A frequency domain approach to groundwater recharge estimation in karst

This paper presents an alternative method for determining the values of parameters of a groundwater recharge model. The phreatic zone of a karst aquifer is considered as the linear and time-invariant filter that transforms the input signal of groundwater recharge rates into the output signal of spring discharges. Similarities between transfer functions of total rainfall rates and transfer functions of groundwater recharge rates are the basis for developing the parametric periodogram depending on parameters of a groundwater recharge model. The values of parameters are estimated by minimizing the differences between the parametric periodogram and a periodogram of spring discharges. The approximate Whittle log likelihood function is the criterion for determining the optimal values of the parameters. By using this frequency domain approach, groundwater-balance calculations are avoided so the method can be applied on unexplored karst aquifers when groundwater-balance cannot be achieved without extensive geologic and hydrogeologic investigations. The results of the applications on two springs located in the Dinaric karst area in Croatia are discussed.