Regional groundwater discharge: phreatophyte mapping, groundwater modelling and impact analysis of land-use change

The relationship between groundwater recharge and discharge is one of the most important aspects in the protection of ecologically valuable areas. Knowledge of groundwater systems is therefore a pre-requisite for up-to-date integrated land and water management. A methodology is presented for assessing the relative importance of different recharge-discharge systems, with respect to ecological status or development, including mapping of regional groundwater systems, and recharge and discharge areas. This methodology is applied to a land-use planning project in the Grote-Nete basin, Belgium. Discharge regions are delineated on the basis of their spatial discharge contiguity, position in the landscape and alkalinity of the plants habitat. The simulated discharge areas are verified by field mapping of phreatophytic vegetation. Particle tracking is used to delineate the recharge area associated with each discharge area, and to characterize each recharge-discharge groundwater system. Three groundwater flow and two vegetation parameters are used in a cluster analysis to obtain four different clusters of groundwater discharge systems. It is shown that the discharge clusters are significantly different in discharge intensity and alkalinity. The effects on the groundwater system due to anthropogenic impacts on the land-use are studied by simulation of the present, pre-development, and future situation. The results indicate the sensitivity and impact of the changes on the recharge and discharge areas, and groundwater discharge fluxes. The impact of the changes for the different areas for both the pre-development and the future situation appears to differ from large decrease to large increase in total groundwater discharge. Of additional ecological importance is the fact that some areas show an opposite behaviour regarding the changes in groundwater discharge area and fluxes. The delicate shifts in the groundwater systems, which cause the changes in the recharge and discharge, clearly show the need for hydrological modelling. The synergy of hydrological modelling and vegetation mapping proves advantageous and reveals some of the ecological differences in the catchment.