Relative importance of weather, land use and slurry spreading regulations for stream water quality in agricultural catchments

Despite long-term regulatory controls on fertiliser management that effectively close and open spreading periods, there are still ongoing stream water quality issues in agricultural catchments. Adjustments to these regulations largely relate to application rate and set-back distances from watercourses at the start of the open period to avoid sudden water quality impacts. Within this regulatory framework and using long-term datasets the aim of this study was to investigate the relative importance of weather, land use and policy effects on stream water quality during the first weeks of the open spreading period. Fortnightly stream water samples were collected over 2009-2023 in twenty-four agricultural sub-catchments of major Northern Ireland rivers. Random Forest Regression models were developed to predict baseline stream water total phosphorus (TP), soluble reactive phosphorus (SRP) and total oxidised nitrogen (TON) concentrations. Results showed that weather and land use were the primary drivers of changes in phosphorus concentrations while land use was the primary driver of changes in TON concentrations. Furthermore, weather was a more important driver of changes in nutrient concentrations in the more intensively farmed sub-catchments. In the less intensive sub-catchments, land use was at least 30% (for TP) to 85% (for TON) more important than the weather and policy predictors for explaining these changes. The study highlights the need to reduce the nutrient source pressure as a more effective step to improve water quality compared to small adjustments to fertiliser spreading protocols. It further supports the need to ensure slurry is spread when weather conditions are appropriate and for policy reviews to account for changes in weather pressures.