PCDitch

As a result of intensive agriculture in catchment areas, many polder ditches have turned from a clear water state with submerged plant dominance into a state where the water is completely covered with duckweed. Dominance of free-floating plants poses a threat to biodiversity by causing dark an anoxic conditions in the water column, and is generally associated with poor ecological quality.

PCDitch predicts the existence of a 'critical nutrient load' above which one may expect duckweed coverage.[1] It can be used by water resource managers to estimate the critical nutrient loading for ditch systems,[2] and to evaluate the effectiveness of restoration measures such as nutrient loading reductions. A meta-model has also been developed [3] for use by water managers to derive an estimate of the critical nutrient loading based on only a few key parameters, without any need to run the full dynamic model.[4] PCDitch is also used by scientists to investigate the more general effects of eutrophication in ditch ecosystems,[5] or to study the competition between different aquatic plant species.

In essence PCDitch is a set of coupled ordinary differential equations. With more than 40 state variables and more than 400 parameters the model is considered fairly complex. The model describes a completely mixed water body comprising the water column and the upper sediment layer. The overall nutrient cycles for nitrogen and phosphorus are described as completely closed (except for in- and outflow, denitrification and burial). Inputs to the model are: water inflow, evaporation, nutrient loading, light intensity, water temperature, sediment characteristics and depth of the ditch. Six functional groups of water plants are modelled: floating-leaved plants, emerged plants, non-rooted floating plants, non-rooted submerged flowering plants, rooted submerged flowering plants, Charophytes, and one phytoplankton group.

The default configuration of PCDitch does not take spatial heterogeneity into account. However, PCDitch can be coupled with spatial explicit hydrodynamical models to model networks of ditches. Although PCDitch is primarily used for Dutch ditches, the model can be applied to other non-stratifying freshwater ecosystems where competition between primary producers is a key determinant of ecosystem functioning, if parameters are adjusted or some small changes to the model are made. PCDitch was calibrated with data of experimental ditches with sand and clay sediments that were exposed to different nutrient loading treatments. As phosphorus in PCDitch is related with aluminium, iron, lutum content (sediment particles smaller than 2 µm), porosity and organic matter content in de sediment, it is assumed that the model can also be used to describe peat ditches.

PCDitch is the twin-model of PCLake, which is an ecosystem model for shallow lakes. Both models were developed by Dr. Jan H. Janse and colleagues at the Netherlands Environmental Assessment Agency (PBL), formerly part of the Netherlands National Institute for Public Health and the Environment (RIVM).[6] Since 2009, the model has been jointly owned by PBL and Wageningen University & Research Centre, where further development and application of the model is taking place.

• Ecosystem model
• Water quality modelling
• Ecopath
• PCLake