Groundwater-surface water relations in regulated lowland catchments; hydrological and hydrochemical effects of a major change in surface water level management

Abstract

In lowland deltas with intensive land use, surface water levels are human controlled letting in river water during dry periods and discharge by pumping during wet periods. The water levels are usually maintained at a fixed level year-round or at fixed winter and (higher) summer levels. Several water authorities in The Netherlands consider implementing a more natural and flexible water level regime in nature reserves, with low levels in summer and high levels in winter. The objective of this study was to assess the catchment-scale hydrological and hydrochemical effects of such a change using water and solute balance modeling.We focus on ten study nature reserves where a conversion to flexible water management was planned or recently implemented. Monitoring data from the catchments were used for validating the water balance and as boundary condition input for the solute balance calculations. For all catchments, the results show relevant changes after implementing flexible water level management. For example, the surface water residence times increased (avg. +25%), the inlet and outlet fluxes reduced (avg. –38% and –72%), the chloride concentrations reduced (avg. –14%), and the N-tot concentrations increased (avg. +13%). Both the quantification of water flux changes and the detection of water quality risks were highly relevant for the water authorities. Customizing our approach to the specific circumstances in other low-lying artificial catchments worldwide may help local water managers in optimizing their water level management.