Estimating hydraulic conductivity of discrete zones from fracture networks using hydraulic head observations: Application to the N’zo watershed (western Côte d’Ivoire)
Estimating the hydraulic conductivity of hard rock aquifers at a regional scale remains a major challenge. At this scale the data are scarce in quantity and quality. This paper provides an approach to estimate the hydraulic conductivity of hard rock aquifers from the calibration of a numerical model using hydraulic heads. This approach is applied to the N’zo watershed, which extends over an area of 4 300 km2. The methodology assumes that the permeability of the aquifer is related to vertical fractures, which are permeable over a depth of 100 m down from the surface. It is also assumed that these fractures can be identified as fault lines and all fault lines are fractures and are permeable. The first step of this methodology is mapping the permeability in discrete zones of the hard rock aquifer. These are identified from analysis of the density and orientation of « fractures » at the mesh scale, which represent the work scale. The fractures are linked to fault lines that are mapped from a radar image. Six spatial configurations of discrete permeability zones are generated with mesh sizes from 2 to 12 km. The second step concerns the construction of the numerical model from the external limits of the watershed, the DEM (Digital elevation model), the river network and a uniform recharge. Lastly, the third step consists of transferring each configuration of the discrete permeability zones to the numerical model and calibration in a steady-state flow simulation, using hydraulic heads. The hydraulic heads are observed at 86 discrete measurement points. The results indicate that the optimal model of discrete permeability zones is obtained with a mesh of 4 km. Calculated hydraulic conductivities oscillate between 1.1 x 10-6 and 2.4 x 10-5 m∙s-1. However, these values should be taken with caution because the identification of the homogeneous zones from the fracture network is based on the controversial approach of using fault lines and the calibration is performed only with hydraulic heads and not with hydraulic fluxes. The conceptual model developed in this work could be improved by characterizing heterogeneity of the hard rock aquifer from other assumptions such as lithology and by the introduction of other types of data into the calibration (ex.: fluxes, groundwater maps).