KARST HYDROLOGY & ECOLOGICAL CONDITIONS: THE SIGNIFICANCE OF THE SOIL SUB-SYSTEM (Invited Presentation)

The main life-relevant fluxes through karst are water, air, heat, nutriments (C, N, P), CO₂, live-mass, etc. In temperate regions, water fluxes are assumed to dominate exchanges with the external environment.

Characterizing karst as a dynamic aquifer system requires three sets of conditions: 1) to define initial conditions, 2) to define the 3D-field of parameters controlling flow and, 3) to know the time variable input/output conditions.

Because karst is extremely heterogeneous, the spatial distribution of flow parameters is poorly known. Speleogenetic rules must be used to generate the spatial distribution. In many cases the overall distribution of flow is mainly controlled by karst conduits. Two methods (KARSYS and KarstALEA) were recently developed for assessing the position and characteristics of the conduit network within karst massive.

Flow simulation (i.e. time-variations of heads and fluxes) is carried out using two main types of models: 1) Assuming that flow in low permeability volumes is negligible, pipe flow (e.g. SWMM) can be directly applied on the generated network of karst conduits. 2) Assuming that flow in low permeability volumes cannot be neglected, the entire volume of aquifer must be included in the simulation. Various modelling approaches were developed for this type of simulation (e.g. Modflow CFP or Feflow).

In any modelling approach recharge must be assessed as input parameter. Studies on soil and epikarst clearly show that a significant part of the slow flow component is controlled by storage in the vadose zone, and that this type of flow is highly variable in time and space. Thus, recharge cannot be assessed quantitatively. More attention should therefore be dedicated to the dynamics of flow in soil and epikarst.

As of today the order of magnitude of water fluxes are still unknown in many compartments of a karst massive! Matter (e.g. nutriments) or energy (i.e. heat) transported by water can hardly be assessed.

Fluxes of nutriment and CO₂ are further controlled by their production rate, i.e. by biological activity in soil (and possibly epikarst). It is why karst upper layer (soil and epikarst) should not only be better characterized from a hydrological perspective, but also from a biochemical one.

Observations and models from the Milandre karst laboratory will provide various examples.