Agricultural ditch remediation strategies - Integrated hydrological and ecological methods for decision making

Agricultural drainage ditches are critical for the removal of excess water from fields. Their traditional trapezoidal shape is effective for this purpose, while also minimizing their footprint and being easy to maintain. However, these drainage ditches also act as transport pathways for phosphorus and nitrogen. Moreover, their steep banks are susceptible to erosion during high flows, which can be a source of additional sediment and phosphorous mobilisation in river systems.  Within Sweden, 60% of water bodies are classified as having poor chemical and ecological status, with diffuse agricultural pollution and hydromorphological pressures being key drivers. Further, the European Green Deal has the ambition to reduce nutrient losses from agricultural catchments by 50%. Therefore, mitigation strategies in agricultural catchments are urgently needed. Ditch remediation, through the construction of two-stage or shallow slope ditches, has been proposed as a solution to reduce nutrient exports and hydromorphological pressures, while maintaining good drainage. Numerous ditch remediation actions have taken place within Sweden, with encouragement and funding from agricultural agencies. However, we currently lack an understanding of the factors controlling the effectiveness of ditch remediation strategies. Further, as aquatic ecosystems provide services beyond simple water conveyance, we are also limited in our understanding of benefits/trade-offs that may occur to these ecosystems as a result of remediation. Therefore, through the synoptic sampling of traditional and remediated ditches, we will analyse and compare channel properties, stream chemistry and macroinvertebrate communities to assess the effect of ditch remediation on both pollution reduction and ecosystems. The obtained data will then be used to model the effectiveness of various remediation strategies. This unique integration of hydrological and ecological methods will increase our understanding of ditch remediation and ultimately support farmers, landowners, and authorities in the design of cost-effective mitigation measures.