b'A next-generation national critical source area map ofphosphorus losses in Irish agricultural catchments fordecision supportThomas IA1,2, Mockler E2, Kelly E.3, Mellander P-E4,5, Daly K4 and Bruen M11Dooge Centre for Water Resources Research, University College Dublin, School of CivilEngineering, Dublin, Ireland2Catchment Science and Management Unit, Environmental Protection Agency, Dublin, Ireland3Agri-Business & Rural Development, University College Dublin, School of Agriculture and Food Science, Dublin, Ireland4Crops, Environment and Land Use Programme, Teagasc, Johnstown Castle EnvironmentResearch Centre, Wexford, Co. Wexford, Ireland5Agricultural Catchments Programme, Teagasc, Johnstown Castle Environment ResearchCentre, Wexford, Co. Wexford, IrelandFarmers, catchment managers and policymakers can benefit from up-to-date, high resolution national maps of critical source areas (CSAs) of phosphorus (P) losses from agricultural land to waterbodies to improve land management decisions and enhance water quality. To achieve this for Ireland, the DiffuseTools project updated the existing Environmental Protection Agencys Catchment Characterisation Tool to improve CSA delineations and predictions of phosphorus losses via surface runoff and leaching. The tool now incorporates the latest nutrient source and mobilisation datasets/science. Furthermore, it uses a new national 5m resolution runoff risk map (soil topographic index), derived using a hydrologically corrected 5m digital elevation model (DEM), a new soil drainage class map for Ireland, and hydrological connectivity modelling. The national next-generation P CSA map was then generated and evaluated using water quality monitoring data from the Environmental Protection Agency and Teagasc Agricultural Catchments Programme. A new sub-model identified all delivery points across the country, where runoff and diffuse pollutants such as P enter the waterbody, and calculated attributes such as CSA size, risk score and receiving waterbody status, in order to further prioritise the targeting of mitiga-tion measures, inform design requirements, and increase cost-effectiveness. The output of this research will contribute to functional land management and the growing evidence base to help ensure that agriculture is sustainable by reducing diffuse pollution to improve and protect water quality.31'