Evaluating diffuse and point phosphorus contributions to river transfers at different scales in the Taw catchment, Devon, UK

This paper describes an empirical study carried out to improve understanding of how small scale transfers of mobilised phosphorus (P) recorded at plot to field scale are mirrored by P fluxes measured at the river catchment scale. Phosphorus transfers were studied in partially nested sites that range in area from 30 m2 to 834 km2. The work was carried out within the grassland dominated catchment of the River Taw, south west England. The research sought to determine the characteristics of the link between plot and river scale P fluxes for different P forms. The highest point source inputs of P were generally recorded in the largest sub-catchments of the river. However, diffuse sources of P were estimated to constitute at least 60% of the annual P flux at all sites. The magnitude of the diffuse P input varied with catchment land use. Detailed studies focused on the lowland catchments showed that diffuse fluxes can be well represented by plot and field scale P transfers. The research produced no evidence for significant inputs of P from river channel banks. However, at the hillslope scale, additional transfers of P from ‘incidental’ sources such as re-seeded fields, open access ditches, and farmyard point sources were recorded. These inputs did not appear to significantly affect river P fluxes, suggesting that they are either of low frequency, or that there is a corresponding decrease in overall P transfer as the research scale is increased from field through to sub-catchment. Additional evidence of scaling was observed where riverine P fluxes were diluted by upland or groundwater flows.