A unified and multi-scale Source: Pathway Priority Index for diffuse pollution management

Diffuse pollution is a global issue where management, particularly relating to phosphorus (P) transfers from agricultural land to water, needs to consider the magnitude of the source pressure and the connectivity of the hydrological pathway pressure. Combined, these pressures are considered as critical source areas (CSAs) and where mitigation resources should be focused as part of landscape targeting. However, data requirements and lack of a unified method have made this difficult to implement at national scales. To overcome this, a unique and transferrable national workflow is presented for this purpose at three scales to aid in prioritisation. First, macro- or basin-scale (100-600 km²) water quality data (soluble reactive P-SRP) were used as an initial indicator of pressures at a national river basin scale in Northern Ireland. Second, within these macro-scale catchments, meso‑scale catchments (10-100 km²) used for Water Framework Directive surveillance (n > 230) were prioritised using a validated relationship between long-term river SRP and soil test phosphorus (STP-Olsen P) as the source pressure in over 300,000 agricultural fields tested as part of a national monitoring programme. These meso‑scale catchments were also screened for persistent point source pressures using ammonium (NH₄) concentration data. Within each meso‑scale catchment, micro-scale catchments (0.02 - 1.6 km²; 5th - 95th percentile) were identified (> 1.9 million) that combined summaries of STP and a runoff risk metric that was developed with a high-resolution (16 points m^-2) LiDAR derived soil topographic index (STI) into an anonymised and dimensionless Source:Pressure Priority Index (SPPI). Exemplar outputs are shown in detail that weight the source and pathway pressures equally, and further emphasise source over pathway pressure, and vice versa, to ensure advisory and mitigation resources can be allocated effectively. The SPPI is a more robust diffuse pollution risk assessment and management tool as it recognises the importance of managing the magnitude of the source pressure, in combination with reducing pathway pressures, rather than focusing on the latter in isolation. This will ensure a faster route to diffuse pollution reduction and offer resilience as pathway mitigations become vulnerable to weather patterns and runoff responses in a changing climate.