Karstic Carbonate Aquifers: Are the Conceptual Models Adequate for Quantitative Hydrology?

Quantitative descriptions and models of karst aquifers are only as good as the conceptual models on which they are based. The base conceptual model is that of fluviokarst with allogenic and autogenic recharge and a variety of conduit systems embedded in the fracture and pore components of the aquifer. The widely-used Darcy-Weisbach pipe-flow approach is limited by difficulty in defining a friction factor, and by the need to deal with blockages (breakdown and sediment fills) and changing, irregular cross-sections even for a single conduit. As improved and more complete cave maps become available, real conduit systems are seen to contain bypasses, spillovers, and other components that make the conduit progressively less pipe-like. Further, there is a large collection of pathways with apertures between that of a cave explorer and the 10-mm aperture marking the onset of nonlinear flow characteristics, Gradient and conduit pattern determine the cross-over from basin characteristics to something like aquifer characeristics, The most intractable aspect of karst aquifers is their response to storm inputs. Storm flows shift the balance of open channel and pipe flow conduits and increase the hydraulic heads in the pipes. The time scale for storm events is short - minutes to hours - compared with usual aquifer response times. Many conduit systems have a finite carrying capacity that can be probed by observing swallet overflows into surface routes. Sediment transport during storms both forms and clears blockages. Storm flow and the non-existence of generic conduits are a substantal challenge to modeling karst aquifers.