Dual-domain modeling of discharge dynamics in a laboratory-scale fractured porous matrix system

Fracture networks often provide rapid pathways for water infiltration and play an important role for the time-dependent recharge in the vadose zone of consolidated fractured rock and karst formations. Such systems are often conceptualized using a dual-domain approach, since they can be divided into a fracture and a matrix domain. The fracture domain, especially when well connected, provides fast preferential flow paths, whereas the matrix domain usually acts as a storage due to the high contrast in hydraulic conductivities. Under partially saturated conditions, fracture-matrix interactions, i.e., imbibition of water from the fracture system into the matrix, strongly control the fracture flow progression. We conducted infiltration experiments in simple fracture-matrix systems of varying vertical length consisting of sandstone blocks, and use a dual-porosity non-equilibrium model to model the discharge dynamics and the internal fracture-matrix mass exchange. The results show strong deviations from the experimental observations when the original parameterization and model assumptions are not modified. The domain coupling, i.e., the (activated) interface area for fracture-matrix interaction, described by the matrix-fracture volume ratio (κ) was found to be the critical parameter in order to reproduce the data. While the original model assumes a perfectly coupled fracture and matrix domain, in the experiments the discrete nature of the fracture network leads to a much stronger dominance of the rapid flow domain and hence to a reduction of κ. The newly introduced (calibrated) parameter κ^* includes additional effects and processes related to the time dependent evolution and smaller dynamic size of the fracture-matrix interface. Furthermore, experiments of varying total vertical system size reveal convergence toward a unique parameter set and the existence of a representative elementary volume (REV) for the chosen setup. Though it performs less well for very small systems below REV scale, the unique parameter set describes discharge dynamics in sufficiently large systems with high accuracy.