Rostock University’s modeling study on the impact of P-filters at catchment scale for the Warnow river
The Warnow river basin - and its sub basins - (3041 km²) is located in north-eastern Germany, it is agriculturally dominated and 18% of the fields are tile-drained. Data indicate that DRP- and TP- concentrations decreased drastically in the 1990s and target values according to EU-WFD and national laws are mostly achieved nowadays. Nevertheless, according to the Baltic Sea Action Plan (BSAP), Germany has to reduce annual P loads into the Baltic Sea by 110 ton.
Broken down to the Warnow river basin, Rostock University calculated a reduction requirement of 13 ton P/y meaning a required P load reduction of 26%. The eco-hydrological Soil and Water Assessment Tool (SWAT) was employed to estimate the reduction effect of P filters in tile-drained areas on the overall P load.
Hydrology and P loads in the Warnow river basin were simulated using a multi-site calibration and validation approach. It was found that the main P sources in the catchment are groundwater (72%) and tile-drainage water (23%). P originating from surface runoff accounts for 5% only.
Figure 1: Total Phosphorus (TP) sources (pie chart) and annual TP loads of the river Warnow basin based on observed (black bar) and simulated (red bar) data. The blue bars indicate TP loads due to different P-filter settings.
A simulation for the reduction effect of various P-filters was done by UROS as tested in the field in Germany (Rostock University, iron-coated sand, scenario 1), Denmark (Aarhus University, DiaPure®, scenario 2) and Belgium (Ghent University, iron-coated sand, scenario 3).
The scenario analyses revealed that P filters installed at the outlet of drain collectors have the potential to reduce the overall P load in the Warnow river basin by 5 ton per year or 10% of the total P load (Figure 1).
The modeling study indicates that P filters can contribute to the reduction of the P load of surface waters but also that additional measures are required in order to meet desired quality standards in receiving coastal waters.