A paired-catchment evaluation of voluntary agri-environmental scheme measures targeting diffuse phosphorus and sediment pollution

Agri-environmental schemes supplement regulations by encouraging on-farm actions to deliver environmental improvements. Investments can be significant but assessment of their effectiveness at improving stream water quality at catchment scale remains limited. Using a paired-catchment and before-after-control-impact approach, this study evaluated water protection measures within Northern Ireland’s Environmental Farming Scheme in reducing phosphorus (P) and suspended solids (SS) pollution. Four years of P and SS data were collected before and during implementation of pathway interception measures (e.g., riparian fencing, 2 m buffer strips) at catchment scale and compared with a control catchment. Seasonal Mann-Kendall trend tests were performed to assess the impact of measures on concentrations while locally weighted regressions were used to estimate response times following implementation. Wilcoxon signed-rank tests for different discharge intervals were performed to evaluate the impact of flow conditions on the effectiveness of measures. The measures decreased SS (>42 %) and, to a lesser extent, particulate P (PP) (>19 %) concentrations, but did not reduce soluble reactive P (SRP). The response time to the implementation of the measures was shorter for SS (∼17 months) than PP (∼22 months). No response was discernible for SRP which in turn impacted response for TP. Decrease of SS and PP concentrations was observed for low to average discharges, but high discharges decreased (SS) or suppressed (PP) the effectiveness of measures. The findings reveal the need to better target pathway interception measures and combine them with source reduction to deliver sustained and climate-resilient diffuse pollution mitigation and restore good trophic status in watercourses.