Catchment Characterization and Targeting of BMPs to Control Sediment and Phosphorus in Land Runoff Using the PSYCHIC Decision Support Tool in the UK.
Paul Withers and Eunice Lord. ADAS Consulting UK, Sunnyhill House, 1 Thistlebarrow Road, Salisbury, SP1 3RU, United Kingdom
The EU Water Framework Directive (WFD) requires the preparation of river basin management plans to implement a programme of measures to achieve good water quality based on a range of ecological, and associated chemical, indicators. These measures will need to control both point source and diffuse pollutants in catchments, including Suspended Sediment (SS) and Phosphorus (P) loss from agricultural land. To facilitate the cost-effective targeting of a range of land management options to mitigate SS and P loss, a prototype catchment-based decision support tool called PSYCHIC (Phosphorus and Sediment Yield CHaracterization In Catchments) was developed and applied to a number of catchments in the UK with siltation and eutrophications problems. PSYCHIC quantifies the loads of SS, particulate and dissolved P in runoff from agricultural land along different surface and sub-surface hydrological pathways and assesses their contribution relative to point sources. PSYCHIC adopts a process-based modelling approach to ensure sensitivity to land management practices that have a large influence on the mobilisation and delivery of SS and P. The model takes account of climate, landscape and land management factors, and utilises current knowledge on the processes of SS and P loss, to predict the spatial and temporal distribution of flow and SS and P loads on a monthly time step and at both 1 km2 and field scales within a GIS environment. The decision support tool integrates model predictions of catchment areas with a high risk of loss or ‘hotspots' (Tier 1 screening tool) with field scale risk assessment of contributing areas on farms (Tier 2) to help identify appropriate Best Management Practices (BMPs) associated with improved nutrient and land management. PSYCHIC then assesses the impact of the selected BMPs on SS and P loss at the field and catchment scale. This enables the end-user to identify and prioritize where control measures need to be adopted, and to help justify their adoption in line with policy mechanisms (e.g. catchment sensitive farming) to reduce diffuse agricultural pollution. In combination with an analysis of historic water quality records, and enhanced by a targeted water chemistry monitoring programme, PSYCHIC was run in a range of catchments and the spatial and temporal risk of loss identified and apportioned according to various sources. The paper describes the data requirements, results, experiences, uncertainties and challenges of developing such a pragmatic catchment-based decision support system and makes recommendations on future development and operation within the context of integrated river basin management.