of G2^G3,G3^G4 and G4^G5.Results of study show that:?1?Hydrochemical type of stream water is Ca-HCO₃^--type.CO₂ degassing from the streams caused decrease of pCO₂ along the flow direct and increase of SIc and pH in March and September,2017,while pCO₂ declined unstably in December,2016 and July,2017.On seasonal scale,the mean value of pCO₂ show a order of July>December>September>March for whole stream.HCO₃^-concentration show a same trend for each sampling point,and the maximum value occurred in December,2016,the minimum value occurred in March,2017.On spatial scale,the HCO₃^-content declined along the flow direction.Biogeochemical processes caused the spatio-temporal difference of hydrochemistry each sampling point.?2?The range of k was from 2.0 to 59.58 cm·h^-1,with an average of 19.03 cm·h^-1.On seasonal scale,variations ofkin reach of G2^G3 showed a order of December<March<July<September,while both G3^G4 and G4^G5 reaches showed a k order of December<July<March<September.On spatial scale,besides the samples in September due to storm event influence,the k of G3^G4 reach was higher than the other reaches.The k of G2^G3reach was smaller than that of G4^G5 reach.Although the measured k₆₀₀00 values were related to the calculated value based on empirical formula in the G2^G3 section,there were no correlation between reach of G3^G4 and G4^G5 reach.It shows that there was a great difference between measured k₆₀₀00 values and calculated k₆₀₀00 values from the empirical formula in a karst water body.The main influence factors on k are hydrological parameters such as flow,water depth and CO₂ concentration in G2^G3 reach.In G3^G4 reach,there are three main influence factors including hydrochemistry,hydrological conditions and aquatic organisms.In G4^G5 reach,the main factors on k also are hydrochemistry and hydrological parameters.During monitoring periods,the k was not related to water temperature in whole stream,but it had a negative correlation between k and DIC?R²=0.60,p<0.01?.The k₆₀₀00 in studied karst stream is larger than that of large rivers,lakes and reservoirs,which also showed large variations range.Therefore,spatial variation of the k₆₀₀00 should be considered when use the TBL model to calculate the CO₂ exchange flux of a stream.We should also avoid use a unified k₆₀₀00 to calculate CO₂ exchange flux in a stream.Comparing with other water bodies,influencing factors of k₆₀₀00 in Guancun karst stream included hydrochemical parameters?pCO₂ and DIC?and hydrodynamic conditions.In addition,aquatic organisms may also affect the CO₂exchange coefficient across water-air interface in study area.?3?Floating chamber method and TBL model were used to calculate the exchange flux across water-air interface of the three reaches of the river during four quarters.There is a significantly positive correlation between calculated results from two approaches?R²=0.34,p<0.05?.In general,the CO₂ degassing flux value calculated by TBL equation is 4.8 times higher than the floating chamber method.Comparing the characteristics of the two methods,it is found that the TBL model method is suitable for streams or mountain streams,while the floating chamber method is suitable for rivers and lakes which are more open in water bodies.