A tiered risk-based approach for predicting diffuse and point source phosphorus losses in agricultural areas

Implementation of the European Union Water Framework Directive requires an assessment of the pressures from human activity, which, combined with information on the sensitivity of the receiving waterbody to the pressures, will identify those water bodies at risk of failing to meet the Directive's environmental objectives. Part of the process of undertaking the risk assessment for lakes is an assessment of diffuse agricultural phosphorus (P) pressures. Three approaches of increasing sophistication were developed for this purpose: a basic 'risk screening' approach (tier 1) applicable to all lakes in Great Britain (GB) and based on export coefficients for different land cover classes and animal types; the Pressure Delivery Risk Screening Matrix approach (tier 2) that differentiated between pressures in surface water and groundwater river basins; and the Phosphorus Indicators Tool (PIT), a simple model of locational risk and P delivery potential (tier 3). Application of the three approaches to a range of lake catchments in England demonstrated that a tiered risk assessment approach was appropriate which was tailored to the quality of the available data. A step-wise procedure was developed whereby if the tier 1 and 2 approaches showed a catchment to be at high risk of failing to meet the Directive's environmental objectives with regard to P, it was justifiable to undertake a more detailed assessment using the tier 3 approach. The tier 1 approach was applied to all lakes in GB greater than 1 ha in size on the assumption that the boundary between the good/moderate status classes under the Water Framework Directive guidelines represented a doubling of the total P (TP) reference conditions. The initial outputs suggested that 51% of lakes in GB are predicted to not meet the TP targets identified for high or good status and must, therefore, be considered at risk. There were regional differences in numbers of lakes at risk. Scotland appeared to have the fewest sites at risk (18%); England the most (88%), with Wales having an intermediate percentage (56%). A comparison of P pressures on freshwaters using the tier 2 approach with other pressures on waterbodies (e.g. nitrate, sediment) in GB is shown as risk maps on the Environment Agency website at: . The tier 3 approach was applied to data-rich catchments and identified at the 1 km(2) areas of relatively high risk of P delivery to water.