An agricultural drainage channel classification system for phosphorus management

• Surface channels were classified into fine sediment retention/transfer classes.
• Ditches with low slopes are likely to primarily retain sediment.
• Ditches with high slopes are likely to mobilise sediment during storms.
• Tailoring management according to channel classes may reduce P transfers downstream.
• Ditches were over-engineered with potentially positive effects on P transfer.

In agricultural landscapes, surface ditches and streams can significantly influence the attenuation and transfer of phosphorus (P) from upstream sources to receiving water-bodies. The magnitude of P attenuation and/or transfer within channels can vary considerably according to fine sediment retention and/or transfer processes. Fine sediment retention and/or transfer processes can, in turn, vary considerably according to channel physical characteristics. An understanding of channel physical characteristics, their effect on fine sediment retention/transfer and their spatial distribution, could be used to develop channel-specific management strategies for the reduction of downstream P transfers. Using a detailed field survey of surface channel networks in a well-drained arable and a poorly-drained grassland catchment, this study (i) characterised the surface channels in both catchments, (ii) classified the channels into four classes of fine sediment retention and/or transfer likelihood based on a comparison of physical characteristics (slope and drainage area) with observations of fine sediment accumulation and (iii) considered P management strategies that are suited to each class. Results of the survey demonstrated that ditch dimensions were not closely related to their indicative flow volumes and were over-engineered, which likely reduces downstream P transfer. Net attenuation of fine sediment and associated P was predicted for 40% of the total channel length in the grassland catchment, compared to 13% of the total channel length in the arable catchment. Net transfer of fine sediment and associated P was predicted for 24% of the total channel length in the grassland catchment compared to 58% of the total channel length in the arable catchment. For eutrophication management in headwaters, periodic removal of fine sediment and maintenance of channel bank vegetation in net attenuating and transferring channels respectively would help to minimise P transfers from these catchments.