Impacts of the transition to a Nordic bioeconomy on streamflow and nitrogen loads in the Odense Fjord Catchment, Denmark

Downscaling and extending the global Socioeconomic Shared Pathways (SSPs) into a set of storylines focusing on the Nordic land-based bioeconomy (Nordic Bioeconomy Pathways (NBPs). In short, the NBPs stand for sustainability first (NBP1), conventional first (NBP2), self-sufficiency first (NBP3), city first (NBP4) and economy first (NBP5). Each of the five NBPs includes a set of linked agricultural and forestry attributes that provide a framework for the BIOWATER researchers for generating input data to catchment models by translating qualitative narratives into quantitative values by means of stakeholder workshops.

A societal transformation towards a bioeconomy in the Nordic countries will have extensive implications for the environment and might conflict with the goal of the European WFD to achieve good ecological status of the majority of European water bodies. This study aims to explore the environmental impact of different bioeconomy scenarios combined with climate change on a Danish estuary, the Odense Fjord. We used the Nordic Bioeconomy Pathways (NBPs), which describe five possible future scenarios for a Nordic bioeconomy in 2050, to identify plausible changes in land use in response to the transition. The catchment of the Odense Fjord is intensively farmed, so the attributes selected for this study included changes in farming intensity (chemical fertilizer and manure amount), land cover change (agriculture vs. forest), and nutrient loss mitigation (buffer strips and wetlands).

We used the catchment model SWAT to model the hydrology and nitrogen (N) dynamics in the intensively farmed River Odense catchment. The water monitoring at the river outlet is the most comprehensive in Denmark, and daily data recordings of N concentrations are available for the baseline period 2001-2008. The SWAT model setup for the River Odense catchment includes 23 sub-catchments and 3,882 Hydrological Response Units (HRUs). Scenarios for the baseline and four selected agricultural attributes with and without climate change (RCP4.5 and RCP8.5) were simulated for the period 2041-2070.

The NBP narratives were translated to quantitative values that can be modelled at catchment scale by local stakeholders. The semi-distributed Soil and Water Assessment Tool (SWAT) was used to simulate the land use and climate scenarios. First, extreme values of each attribute were simulated to ensure plausibility of the model response to the changes. Subsequently, the combined effects of all changes were quantified for each NBP with and without climate change. The differences in simulated streamflow between the five NBPs were very small, whereas the impact of the different pathways on the simulated nitrogen loads was more pronounced, especially during the winter months. In both climate change scenarios using the median of an ensemble of climate models conducted for the period 2041-2070, the average annual total N loads from the River Odense catchment decrease slightly in both RCP 4.5 and RCP 8.5. The NBP scenarios showed that the needed reductions in total nitrogen loads to the Odense Fjord for obtaining a good ecological quality could only be reached when following the trajectory of NBP1 being sustainable first scenario and including restoration of all previously drained wetlands in the catchment.