Modeling as a means to plan river basin management

Introduction

Emission models

 Water quality models

Aquatox

SWAT

Introduction

River basin management consists of measures that are implemented to reach certain goals in terms of improving surface water quality. Hence the necessity to be able to predict the impact of those measures in order to wisely invest the limited resources available for maximum improvement. In practice the use of models is very limited in current planning processes, in essence because there are no dedicated water quality models that are able to produce results in metrics that are relevant for the Water Framework Directive. Since these metrics are predominantly biological, water quality models need to make the link between non-living parameters (nutrients, oxygen content, pollutants) and their effects on biological communities that serve as benchmarks. These features are currently not available or only in development under different model platforms or statistical approaches. The same applies for the impact of micro-pollutants, which can only insufficiently be tackled by existing models.

Another hurdle that makes water managers shy away from modeling is the need of substantial amounts of targeted monitoring data which will allow for model validation.

In this context the M3 project conducts some exploratory modeling actions on pressures that are seldom addressed in current planning processes. M3 uses an array of different models with different complexity and data needs. The monitoring campaigns are adapted to these models’ input and validation needs.

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Emission models

Mitigation at the source is still the most efficient way of dealing with pressures, hence one needs to identify their location and amplitude. Substance flow and mass balancing models are a robust estimation method as long as the pollutant loads are not too variable (strong seasonal or yearly dynamics). They can rely on sound calibration efforts in representative catchments and extrapolate these estimates to unknown entities. Erftverband’s own DatenFluss Model uses this approach to balance the impact of WTTPs, overflow structures, surface runoff and interflow. The model has been extensively calibrated in the Erftverband’s catchment to provide robust estimates where monitoring data are not available. M3 will test the applicability of this model in non-calibrated catchments.

The Soil Water Assessment Tool SWAT is a distributed process model which is basically a precipitation-runoff simulator featuring very detailed descriptions of nutrient, suspended matter and pollutant fate at the source. The generated loads are then routed through the catchment’s river network with further transport processes considered. SWAT needs sound hydrological calibration and good knowledge on parameter sensitivity as well as output uncertainty to be applied in a meaningful way. M3 will explore the usefulness of such a complex model in the context of pesticide emission prediction.

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Water quality models

Water quality models have a long history of application for oxygen depletion and eutrophication phenomena. They have also served to simulate the transport and fate of pollutant plumes. The introduction of the biological metrics as a benchmark in the WFD has created a need for more holistic ecological models that ideally would also cover toxic effects of pollutants on river ecosystems and their communities. The majority of models predicts pollutant concentrations but fails to evaluate the impact which is often just estimated manually from tabulated thresholds. Moreover, there are no convincing solutions yet for dealing with multiple pressures in one model frame. Since biological metrics are by definition integrative the link to water quality models poses a problem.

In the M3 project two models are explored with a special focus on their pertinence for WFD related issues. The DWA-Water quality model is a traditional transport oriented model which has acquired a certain acceptance in the German community and can be used to simulate nutrient and pollutant fate in surface waters in a comprehensive framework. It is an ideal tool to assess nutrient or pollutant propagation in a river network. EPA’s Aquatox is an ecological model with a dedicated toxicity module which allows for comparative evaluation of substance related impacts. It has capabilities of simulating long term and chronic impacts but has weaknesses in transport and network simulations.

Both models will be used in a comparative manner to identify conceptual differences in fate modeling and impact assessment. The respective sensitivities to parameters and uncertainties in outputs will be evaluated.

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