A functional model of a ditch ecosystem has been developed, aimed at describing the relation between nutrient input and water quality and dominant vegetation in drainage ditches. Its aim is the derivation of the ‘critical nutrient loading’ for a shift from submerged vegetation to duckweed dominance. The model, called PCDitch, describes the competition between several functional groups of macrophytes, as well as algae. The macrophyte groups were defined according to the layer(s) in which they grow: submerged, floating or emergent, rooted or non-rooted. The model also includes the cycling of nutrients within the water, the sediment top layer and the vegetation. The model has been applied to the data of 8 experimental ditches located at Renkum (The Netherlands), which received different levels of nutrient loading during 4 years. The controls and the low- and medium-loaded ditches remained dominated by submerged plants, while in the high-loaded ones a dense cover of duckweed developed. In the sand ditches, submerged biomasses were lower than in the respective clay ditches. An optimization study has been performed for a number of sensitive parameters, minimizing the total sum of squared differences between simulated and measured values for all ditches, resulting in a set of parameter values that gives the best overall fit. The parameters included the maximum growth rates, the minimum phosphorus contents and the overwintering fraction of the plant groups. The model simulations by PCDitch were grossly comparable to the field observations, with duckweed in the high-loaded ditches and submerged plants in the other ones. The fit for algae and charophytes remained poor. Further calibration as well as testing the model in field situations are recommended to improve the model's predictive value.