Toward the estimation of the transfer coefficient in karst systems: Using baseflow recession coefficient under matrix-restrained flow regime

Abstract

Using the reservoir model theory, this study developed an equation for the relationship between the transfer and recession coefficients under a matrix-restrained flow regime. Assuming Darcy's law, the proposed equation was developed based on conventional parameters of karst systems, such as recession coefficient, specific yield, and catchment area. Firstly, the equation was evaluated through forward and inverse modeling of an idealized karst system with MODFLOW-2005 Conduit Flow Process (CFP) and KarstMod, respectively. Then, calibration of a KarstMod and a CFP model of the Baget karstic watershed in France was carried out to check the efficiency of the developed equation.Results of the first step showed that the optimal value of the transfer coefficient based on KarstMod for the linear transfer flow is very close to the baseflow recession coefficient values. Therefore, the transfer coefficient in KarstMod may be considered as the recession coefficient from the matrix to the conduit compartment. Moreover, substituting these two coefficients in the developed equation caused less than 10% errors in estimating the transfer coefficients assigned to the idealized models in CFP. In addition, adding the recession coefficient as prior information in the calibration process affects the recession coefficient of the simulated time series and provides a reliable evaluation of the internal dynamics of the system. Therefore, the results of this research show that the recession coefficients can be used as an initial estimate of the transfer coefficient in the baseflow condition. This achievement can significantly assist the discrete-continuum model calibration and further support the water balance calculations based on the recession data.