Investigation of Development and Hydrodynamic Characteristics of Karst Systems Using Analysis of Hydrograph Recession Curve, Case Study: Karstic Aquifers of Alvand Basin

Introduction
Hydrograph of springs represents clearly all physical processes that control ground water flows inside aquifer (Kuto et al., 2012: 41). Study of the curve can reveal information about structure and function of aquifer, particularly hydrodynamic parameters including permeability and storage (Troch et al., 1993: 228). Analysis of recession curve has the advantage to calculate parameters of drainage mechanism such as recession coefficients. The volume of the primary discharge is sum of rapid and base flows and the specification of sub-regimes (Malık and Vojtkova, 2012). Identification of base flow properties and its prediction in dry period is necessary to determine the storage and to prevent it from pollution (Dewandel et al., 2003). Karstic aquifers are an enormous reservoir of water in many regions of the world and with decline in quality and the amount of alluvial aquifers they got important more as a strategic hoard of water (Suta, 2008: 856). The karstic aquifers are the main source of water for human society and ecosystem of Alvand Basin. They also provide the base flow of Alvand and are important in initiation and continuation of civilization in this basin. The purpose of the study is to assess the development of karst in the aquifers of Alvand using the analysis of recession curve hydrograph.
 
Materials and Methods
The methodology of this research is based upon literature evidence, field work data and analysis of the recession curve hydrograph. Thus, we have used qualitative and quantitative data from hydrographic and precipitation stations of Power Ministry from 1999 to 2010. Coefficient of variations of the springs’ discharge (CV), the quality of water in the aquifers, and the hydrochemical parameters has been analyzed. Given the unequal development of karst in different parts of the aquifer, five springs (aquifers) have been selected to assess development of karst. These aquifers are including Rijab, Marab, Gelin, Sarab Garm and Gelodareh. To specify the type of flow system in karstic aquifers, hydrograph of recession curve have been analyzed. In order to calculate sub-regimes of laminar and turbulent flow, equations 1 and 2, respectively have also been used. Equation 1 is an exponential function that was suggested for laminar flows by Froccoyatsh and Palouk (1967). Equation 2 is linear function that was outlined by Kolmann for turbulent flows. 
Equation 1: Q_t=Q₀e^-αt
Equation 2: Q_t=Q₀(1-βt)  
 
Results and discussions
The Rijab Spring has two laminar and turbulent sub-regimes. The equation of recession curve for this spring is as following: Q_t = 2.1e^-0.008t + 1.25e^-0.001t + 510(1-0.29t). The degree of karstification of the Rijab aquifer is 5.5. There are also karstic channels in this aquifer. The rapid flow in Rijab aquifer is charged mainly by sinkholes and large cavities. The flow is also discharges by channels inside the aquifer. Sarabgarm Spring based on equation of recession curve of Q_t = 2.01e^-0.001t + 1.25e^-0.003t  has two sub-regimes and the karstification is 2.7. Golin Spring has two sub-regimes and recession curve as Q_t = 0.62e^-0.009t + 0.46e^-0.001t. Karstification is also about 3.7 with little channels. The equation is Q_t = 0.263e^-0.009t + 0.175e^-0.001t for Gelodareh with two laminar sub-regimes and 3.7 karstification degree. There is just one flow regime (laminar) and many small channels. The equation is as Q_t = 0.725e^-0.002t + 0.620e^-0.008t + 285(1-0.37t) for Marab spring. The equation indicates two laminar sub-regimes and a turbulent flow system with 5.5 karstification degree. In Marab aquifer turbulent flow is prevailing. The aquifer is charged by sinkholes and is discharged by channels inside aquifer.
 
Conclusion
Local differences in factors of karstification including lithology, tectonic, climate, elevation, slope and also physiographic characteristics of the drainage areas charging the aquifers in Alvand Basin cause variations in karstification and hydrodynamic behavior of aquifers. Analysis of rates of karstification, the values of Alpha, and also the number and the types of sub-regimes indicate heterogeneous karstic aquifers in Alvand Basin. Aquifers of Marab and Rijab are the most developed in karstification and Sarab Garm is the least in this terms. Geomorphology of surface terrain plays a major role in hydrodynamic behavior of aquifers. Physiography of the drainage basin where is charging the aquifers plays also a role in hydrodynamic characteristics and the quantity and kinds of sub-regimes. In high elevation areas, precipitation is more and evaporation is low. This cause more input into the aquifer that affect, in turn, the aquifer sub-regimes. Finally, this can be stated that three factors have the most importance in hydrodynamic properties and karstification of aquifers in Alvand Basin. These are geomorphology of surface karst, physiography of recharging basins, and elevation of the area.