Diffuse pollution management in agricultural landscapes – a combined Source:Pathway Priority Index to target advice and resources for impact.

Diffuse pollution is a global issue where management, particularly of phosphorus (P) loss from agricultural land to water, must address both source and pathway pressures concurrently as part of effective mitigation. Where this is a widespread issue, policy makers, agri-environmental managers and farmers need a process of prioritisation that places the delivery point for diffuse P to a waterbody into a wider context of risk and maximises the impact of any mitigation for the limited resource available. However, data requirements and lack of a unified method have made this difficult to implement.

This study considers this challenge using field-by-field soil test P monitoring and high-resolution LiDAR runoff risk modelling being developed for all agricultural land in Northern Ireland through the Soil Nutrient Health Scheme. We combine long-term available water quality data for macro- and meso-scale catchments with this unique spatially explicit data set on soil test P and runoff risk (Hydrologically Sensitive Area (HSA)) combinations to rank risk to water quality down to a base unit of a micro-catchment scale (0.02 – 1.6 km²) delineated upslope from each delivery point to a waterbody. This is expressed as a dimensionless Source:Pressure Priority Index (SPPI) which conveys the combined source and pathway risk at a location but without any dimensioned values that would link to soil test P in specific fields and affect confidentiality of field-scale nutrient status information. With an average of 250 delivery points km^-2 this approach can filter the highest category SPPI areas to ~1% of those micro-catchments where measures should be targeted first.

This combination and analysis of “big data” provides a whole-landscape risk ranking method for diffuse pollution management that can be directed centrally and rolled out more locally as part of catchment level agri-environmental schemes (AES) and in targeting advisory and extension services. 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.