Overfertilization by agriculture loads polder ditches in such a manner as to allow transition from submerged macrophyte-dominated clear water to water completely covered with duckweed, the latter disturbing aerobic life and diminishing biodiversity. PCDitch, representing a functional model of a ditch, describes the nutrient fluxes in the water, the sediments and the vegetation, as well as the competition between different groups of vegetation. PCDitch was calibrated using data from the experimental ditch system at Sinderhoeve, in cooperation with the Wageningen Agricultural University and the Winand Staring Centre. The observed dominance of submerged macrophytes at a low nutrient loading, in contrast to duckweed dominance at a high one, was simulated reasonably by the model. This was followed by a model application on the ditches in the province of Noord-Holland, using field data from the Water Authority in Hollands Noorderkwartier. The final result was a method to relate nutrient load to dominant vegetation. The model predicts the existence of a 'critical load' or 'critical concentration' in the water above which one may expect duckweed coverage. These 'critical' parameters depend on type of soil, concentration of inlet waters, hydraulic residence time and nutrient-uptake by vegetation. The dose-effect responses calculated in this stage are preliminary.