Selection of a watershed model used to predict the effects of management decisions on water quality based on multi-criteria comparison


The Aquisafe project aims at mitigation of diffuse pollution from agricultural sources to protect surface water resources. The first project phase (2007-2009) focused on the review of available information and preliminary tests regarding (i) most relevant contaminants, (ii) system-analytical tools to assess sources and pathways of diffuse agricultural pollution, (iii) the potential of mitigation zones, such as wetlands or riparian buffers, to reduce diffuse agricultural pollution of surface waters and (iv) experimental setups to simulate mitigation zones under controlled conditions. The present report deals with (ii) and aims at identifying numerical modelling tools that can assess the origin of contaminants as well as the impact of different mitigation measures regarding water quality aspects on a catchment scale. In order to test the identified modelling tool in the further course of the Aquisafe project a case study was found in Brittany (France) in agreement with Veolia Eau: the small watershed of the river Ic. Due to intensive agricultural land use the nitrate concentration exceeds the threshold for surface water used for drinking water purpose (which is the main concern of Veolia Eau). Additionally, trace contaminants (pesticides) were detected in the surface water ever since measurements have been carried out. Therefore modelling shall mainly support the water supplier in actions aiming at reducing the nitrate concentration in the surface water. An additional task could later on be the application of the model in order to assess the effectiveness of mitigation measures against trace contamination. In order to choose the most appropriate model a model comparison was carried out using a three step approach. The first step was a screening of different information sources and resulted in the identification of 44 existing models. The second step was a pre-selection according to essential criteria in order to identify models that fulfil the basic requirements for a) the Ic nitrate issue and b) the Aquisafe trace contaminant issue. In a third step a multicriteria analysis was carried out using 6 additional criteria followed by a final recommendation. The essential criteria used for the pre-selection of the models were a) the inclusion of major hydrological processes, b) the inclusion of the nitrogen cycle (for the Ic nitrate issue) or the inclusion of trace contaminants (for the Aquisafe trace contaminant issue) c) the size of catchments that can be modelled, d) the temporal and spatial resolution and e) the possibility to include management options and/or mitigation measures. For the Ic nitrate issue this resulted in the selection of the models: HBV-NP, HSPF, SWIM, SWAT, WASMOD and Mike She. For the Aquisafe trace contaminant issue only four models remained after the pre-selection process: DRIPS, HSPF, SWAT and Mike She. Additional criteria were then applied and resulted in the recommendation to use the model SWAT for further investigations in both cases due to sufficient accuracy and included processes (full hydrological model with water quality simulation (nutrients and trace contaminants) as well as a wide range of successful applications (amongst others). This report presents a wide range of models with their capabilities and limits. It contains criteria which were identified with the stakeholders in order to choose the most appropriate model. The approach presented in this report shall support the decision process of selecting a model for a certain problem regarding water quality and includes only a recommendation. The final decision on which model shall be applied, will be taken in agreement with the stakeholders Veolia Eau and Goel’Eaux.

Kompetenzzentrum Wasser Berlin gGmbH