Despite substantial efforts, the leaching of nutrients from agricultural land is still a serious and costly environmental problem in Denmark and elsewhere. The quality goals of the European Water Framework Directive (WFD) for the aquatic environment require a substantial reduction of diffuse nutrient loads from farmland in Denmark. Tile drains connecting fields to receiving waters thus acting as subsurface highways for nitrogen (N) and phosphorus (P) as well as other agricultural contaminants. Drainage losses of nutrients contribute to estimated 45-60% of total N losses, and 33% of total P losses in Denmark. Hence, for a large number of recipients, drainage water nutrient loads have a major impact on water quality, and measures targeting agricultural drainage losses may potentially be more cost-efficient measures compared to traditional agricultural measures. This calls for a paradigm shift in the environmental regulation to mitigate site-specific nutrient losses in agricultural drainage water.

The Danish research and innovation project “iDRAIN” (2011-2017) (www.idrain.dk) aims at improving the understanding on nutrient (N and P) fluxes and transformations at subcatchment scale to facilitate a more targeted cost-efficient environmental regulation. The subcatchment study site (Fensholt, 6 km²) is located with the larger Norsminde Fjord catchment (100 km²) on the east coast of Jutland, Denmark. The Fensholt subcatchment is characterized by hilly terrains and with riparian lowland bordering the main stream which discharges into Norsminde Fjord. The subcatchment is moraine clay xxxxxxx. The major land-use is agriculture (78%) and the subcatchment is heavily tile-drained. To monitor the nutrient fluxes the subcatchment was instrumented with a number of drainage stations, stream stations and shallow piezometers to continuously monitor drainage- and stream discharge, nutrient (N, P) transport and water table dynamics. Based on four years of continuous monitoring (2012-2016) of nutrient fluxes and transformations, a scenario analysis was conducted to compare a targeted mitigation strategy with general measures to reduce agricultural nutrient losses to environmental thresholds.