Hydrological Recharge Mechanism Of Epikarst In Small Karst Catchment

The special binary three-dimensional spatial structure and complex hydrogeological background in karst area form a dual pattern of surface loss and underground leakage.The epikarst composed of highly weathered carbonate bedrock has important hydrological regulation and storage functions.The mechanism of how the structural system of epikarst affects the near surface hydrological process of karst is not clear.It is of great significance to explore the hydrological recharge mechanism of epikarst for the restoration and reconstruction of karst fragile ecological environment and the coordinated utilization of water and soil resources.Based on the characteristics of karst hydrogeological structure and hydrological cycle,combined with environmental isotope technology and hydrochemical research theory.In this paper,the hydrological recharge mechanism of epikarst in Karst small catchment is discussed.In Guizhou Plateau,which represents the overall structure of karst rocky desertification environment in southern China,three typical rocky desertification comprehensive control demonstration areas of Guanling-Zhenfeng,Shibing and Bijie salaxi are selected as the study area.In the corresponding study area,the epikarst in Chaeryan small catchment,the epikarst in Heichong small catchment and the epikarst in Chongfeng small catchment are selected as the monitoring areas.Through field fixed-point long-term monitoring,the concentrations of hydrochemical elements and?D and?¹⁸O from different hydrological sources such as runoff confluence,groundwater and soil flow are measured.Analyze the hydrochemistry,runoff production and?D and?¹⁸O dynamic variation characteristics under different rainfall conditions.The proportion of new water and old water in epikarst of small catchment under different rainfall conditions is analyzed by using two terminal element mixed model,and the Iso Source model is used to determine the recharge contribution rate of different water recharge sources to the runoff production of epikarst in small karst catchment,and then reveal the hydrological recharge mechanism of epikarst in small karst catchment.It provides a scientific basis for the restoration and reconstruction of karst ecological environment and the coordinated utilization of water and soil resources.(1)It is found that the geological and lithologic differences of the epikarst in small catchment under different rocky desertification environment lead to the differences of water chemical characteristics in the rainfall process of the epikarst in small catchment.The hydrochemistry of the epikarst in Chaeryan small catchment is mainly HCO₃-Ca type,and the hydrochemistry type of the epikarst in Heichong small catchment is mainly HCO₃-Mg-Ca type,and and the hydrochemistry type of the epikarst in Chongfeng small catchment is mainly HCO₃-Ca type.The anions in the three small catchments are the most abundant in SO₄^2-and HCO₃^-.Among them,the average content of SO₄^2-is as follows:Chongfeng small catchment(26.00 mg/L)>Heichong small catchment(23.01mg/L)>Chaeryan small catchment(12.30 mg/L),and the average content of HCO₃^-is as follows:Heichong small catchment(409.77mg/L)>Chaeryan small catchment(353.60 mg/L)>Chongfeng small catchment(246.00 mg/L).Ca²⁺and Mg²⁺are the most abundant cations in the epikarst of Chaeryan small catchment and Heichong small catchment,and Ca²⁺is the most abundant in the epikarst of Chongfeng small catchment.The average content of groundwater Ca²⁺in the three small catchments is shown as follows:Chaeryan small catchment(110.50mg/L)>Chongfeng small catchment(109.93 mg/L)>Heichong small catchment(84.64 mg/L),while the average content of groundwater Mg²⁺is realized as follows:Heichong small catchment(28.81 mg/L)>Chaeryan small catchment(8.27 mg/L)>Chongfeng small catchment(2.90 mg/L).(2)The difference of runoff yield in the epikarst of small catchment under different rocky desertification environment is found,and the dynamic variation characteristics of runoff yield in medium intensity rocky desertification area are clarified.In the rainy season of 2021,the epikarst of Chaeryan small catchment produces 53696.601L,the epikarst of Heichong small catchment produces 1537061.621L,and the epikarst of Chongfeng small catchment produces 546134.4L.The runoff production coefficients of the three study areas are Heichong small catchment(0.148%)>Chongfeng small catchment(0.116%)>Chaeryan small catchment(0.010%),the runoff coefficient of Chaeryan small catchment is the lowest,indicating that the watershed is easy to form underground leakage.As Chaeryan small catchment belongs to the area of intense rocky desertification,with high exposure rate of bedrock and wide distribution of fissure karst ditches,it promotes the vertical leakage of water,and rainfall is difficult to form surface runoff.Heichong small catchment is a non potential rocky desertification area.There are few fissure karst ditches in the area,which is difficult to form underground leakage and easy to form surface runoff.The dynamic monitoring of runoff production at runoff points and confluence in the epikarst of Chaeryan small catchment in the medium intensity rocky desertification area shows that:the runoff generation time shows confluence(787.14 min)>runoff point 2(598.57min)>runoff point 1(462.84 min).The peak runoff shows confluence(335.50 L/s)>runoff point 2(137.94 L/s)>runoff point 1(30.36L/s).The peak cumulative rainfall shows confluence(20.3 mm)>runoff point 1(18.28 mm)>runoff point 2(17.08 mm).The rainfall response sensitivity is:runoff point 2>confluence>runoff point 1.It shows that compared with the runoff point at the confluence,the response of runoff point to rainfall lags,but its runoff generation duration and runoff are significantly higher than that at the runoff point.It also reflects that runoff1 and runoff 2 have obvious water supply effect on the confluence.The water source at the confluence point may be complex,there are many supply paths,and there is a mixed mechanism in the supply process.(3)Reveal the water supply source and supply mechanism of the epikarst in small catchment.The structure of epikarst and early hydrological conditions control the runoff supply process of the catchment.The groundwater and rainfall in the epikarst of Chaeryan small catchment are the main sources of runoff in the basin.With the increase of rainfall events,the proportion of rainfall supply at the confluence of Chaeryan small catchment is increasing.In the continuous rainfall events,rainfall is the main supply source of the watershed.When the small catchment has less precipitation in the early stage and is in a relatively unsaturated state,the runoff is mainly supplied by groundwater.The results show that the slope runoff generation mode of small catchment is mainly full storage runoff,and the water supply source of small catchment is affected by early rainfall conditions.The super seepage runoff and full storage runoff of watershed hydrological process are intermittent.In the runoff production process of small catchment,based on?D divide the proportion of new water and old water at runoff points and confluence.The results show that the supply source is dominated by old water,accounting for more than 85%,while the proportion of new water is less than 15%.The water source of the sinkhole at the outlet of the epikarst in Heichong small catchment is dominated by old water,accounting for more than 75%.Rainfall is the main recharge source of the sinkhole at the outlet of the epikarst in Heichong small catchment.The water source of the sinkhole at the outlet of the epikarst in the small catchment of the charge is dominated by the old water,accounting for more than 80%.Rainfall is the main recharge source of the sinkhole in the small catchment of the charge,and the average recharge contribution rate is 42.2%.The second is interflow,with an average recharge contribution rate of 32.67%,while the average recharge contribution rate of groundwater is 25.13%.