Introducing Life Cycle Assessment in Costs and Benefits Analysis of Vegetation Management in Drainage Canals of Lowland Agricultural Landscapes

Looking back to move forward: Restoring vegetated canals to meet missing Water Framework Directive goals in agricultural basins

Nitrate pollution and eutrophication remain pressing issues in Europe regarding the quality of aquatic ecosystems and the safety of drinking water. Achieving water quality goals under the Water Framework Directive (WFD) has proven to be particularly challenging in agricultural catchments, where high nitrate concentrations are the main reason for the failure of many water bodies to meet a good ecological status. Canals and ditches are common man-made features of irrigated and drained landscapes and, when vegetated, have recently been identified as denitrification hotspots. By combining experimental data and GIS-based upscaling estimation, the potential capacity of the canal network to reduce nitrate loads was quantified in several scenarios differing in the level of nitrate pollution and in the extent of the canal network length where conservative management practices are implemented. The analysis was carried out in the irrigated lowlands of the Po River basin, which is the largest hydrographic system in Italy and a global hotspot for nitrogen inputs and eutrophication. Scenario simulations showed that maintaining aquatic vegetation in at least 25 % of the canal network length, selecting sites with high nitrate availability (>2.4 mg N L-1), would promote a greater potential for permanent N removal. The increased denitrification capacity would meet the load reduction target required to achieve a WFD good ecological status in waters draining into the Adriatic Sea during the spring-summer months, when the eutrophication risk is higher. Promoting denitrification in the canal network by postponing the mowing of in-stream vegetation to the end of the growing season could be an effective mitigation strategy to improve water quality in agricultural basins and contribute to achieving the WFD goals.

The achievement of Water Framework Directive goals through the restoration of vegetation in agricultural canals

Decreasing nitrate concentrations is one of the most relevant Water Framework Directive (WFD) goals, which today is still unreached in several European countries. Vegetated canals have been recognized as effective filters to mitigate nitrate pollution, although rarely included in restoration programs aimed at improving water quality in agricultural watersheds. The Po di Volano basin (713 km², Northern Italy) is a deltaic territory crossed by an extensive network of agricultural canals (~1300 km). The effectiveness in buffering nitrate loads via denitrification was assessed for different levels of in-stream emergent vegetation maintenance by employing an upscale model based on extensive datasets of field measurements. The scenarios differed for the canal network length (5%, 20%, 40%, and 60%) where conservative management practices were adopted by postponing the mowing operations from the middle of summer, as nowadays, to the early autumn, i.e., the vegetative season end. The scenario simulations demonstrated that the capacity to mitigate diffuse nitrate pollution would increase up to four times, compared to the current condition (5% scenario), by postponing the vegetation mowing to the end of the vegetative season in 60% of the canal network length. By preserving the in-stream vegetation in 20% of the canal network, its denitrification capacity would equal the nitrate load reduction target required for achieving, from May to September, the good ecological status according to the WFD in waters delivered to the coastal areas. Changing the timing of vegetation mowing may create a large potential for permanent nitrate removal via denitrification in agricultural landscapes, thus protecting the coastal areas when the eutrophication risk is higher. Conservative management practices of in-stream vegetation might be promoted as an effective low-cost tool to be included in the WFD implementation strategies.