COUPLED DISCRETE-CONTINUUM REPRESENTATION OF DUAL KARST FLOW SYSTEMS

Karst aquifers are frequently conceptualized as dual flow systems comprising localized and often turbulent flow in solution conduits and Darcian flow in the fractured porous rock. In order to translate this dual-flow concept into a numerical model, a discrete pipe network, representing solution conduits, was coupled to a continuum flow model, simulating Darcian flow. Hydraulic heads are iteratively calculated in both flow systems, and flow between the two systems is assumed as linearly related to the head difference between them. This model has been successfully applied to various hypothetical settings in order examine the fundamental processes of conduit development in karst rocks as well as the mechanisms determining karst spring responses to storm events. Application to real karst aquifers, however, is highly challenging, as the location and geometry of karst conduits is generally poorly known. First attempts to apply the model to real karst catchments are presented, and a strategy for model design and calibration based on manifold field data is discussed. Artificial tracer tests permit selective verification of assumed conduit pathways and are found to be very useful for model calibration. However, tracer-test interpretation alone is ambiguous. Results are particularly biased by assumptions made about the relative importance of conduit flow versus Darcian flow and about the general structure of the conduit network. Therefore, complementary information is needed in order to establish reliable model representations of real karst aquifers. This includes structural and geomorphologic data, which can be used to infer hypothetical structures of conduit networks, as well as hydraulic and physicochemical data from karst springs, which provide integral information about flow and transport processes within karst catchments.