| Karst areas are widespread in South China,with highly karstification and complex structures in karst water systems,and the spatial and temporal distribution of karst water resources are extremely various,which makes it challengeable to find suitable methods to evaluate karst water resources.This paper is trying to delineate the structures of typical karst water systems in South China,and reveal the physical mechanism of karst water circulation,finally propose a mathematical model that can evaluate karst water resources in South China.Firstly,identifying the spatial structures of karst water systems is the fundamental work to reveal the physical mechanism of water circulation in karst areas.The physical mechanism is the transfer relationships of waters in each circulation step,including recharge,regulation,response and discharge,etc.Hydrogeological mapping,hydrogeological drilling,groundwater tracing,hydrochemistry and stable isotope methods were used to identifying the aquier structures and hydrodynamics features of karst water systems in Xiangxi River basin,and several conclusions were drawn as below:1.The karst water systems can be summarized into the concentrated drainage type and the decentralization drainage type in the Xiangxi River karst basin,whose aquifer medium are dominated by karst conduits-fissures and fissures,respectively.A conceptional physical structure model of a karst water system was delineated in the case study of Huangliang karst trough valley area.There are two recharge patterns of precipitation in the karst areas of South China: one is the concentrated recharge through karst depressions and sinkholes;another is the decentralization recharge through fissures.Once fast flow is formed by concentrated recharge in wet season,the actual mean groundwater velocity can be up to 209-841 m/h that determined by artificial tracer tests,and the solute transport performs in advection-dispersion.The karst conduits are extremely complex in the study area,especially a well-developed horizontal karst layer was recognized in early Cambrian formation,and several vertical karst belts form the main flow paths for several karst springs,which shows complex structure patterns of “single recharge multiple discharge” and “multiple recharge single discharge”.2.In the study area of Gufu syncline,a regional karst water system was found in this area,and a conceptional model of multiple karst water systems was delineated.Recharge elevations and circulation depths of three typical karst water systems were calculated using a mean ?18O elevation gradient of-2.4‰/km in this mountainous area.Mean groundwater residence time of 230 days and ~2 years were estimated for local and regional karst water systems,respectively,using the damped running average model.The input signals of precipitation were significantly damped in regional system,while the local systems displayed distinct response to inputs.Groundwater in the regional karst water system has undergone the greatest amount of water–rock interaction as indicated by its high Mg2+ concentrations,Mg/Ca ratios and dissolved solids,and even exchange 18 O with carbonate bedrock.Secondly,this paper analyzed the water transfer relationship in each step of karst water circulation using hydrological and meteorological data,and discussed the assessment methods of water quantity,which forms the basis for proposing physical conceptual model and mathematical model.Several conclusions were conducted based on water balance theory and spring recession theory as below:1.Hydrographs of local karst water systems respond very sensitively and rapidly to rainfall events in the study area.Response times for hydrographs and conductivity curves are 4.0~7.5 and 5.9~10.3 hours,respectively,and the peaks of conductivity curves always occur later than those for hydrographs.Recessions of karst water displayed in several segments,indicating the karst medium is highly anisotropic.Fast flow contributes about 75% to the total amount at peak status,but for the entire recession process,fissure water accounts for more than 70% of total storage quantity in karst aquifer.2.This paper improved the methods of calculating recharge coefficient of precipitation event based on spring recession analyses.A mean threshold of 7.1 mm for effective recharge precipitation events was calculated by the positive relationship between event recharge coefficient and event precipitation.Decentralization recharge will be firstly produced when event precipitation is larger the threshold value,and concentrated recharge occurs when runoff yield under excess infiltration.Seasonal distribution of precipitation is particularly various in monsoon areas,leading to significant variety in recharge coefficient at different months.Mean monthly average recharge coefficients range from 0.07 to 0.62 for Wulongdong spring,with the highest value in wet season and lowest value in dry season,and annual mean recharge coefficient is 0.48.3.This paper proposed the conceptions and evaluation methods of regulating groundwater resources and regulating coefficient.Annual regulating quantity is the total amount of monthly storage or releasing water quantity,and annual regulating coefficient is the ratio of annual regulating quantity to total runoff.According to the differences in climate and karst development,three typical karst watersheds were discussed and compared.It shows that those karst watersheds with concentrated precipitation,higher terrain slope and highly karstified systems exhibit stronger capacity of runoff generation but weaker regulation.Karst fissures control the regulating function of the groundwater in karst water systems,and it shows high regulating capacity in those karst aquifer systems with high rate of karst fissure medium.Thirdly,a conceptual physical model and a mathematical model were proposed to simulate karst hydrological processes based on the analyses of karst water system structure and physical mechanism of karst water circulation.1.In the areas of South China,multiple flow paths are formed in highly anisotropic karst medium,leading to superposition of multi-pulses in response and discharge.Combining hydrological pulse function and water balance theory,a rainfall-runoff model was proposed,and found a successful application in the runoff simulation in Wulonglong spring.2.A ratio of 0.5 was determined for transferring evaporation to evapotranspiration in the study area,which can be used to calculated the deficit in soil moisture for each rain events in the model.Calculated evapotranspiration plays an important role in filtering the simulated hydrographs.3.Daily and hourly runoff were simulated and compared,with a best model parameter M of 0.022×10-3m2/s and ? of 0.8 days for daily simulation,and a best M of 0.0195×10-3m2/s and ? of 0.85 days for hourly simulation.Temporal variations in hydrographs were well matched in both daily and hourly simulation,but hourly simulation performs better in catching the flood peak time and peak amounts.It will get better simulation results if the simulation time step is less than the theoretical response time(2?/3)of a karst water system.4.Model parameter ? comprehensively represents basin scale of a karst water system and aquifer medium features,and M is determined by ? and recharge area.This model is particularly suitable for those karst water systems with moderate or small watersheds and respond rapidly to rain events in the meantime.Furthermore,this model can be used to estimate recharge area of a karst water system according to its best calibrated parameters M and ?.The main innovations of this paper can be concluded as below.Firstly,this paper characterized the structures of typical karst water systems in South China,and revealed the complex flow pattern of “single recharge multiple discharge” and the exsit of regional karst flow system.Secondly,the concepts and methods of regulating water resources and regulation coefficient were proposed,contributing to new ideas for karst water resources assessment.Thirdly,a rainfall-runoff model that is applicable for medorate or small watersheds was proposed,and the physical functions of ? for karst water systems were analyzed,which offers a tool to predict floods in karst areas and should find wide application for flood risk analysis and karst water resource evaluation in South China. |
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