Proceedings of the first Geologica Belgica International Meeting, Leuven, 11-15 September 2002 Aardk. Mededel., 2002, 12, 207-210 207 Development of a Groundwater Model for some Small Sub-Catchments of the Dender Basin B. Verbeiren, O. Batelaan & F. De Smedt Vakgroep Hydrologie en Waterbouwkunde, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussel, bverbeir@vub.ac.be ABSTRACT To characterise the ecological condition of valleys hydrological variables such as groundwater level, fluctuation, discharge zones, etc. are required. In this study a groundwater modelling approach is used for the estimation of these variables. Within the framework of the action plan “Development of ecosystem approaches for different valleys in Flanders” a MODFLOW groundwater model was developed, for three sub-catchments of the Dender River. GIS was used to prepare the input, to analyse the output and to present the results of the groundwater modelling. The results of the groundwater model, a set of hydrological variables, was used to analyse the relationship between vegetation and groundwater. Both the present groundwater system as well as a future optimal situation were simulated. The model and its analyses serve, within this ecosystem approach, as a basis for the formulation of policy measures for nature conservation and (potential) nature management in the area. KEYWORDS Dender basin, Geographical Information Systems (GIS), Groundwater modelling, MODFLOW, Nature conservation. Objectives A groundwater model was developed within the framework of an ecosystem approach. Its purpose is to obtain different hydrological variables and to estimate the hydrological boundary conditions for (potential) future vegetation. Study area The study area includes the catchments of Molenbeek- Terkleppebeek, Molenbeek-Ophasseltbeek and Beverbeek. These are small sub-catchments of the Dender River and are situated in the southeastern part of the province of East- Flanders, their surface area is about 132 km² (Fig. 1). The Dender River forms the eastern border of the study area. The topography of the study varies from more than 100 m at the western hills (Vlaamse Ardennen) to less than 20 m near the Dender River. The tributaries of the Dender River in these sub-catchments run from west to (north-)east. Fig. 1. Location of study area. The geology of the region is dominated by the Tertiary formations of Kortrijk, Tielt, Ghent, Lede and Maldegem. The Dender valley cuts into the Moen (fine sand) and Saint- Maur (clay) member of the formation of Kortrijk. The western hills consist of the sands and clays of the formations of Tielt, Ghent, Lede and Maldegem. The Quaternary loamy top layer is often very thin on the hills. However in the lower parts of the area it can reach a thickness of minimum 10 to 20 m (Maréchal, 1993; De Geyter, 1999). Groundwater model The groundwater modelling was performed with the MODFLOW code (McDonald & Harbaugh, 1988). Since groundwater modelling requires a lot of spatially distributed data, GIS was used to generate and prepare these data. Also, the output of the groundwater model was exported to GIS to be able to perform analysis and to present results in a clear way. An adaptive script environment was built to support the GIS-MODFLOW communication. This environment facilitates considerably the modelling and analysis. Description The model area was discretised in cells of 20 by 20 m. The groundwater model consists of two layers, each with 700 rows and 1060 columns, in total about 330.000 active cells a layer. Hydrogeological concept Based on the (hydro)geological information of the geological map of Geraardsbergen and on the stratigraphical