The controversial role of tile drainage in phosphorus export from agricultural land

Field drainage, in the form of permanently installed pipes or temporary mole drains, is extensively used in Britain to reduce the incidence of waterlogging, and increase the length of the grazing season. Whereas the installation of artificial drains significantly improves the structural stability of the soil, water quality in recipient streams may be adversely affected by the accelerated rate of nutrient transport, and the circumvention of critical storage areas such as buffer zones. This research investigates the importance of phosphorus (P) loss in tile drainage for a mixed agricultural catchment (120 ha) in the UK. Phosphorus concentrations in drain discharge were low (<100 mu g Total P l(-1)) and stable during base-flow periods (<0.5 l min(-1)), and generally lower than in the receiving stream. In contrast, temporary (hours) elevated P peaks exceeding 1 mg Total P l(-1) were measured in drain-flow during high discharge periods (>10 l min(-1).) Large sediment-associated particulate P losses were measured during the first major drain-flow events of the autumn. Field drains are evidently effective conduits for P export from agricultural catchments. Recommendations for controlling P loss from diffuse agricultural sources are therefore critically dependent on a better understanding of surface and subsurface transport pathways. (C) 1999 IAWQ Published by Elsevier Science Ltd. All rights reserved.