types of wetlands, agricultural fields, heathlands and small villages. The heathlands, forests and wetlands are all part of a protected nature reserve [Van Diggelen et al., 1995]. Different types of wetland vegetation are dependent on various water sources, with the biodiversity-rich fen peatlands primarily depending on the groundwater flows from phreatic and semi-confined groundwater aquifers [Grootjans et al., 1993; Van Diggelen et al., 1995]. Everts & De Vries [1991] illustrated the hypothetical groundwater systems in Drentsche Aa based on the vegetation gradients using the theoretical framework from the groundwater systems by Toth, [1963] (Figure 1). These