Cost-effectiveness of targeted riparian management for sediment and total phosphorus considering convergent surface flow pathways: an Irish case study

Diffuse pollution, globally affecting water quality by delivery of sediment, nutrients, pathogens and agro-chemicals from farmland, often has dominant flowpaths connecting to discrete channel delivery points, where field-edge mitigation can be optimally targeted. Accurate representation of field convergent flow paths (CFPs) can inform decisions on riparian mitigation planning. For three fields in Wexford, Ireland, we combined literature, catchment data, field-survey and spatial data methods to derive sediment and P exports (7.4–18.7 tonnes sediment/year and 0.9–6.9 kgP/year), runoff areas and watercourse delivery points (one to six CFPs per field). We moderated exports according to the ratio effective riparian buffer area: CFP contributing area and compared 3 mitigation levels. Low buffer to CFP area ratios highlighted limitations of narrow buffers for larger CFPs. Linear grass buffers (2 m, level 1) were predicted to retain 2–17% of sediment and 1–6% total P exports. Level 2, 5 m buffers targeting CFP delivery points to watercourses retained 4–38% of the sediment and 2–15% total P and improved cost-effectiveness two- to three- fold relative to level 1 (20–1761 Euros/tonne sediment and 650–5114 Euros/kgP for level 2). Level 3 scenarios (sediment traps and in-ditch filters; 49% and 33% retention of field sediment and P losses, respectively) improved cost-effectiveness (50–145 Euros/tonne sediment and 108–1498 Euros/kgP). Mitigation cost-effectiveness best informs policy and planning and landowner decisions by including surface runoff behaviour utilising spatial soil and topographic data, accompanied by walk-over ground truthing.