5 Scale-dependent hydraulic characterization by large-
5.5 Conclusions and Discussion
The interpretation of the influence of dimensionless wellbore storage and skin damage factor on drawdown behavior combines the double porosity approach applied for simulation of karst hydraulics and observed skin effects during pumping tests on local scale. The concept is focused on the drainage ability of the conduit concerning the adjacent rock. With different parameterization representing different hydraulic conditions around the conduit a wide range of different type curves can be simulated. The differences of the hydraulic conditions are related to the degree of karstification (conduit porosity). Depending on dissolution, enlargement of conduits, progressive development and interconnection of the fissured/fractured system on local scale (macroscopic heterogeneities) the flow between conduit and adjacent rock shows different degrees of flow restriction. The dimensionless wellbore storage and skin damage factor are able to describe these complex differences of macroscopic heterogeneities and gradually different hydraulics. The results show that an interface between fissured matrix and conduit is important to beware the physical meaning of the matrix conductivity.
The usage of idealized parameters along the conduit (e.g. homogeneous exchange coefficient, CAD-storage width and diameter) and idealized initial conditions simplifies the complex field conditions. The natural head distribution inside a karstic catchment with head differences between matrix and conduit before the start of pumping will change the shape of the drawdown curve. For example, a rainfall event with high percentage of direct recharge into the conduit system just before the start of the pumping test may introduce an apparent negative skineffect due to the additional water volume in the conduit system.
Another important point, which is not focused in this work, is the differences of the drawdown curves for confined and unconfined karstic aquifer or even the change from confined to unconfined conditions during pumping.
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