A guideline for consistent water quality modeling in rural areas

In several hydrological studies the need for consistency in hydrological modeling was highlighted. To achieve model consistency, it is required that all relevant hydrological processes are evaluated for accuracy in their spatio-temporal representation under consideration of available observations. In this study, we transfer the idea of hydrological consistency to water quality modeling. We focus on water quality modelling in rural mesoscale catchments and the interaction with agricultural production systems and their management. Based on several studies, we have developed a guideline which includes the following six challenges:

• Representation of rural landscape: Spatial and temporal patterns of land use and land management are critical to adequately represent water quality in models. Remote sensing and land use models are very useful resources to be exploited.
• Accuracy in model structure and model parameters: The transfer of a model diagnostic analysis to water quality leads to a better understanding of how water quality variables are controlled by model structures and corresponding model parameters.
• Check of multiple model output for consistency: Assessing multiple model outputs regarding their temporal, spatial and process performance using observed time series, remotely sensed spatial patterns, knowledge about transport pathways and even soft data can significantly enhance model consistency.
• Joint multi-metric calibration of discharge and water quality for all magnitudes: Multi-metric calibration using performance metrics and signature measures both for discharge and water quality, such as flow and nitrate duration curve, leads to more balanced model simulations that represent all magnitudes of discharge and water quality accurately.
• Scenarios and storylines for reliable land management: Scenarios and storylines should be co-developed with stakeholders in the river basin to increase realism and the acceptance of model results. They should be coherent in space and time, and provide a mix of available management options.
• Consistent interpretation of impacts on water quality: The interpretation of scenarios can be supported by diagnostic tools to show the effectiveness of measures and their combinations while considering their costs and impacts on ecosystem services.

In our contribution, we give examples and further details regarding each challenge to give insights how to achieve consistency in water quality modelling.