Numerical simulation as a tool for checking the interpretation of karst spring hydrographs

A schematic representation of karst aquifers may be that of a high hydraulic conductivity channel network with kilometre-wide intervals, surrounded by a low hydraulic conductivity fractured limestone volume and connected to a local discharge area, the karst spring. The behaviour of the karst spring (hydrographs, chemical or isotopic composition, etc.) represents the global response of the karst aquifer to input events. <br> The available data on karst aquifer hydraulic parameters are limited. Global response is therefore more easily obtained and is commonly used to make inferences on the recharge and groundwater flow processes, as well as on the hydraulic parameter fields. Direct verification of these interpretations is, obviously, very difficult. <br> We have used an indirect method of verification, consisting of introducing well-defined theoretical karst structures into a finite element model and then analysing the simulated global response according to presently accepted interpretation schemes. As we know what we put into the numerical model, the validity of any interpretation may be checked. <br> The first results indicate that some of the generally accepted interpretations are not necessarily true. In particular: (i) separation of simulated recession hydrographs into several components shows that different exponential components do not necessarily correspond to aquifer volumes with different hydraulic conductivities; (ii) non-exponential parts of recession hydrographs do not always give information about the infiltration process; and (iii) the recession coefficient of the baseflow (i.e. the last, nearly exponential part of the recession hydrograph) depends on the global configuration of the whole karst aquifer, not just on the hydraulic properties of the low hydraulic conductivity volumes.