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 such as The Netherlands, surface water levels are tightly controlled by inlet of diverted river water during dry periods and discharge via large-scale pumping stations during wet periods. The conventional water level regime in these polder catchments is either a fixed water level year-round or an unnatural regime with a lower winter level and a higher summer level in order to optimize hydrological conditions for agricultural land use. The objective of this study was to assess the hydrological and hydrochemical effects of changing the water level management from a conventional fixed water level regime to a flexible, more natural regime with low levels in summer and high levels in winter between predefined minimum and maximum levels.

Ten study catchments were hydrologically isolated and equipped with controlled inlet and outlet weirs or pumping stations. The water level management was converted into a flexible regime. We used water and solute balance modeling for catchment-scale assessments of changes in water and solute fluxes.

Our model results show relevant changes in the water exchange fluxes between the polder catchment and the regional water system and between the groundwater, surface water, and field surface storage domains within the catchment. Compared to the reference water level regime, the flexible water level regime water balance scenario showed increased surface water residence times, reduced inlet and outlet fluxes, reduced groundwater-surface water exchange, and in some catchments increased overland flow. The solute balance results show a general reduction of chloride concentrations and a general increase in N-tot concentrations. The total phosphorus (P-tot) and sulfate (SO₄) concentration responses varied and depended on catchment-specific characteristics.

For our study catchments, our analyses provided a quantification of the water flux changes after converting towards flexible water level management. Regarding the water quality effects, this study identified the risks of increased overland flow in former agricultural fields with nutrient enriched top soils and of increased seepage of deep groundwater which can deliver extra nutrients to surface water. At a global scale, catchments in low-lying and subsiding deltas are increasingly being managed in a similar way as the Dutch polders. Applying our water and solute balance approach to these areas may prevent unexpected consequences of the implemented water level regimes.