Analysis And Simulation Of The Hydrological Process Of The Karst Aquifer With Fracture-conduit Dual Structure

The carbonate rock is widespread in Southwest China with abundant karst water resources. In this area, the karst water is always the main water resource for life and production. Due to the dissolvability of carbonate rocks, conduits often exists in the karst aquifer besides small fractures. Generally, the karst aquifer can be divided into fissure system with high storage capacity and conduit system, which acts as the main drain passage. The groundwater in the aquifer is drained by the conduit network in springs. The research on the hydrological process of the karst aquifer with fracture-conduit dual structure plays a significant role in understanding, management and prediction of the karst resource.MODFLOW-CFP, which is proposed by USGS basing on MODFLOW, is a groundwater model for the karst aquifer with fracture-conduit dual structure. In this model, the fissure system is assumed as the equivalent porous media and the conduit system is conceptualized as several tubes which superposes on the fissure system. The groundwater flow in the conduit system can be considered as turbulent flow or laminar flow. These two systems are coupled by the flow boundary and the exchange flow has a linear proportional relationship with head differece of two systems. According to the dual structure of the karst aquifer, MODFLOW-CFP is used to analyze the hydrological process of the karst spring based on a simple typical karst aquifer. The simulation results show the spring baseflow recession contains the early nonexponential recession and late exponential recession when the spring discharge is not influenced by the exchange coefficient and the conduit network. The initial nonexponential behavior has a close relationship witth rainfall conditions and internal properties of the fissure system. The greater the diffusivity (K/μ) and rainfall intensity are, the larger the initial exponential recession coefficient is. This coefficient also decreases with the spring discharge. The later exponential recession coefficient is a constant which only depends on the properties of the fissure system and is not influenced by the rainfall conditions. The spring recession curve could also be strongly influenced by the permeability of the conduit wall (Kw). However, the later recession process also obeys to exponential recession and the exponential recession coefficient decreases with Kw. Generally, the exponential recession coefficient has a nonlinear relationship with Kw. When Kw is small, little variation of Kw could cause strong variation of exponential recession coefficient. When Kw decreases to a very small value, the initial recession could also be changed to the exponential recession and the recession coefficient has a linear relationship with Kw.Based on the original model, the conduit structure is revised to discuss the influence of conduit on the spring discharge. The storage and conductivity of conduits could all influence the spring discharge and baseflow recession. When ignoring the variation of conduit storage, the decrease of turbulent or laminar average conduit diameter could raise the water head in the conduit and make whole or a part of conduit to deviate the fixed boundary to influence the spring discharge and recession behavoir. Meanwhile, the influence of local conduits on the spring behavoir has an intense relationship with their location and decreases with the downstream movement (far away the spring). When the conduit network is unsaturated, the conduit could temprarily storage the groundwater and decreases the spring discharge peak and increases the following spring discharge. When the spring discharge is influenced by the conduit network, the influence of turbulent and laminar conduit on the spring discharge and recession behavoir has obvious differences. The laminar conduit has obvious influence on whole baseflow recession and its influence on the the later exponential recession coefficient does not change with the spring discharge or rainfall intensity. However, the influence of turbulent conduit decreases with the spring discharge and rainfall intensity. When the spring dischagre or rainfall intensity is smaller than a threshold value, the influence from the turbulent conduit could completely disappear.Generally, the fissure and turbulent system in the karst aquifer can be represented by the different reservoirs. If ignoring the conduit storage variation, the karst aquifer could be conceptualized as a series reservoir-pipe model. According to the characteristics of the karst aquifer, the reservoir could be linear or nonlinear, and the flow in the conduit could be laminar or turbulent. After the relevant analysis, the recession curves of this reservoir-pipe model are similar with the karst spring from the simple karst aquifer in chapter 3. Therefore, the reservoir-pipe model is suitable to reflect the hydrological process of dual-structure karst aquifer.The yaji experimental site locates in the east suburb of Guilin city and belongs to peak cluster depression. The whole experimental site is divided into four sub-catchment and the catchment of spring S31 is the biggest which includes No.l, No.3 and 4 depressions. The reservoir-pipe model and MODFLOW-CFP is used to simulate the spring S31 in this paper. The simulation results show the reservoir-pipe model could finely simulate the discharge of spring S31. The fissure system in the spring catchment is a strong nonlinear system with nonlinear index of 2.95 which is mainly caused by the existing of epikarst. In the storm, the turbulent pipe has strong influence on the spring discharge, whereas the spring discharge is main controlled by the fissure system in low rainfall intensity. This simulation results is also consistent with the field observations. Although MODFLOW-CFP model could finely model the hydrological process in the dual structure of karst aquifer, it hardly consider the hydrological behavior happened in the epikarst zone. Given that, a new hydrological combination model of reservoir and CFP is used to simulate the spring S31. In this model, the reservoir is used to model the flow in the epikarst zone, where the CFP is used to simulate the hydrological process in the saturated zone. The simulation results show a good performance of the combination model on the spring S31. In the storm, more than 33.6%of rainfall is regulated by the epikarst zone and recharge the conduit though the point recharge directly.