Southern Water: Low cost catchment modelling approaches

Since its inception in 2000, the EU Water Framework Directive has fostered a transformation in river basin management. In order to meet the Directive's need for holistic River Basin Management Plans, hydrological catchment models are increasingly used to predict the impact of environmental change on the flow characteristics and water quality of European rivers.

The Water Framework Directive seeks to prevent the deterioration of ground and surface water bodies, and achieve good ecological and chemical status in water courses. This requires the management of a range of chemical contaminants, including pesticides, and their supply to water courses. Metaldehyde (a synthetic aldehyde pesticide used globally in agriculture) has been identified as an emerging contaminant of concern. Metaldehyde is of particular problem because it is highly stable in water, can be very mobile in the environment, and is ineffectively removed by drinking water treatment processes.

This project considered how low-cost catchment modelling approaches (e.g. ArcSWAT) might help to predict where and when water quality issues are most likely to arise, and was a collaboration with Southern Water.

This research aimed to integrate large existing data sets with the Soil and Water Assessment Tool (SWAT) in order to establish a transferable protocol for mapping and predicting aquatic pollutants. SWAT is an open source catchment-scale hydrological model originally developed in the US.

This research modelled metaldehyde in a pilot catchment (the Medway catchment) in order to better assess risk and to establish a transferable protocol for mapping and predicting future emerging aquatic contaminants. As such, the approach allowed the development of more effective mitigation strategies and will provide the water sector with much needed resilience to future climate change.

A SWAT model was developed for the River Medway catchment to predict metaldehyde concentrations at sites with no observational data, under projected future rainfall scenarios and after targeted catchment management. The results of calibration and validation of river flow showed that the SWAT model could simulate water balance in this catchment very well. Concentrations of metaldehyde were also calibrated successfully, although validation of metaldehyde concentrations showed a decreased fit of the data that requires further investigation in future research.

Metaldehyde in-stream hazard maps were produced using simulated data from the model. These maps were then used to identify locations in the catchment at risk from the highest peaks in metaldehyde concentrations. A range of metaldehyde reduction scenarios were simulated for sub-catchments and results suggested that up to 58 per cent of in-stream metaldehyde could be removed at specific intakes with effective management.

Different rainfall scenarios determined from the Meteorological Office’s UK Climate Projections (UKCP09) 2020s and 2050s projections were simulated by the model. The results implied that projected rainfall increases of up to 20 per cent over the winter months could lead to increases of in-stream metaldehyde of up to 38 per cent, with the limiting factor being metaldehyde application quantities. Simulations of projected rainfall decreases in summer months suggested that reduction in rainfall may also lead to increases in in-stream metaldehyde concentrations because of reduced dilution. Simulated rainfall and metaldehyde concentrations were analysed to determine whether the intensity of rainfall over 24, 48 and 120 hours was associated with metaldehyde drinking water standard exceedances (>0.1 µg/L). The results indicated that metaldehyde concentration exceedances cannot be predicted accurately using rainfall data alone. Overall, so far, this study demonstrates the potential of the SWAT model to identify locations for targeted catchment management and also for predicting the impact of selected management strategies.

Knowledge transfer was an important aspect of this project. Knowledge transfer was undertaken in the final six months of the project to transfer capacity to work with the Soil and Water Assessment Tool protocols. A series of knowledge transfer videos were produced to facilitate this in the longer term.