| Karst is widely distributed in Southwest China,with a high degree of karst development.Karst water resources are rich in total,but there are great differences in temporal and spatial distribution.As the main ecological and landscape water source of Lijiang ancient city,Heilongtan springs has frequently dried up in recent years,which has seriously affected the production activities,daily life,and tourism quality of Lijiang ancient city.Heilongtan springs are the main discharge of the karst spring area to the north of the ancient city.The research on the dry-up mechanism of springs needs to analyze the data dynamics and correlation characteristics of rainfall and spring discharge on the basis of clarifying the characteristics of the karst water system and hydrological process in the spring area and carry out comprehensively in combination with hydrogeological principles and statistical methods.The research on the characteristics of the karst groundwater system and the dry-up mechanism of springs can provide theoretical references for the follow-up remediation work and is of great significance to the security of water resources in Lijiang ancient city.Firstly,based on the analysis of the geological environment and karst hydrogeological conditions of the Heilongtan spring area and conbined with a variety of data statistics and analysis methods,this paper studies and obtains the multi-year and intra-year response characteristics of spring discharge to the rainfall,which provides data support for the study of springs cutoff mechanism,and several conclusions were drawn as below:1.Dry-up of Heilongtan springs is fundamentally driven by the decrease in rainfall.From the beginning of 1993 to the end of 2017,the fluctuation of rainfall,spring discharge,and groundwater level rises at the beginning and then decreases,and the declining trend is significantly larger,indicating decreasing groundwater resources in the spring area,which was closely related to the decreasing trend of regional rainfall caused by climate change.2.The discharge of the Heilongtan spring group is related to the rainfall in the first four years,and the rainfall in the first year has the greatest influence on the spring discharge.The time lag of Heilongtan springs discharge relative to rainfall is 3 months.The time lag of groundwater level relative to rainfall has changed from 3 months to 3 or 4 months since 2007.The increase in response time is related to the decrease in aquifer saturation caused by the decrease in regional rainfall.Secondly,by conducting the tracer tests,the tracers are injected into the Jiuzihai karst depression,which serves as the main recharge area of the Heilongtan spring area.Subsequently,the breakthrough curves of tracer are obtained at a total of 9 karst springs including Heilongtan springs.Based on the previous work,the parameter inversion of the curve is carried out,and the hydrodynamic characteristics of karst groundwater and the main structural characteristics of the karst aquifer system in the spring area are analyzed,which provides a hydrogeological basis for the study of the cutoff mechanism of springs.Several conclusions were drawn as below:1.The breakthrough tracer curves can be reasonably reversed by the advectiondispersion model(ADM),multi-dispersion model(MDM),and two-region nonequilibrium model(2RNE).The above models are suitable for symmetric and singlepeaked curves,symmetric and multi-peaked curves,and trailing single-peaked curves respectively.2.Karst conduits and fractures are the main aquifer types of Heilongtan spring area.There is a considerable scale of karst groundwater storage space near the outlet of the springs.There are three major conduits from the recharge depression of the spring area to Heilongtan springs.The groundwater in the channel is in good hydrodynamic conditions and the flow state is turbulent.3.Results of tracer tests obtained at other springs show that there are two and one major conduits from Jiuzihai karst depression to Qingxi spring and other springs(including springs in Lijiang ancient city and the springs located at the east fault)respectively.Moreover,springs in Lijiang ancient city are partly supplied by the Heilongtan springs.The lithologic characteristics in the spring area and the spatial cross-relationship of water control faults provide a good runoff channel for the fault spring group from Jiuzihai to the East.The exchange process of tracers between the mobile region and immobile region leads to the "tailing" of BTCs of springs at the east fault.Thirdly,the reservoir model is used to generalize the Heilongtan karst spring area.The rainfall time series is used as the input signal of the model to simulate the discharge of Heilongtan springs.Based on the water transfer process between reservoirs,analyzing the karst hydrological process in the spring area is helpful to understand the dry-up mechanism of the springs.The conclusions are as follows:1.Heilongtan spring area can be generalized as a reservoir model composed of epikarst regulation reservoir(E),matrix reservoir(M),and conduit reservoir(C).Discharge of Heilongtan springs was successfully reproduced by the model.2.The simulation results show that the recharge of reservoir E to reservoir M is not sensitive to the rainfall,while the discharge of reservoir E to reservoir C is very sensitive to the rainfall,which indicates that the karst conduit in the Heilongtan spring area is developed and well connected,leads to the concentrated recharge and rapid increase of discharge at springs.3.The dry-up of Heilongtan springs can be successfully reproduced by setting the discharge of reservoir E to reservoir C as the hysteretic mode.Most of the discharge at Heilongtan springs is supplied by the reservoir C and the discharge of reservoir C is characterized by strong nonlinear.Finally,according to the research contents,the dry-up mechanism of Heilongtan springs,known as the typical karst springs in South China,is systematically analyzed and summarized.The relevant conclusions can be summarized as follows:1.The dry-up mechanism of Heilongtan springs can be summarized as the control mechanism of rainfall and the influence of structural characteristics of the karst aquifer.The control mechanism of rainfall on the dry-up of springs can be further refined into the annual rainfall control mechanism and annual rainfall distribution characteristic control mechanism.The former can be explained as the dry-up characteristics and duration of springs are jointly controlled by the rainfall characteristic period of the current year and the rainfall of the previous year,while the latter mainly shows that the increase of the coefficient of uniformity of rainfall distribution within the previous year will lead to the increase of the dry-up probability of springs in the current year.2.The influence mechanism of the structural characteristics of the karst aquifer system can be explained as follows:a set of conductive conduits system with weak regulation and storage capacity is developed from Jiuzihai karst depression,which is known as the main recharge area,to Heilongtan spring group.This system will lead to the sharp rise and fall of the discharge curves of Heilongtan springs with the change of rainfall,therefore,increasing the probability of dry-up in the condition of insufficient rainfall.Moreover,the karst hydrogeological process revealed by the reservoir model is helpful to understand the dry-up mechanism of springs. |
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