Résumé
Hard-rock aquifers (HRA) have long been considered to be two-layer systems, with a mostlycapacitive layer just below the surface, the saprolite layer, and a mainly transmissive layerunderneath, the fissured layer. Although this hydrogeological conceptual model now attracts a wideconsensus within the scientific community, it is difficult to prove that it can be efficient in terms ofdeterministic modelling, especially with an equivalent porous medium model, w hich would not be thefirst choice for “fractured aquifers”. In order to investigate the storage capacities of both layers and therecharge, a deterministic hydrogeological model was used on a HRA to calibrate the specific yield ofeach layer. The application site is the Plancoët migmatitic aquifer located in north-western Brittany,France, with piezometric data from 40 observation wells surveyed every two weeks for eight years.When the variance, the most common quality-of-fit criterion used in inverse calibration methods, wasfound to be inefficient for calibrating the specific yield and the recharge together, Advar , a newquality-of-fit criterion based on the seasonal piezometric amplitude variation, was developed andcalculated for a set of 512 simulation results. Contrary to the variance, it is able to define the bestvalues for the recharge and the specific yield in each layer. Adding to the conclusion that this two-layermodel is well suited to such a HRA, it is demonstrated that the saprolite is about 2.5 times morecapacitive than the fissured layer.