Groundwater-surface water interactions in a North German lowland floodplain - Implications for the river discharge dynamics and riparian water balance

Water balance and groundwater dynamics of a floodplain catchment in the Northeast German lowlands are investigated with consideration of the variable interactions between the riparian groundwater and surface water. Based on experimental and numerical investigations, evidence is given for temporally and spatially variable exchange fluxes between groundwater and surface water, which have significant impact on the riparian water balance and groundwater recharge. A coupled soil water balance-groundwater model is used for the quantification of exchange fluxes across the groundwater-surface water interface and the groundwater recharge for nested catchments in a typical floodplain representative for the Central. European lowland catchments. Simulation results indicate substantial exchange fluxes between groundwater and the river, which are subjected to intensive spatial and temporal variability. The intensities and also the directions of exchange fluxes in particular stream reaches are characterised by transient alterations. Groundwater-surface water interactions are found to control the groundwater recharge dynamics in the floodplain and outweigh the influence of vertical percolation and root water uptake. Although groundwater contributions from this river stretch represent only 1% of the annual total discharge within the river its impact is much higher during tow flow conditions in summer when ca. 30% of the river runoff which is generated in the catchment is originated by groundwater discharge from the riparian zone along this river stretch. The results of this study show that groundwater-surface water interactions within the investigated riparian floodplain are far too complex for the traditional classification concept distinguishing between loosing and gaining sections. The temporally variable impact of groundwater-surface water interactions furthermore highlights the necessity to consider seasonal effects when assessing the significance floodplain processes and functions. (c) 2007 Elsevier B.V. All rights reserved.