INTERNATIONAL JOURNAL OF ENVIRONMENTAL SCIENCES Volume 4, No 3, 2013 © Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0 Research article ISSN 0976 – 4402 Received on September 2013 Published on November 2013 296 Relative importance of weathering profiles and major fracture zones to fit the water balance of a hydrogeological catchment in hard rocks Mahamadou Koïta 1 , Hervé Jourde 2 , Yvan Rossier 3 1- Joint Research Center for Water and Climate, International Institute for Water and Environmental Engineering (2iE), 01 BP 594 Ouagadougou, BURKINA FASO 2- Laboratoire HydroSciences Montpellier UMR 5569 Université Montpellier 2 Place E. Bataillon, 34095 Montpellier Cedex 5, France 3- Laboratoire d’étude des Transferts en Hydrologie et Environnement (LTHE), Domaine Universitaire, Bat. OSUB-B, 460, rue de la piscine 38400 Saint Martin d’Hères Mahamadou.koita@2ie-edu.org doi: 10.6088/ijes.2013040300008 ABSTRACT Hard rocks constitute complex hydrodynamic structures in which both weathering products of hard rock and fractures play a major hydrodynamic role. The characterization of the relative importance of weathering products and fractures in groundwater flow is assessed through a numerical groundwater simulation. The methodology employed focuses first on the capitalization of all geological and hydrogeological information available in the Dimbokro catchment. Secondly, all geological and hydrodynamic data obtained are incorporated in a flow simulator. To estimate and possibly quantify the relative importance of weathering profiles and major fractures zones, two approaches of modeling are adopted: Equivalent Continuum (accounting for the weathering profile without fractures) and Continuum/Discrete fractures (incorporating discrete fractures). In the two approaches, the adjustments of simulated and observed hydraulic head are sensibly the same. However, the Equivalent Continuum model shows disequilibrium in the water budget due to the fact that the model is not able to reproduce the drainage condition of the aquifer. The presence of fractures in the Continuum/Discrete fractures approach allowed balancing the water budget. Although the fractures do not show any influence over hydraulic head distribution, they play a major role in groundwater circulations within the flow paths of the aquifer which allows balancing the water budget. Keywords: weathering profile, fractures, hard rock, numerical groundwater simulation, water balance. 1. Introduction The term “hard rock” applies to crystalline or recrystallised igneous or metamorphic rocks associated with Precambrian age. These include ancient Archaean cratonic rock (granite, gneisses, and greenstones) and metamorphic rocks associated with mobile belts (Arcworth, 1987; Jones, 1985; Key, 1992). In terms of hydrogeology, a rock mass can be regarded as a combination of porous rock matrix and secondary openings. Secondary features such as fractures, faults and weathering zones are responsible for the majority of groundwater movement.