Impact of climate change on water management in Dutch peat polders

A spatially explicit dynamic model was developed that combines water quantity and quality processes for an average Dutch peat polder. The model has been used to calculate the consequences of climate change for surface water level, groundwater level and nitrogen and phosphorus fluxes, based on time series (2036–2065) of the two most extreme climate change scenarios for the Netherlands (G and W+) developed by the Royal Dutch Meteorological Institute. It was calibrated using measured data from two existing polders. The results suggest that surface water levels will not change much in both scenarios. Hence, the current practice of letting extra water in during dry summer periods to maintain sufficient water levels will probably remain a viable solution to temporary summer droughts in the near future as the required water volumes are not likely to increase greatly. Late summer groundwater levels decrease substantially in the W+ scenario but not in the moderate G scenario. This drop in groundwater level would accelerate the on-going process of peat oxidation, resulting in increased soil subsidence, greenhouse gas emissions and nutrient release from mineralisation. The W+ scenario also led to higher nitrogen concentrations and lower phosphorus concentrations in the ditch network. Probably the reduced stream velocity in the W+ scenario results in an increased sedimentation of (adsorbed) phosphate. Sedimentation is a less important process in the nitrogen cycle, and the increased nitrogen concentrations anticipated in summer according to the W+ scenario are therefore probably the consequence of a reduced outlet of water due to increased evaporation. Additionally, an increased delivery of nitrogen through mineralisation could also contribute to the increased nitrogen concentrations in summer.