PERMEABILITY IN A LARGE CRYSTALLINE WATERSHED

Sékouba Oulare, Fernand Kouame Koffi, Université d’Abidjan, Côte d’Ivoire
René Therrien, Université Laval, Québec, Canada
Bachir Saley Mahaman, Jean Biemi, Université d’Abidjan, Côte d’Ivoire

Abstract

Permeability, the ease of fluid flow through porous rocks and soils, is fundamental to quantify the sustainability of water resources. Inverse modelling is a useful approach to quantify permeability of the aquifer at watershed-scale from observation data. In this paper, the permeability of a crystalline aquifer at watershed-scale is quantified through calibration of numerical model to piezometric data. The study watershed is located in the mountainous region at the West of Ivory Cost and covers 4100 square kilometer area. The conceptual model proposed is based on geological and hydrogeological interpretation from available borehole logs. The flow model is built with the HydroGeoSphere code, and the resulting three-dimensional steady-state hydrodynamic model of saturated zone was calibrated using the inversion code PEST. Calibration of the flow model in the inverse modeling approach allowed quantifying aquifer parameters from a fit between measured and calculated heads.
Then the map of aquifer permeability and calibration tests are presented. However the
calibration results showed some persistence major differences between the measured and
calculated heads. This anomaly is mainly explained by the influence of secondary perched aquifers in the mountain region which behavior are not possible to reproduce in the model simulation. Notwithstanding the above limitations, the results provide the first global map of subsurface permeability in West of Ivory Cost and will be of particular value for evaluating global water resources and modelling the interactions between the subsurface aquifer and global climate change.

Keywords: permeability, water resources, HydroGeoSphere, PEST, calibration, inverse modelling