Evaluating the critical source area concept of phosphorus loss from soils to water-bodies in agricultural catchments

Using data collected from six basins located across two hydrologically contrasting agricultural catchments, this study investigated whether transport metrics alone provide better estimates of storm phosphorus (P) loss from basins than critical source area (CSA) metrics which combine source factors as well. Concentrations and loads of P in quickflow (QF) were measured at basin outlets during four storm events and were compared with dynamic (QF magnitude) and static (extent of highly-connected, poorly-drained soils) transport metrics and a {CSA} metric (extent of highly-connected, poorly-drained soils with excess plant-available P). Pairwise comparisons between basins with similar {CSA} risks but contrasting {QF} magnitudes showed that {QF} flow-weighted mean {TRP} (total molybdate-reactive P) concentrations and loads were frequently (at least 11 of 14 comparisons) more than 40% higher in basins with the highest {QF} magnitudes. Furthermore, static transport metrics reliably discerned relative {QF} magnitudes. However, particulate P (PP) concentrations were often (6 of 14 comparisons) higher in basins with the lowest {QF} magnitudes, most likely due to soil-management activities (e.g. ploughing), in these predominantly arable basins at these times. Pairwise comparisons between basins with contrasting {CSA} risks and similar {QF} magnitudes showed that {TRP} and {PP} concentrations and loads did not reflect trends in {CSA} risk or {QF} magnitude. Static transport metrics did not discern relative {QF} magnitudes between these basins. In basins with contrasting transport risks, storm {TRP} concentrations and loads were well differentiated by dynamic or static transport metrics alone, regardless of differences in soil P. In basins with similar transport risks, dynamic transport metrics and P source information additional to soil P may be required to predict relative storm {TRP} concentrations and loads. Regardless of differences in transport risk, information on land use and management, may be required to predict relative differences in storm {PP} concentrations between these agricultural basins.