Projects Nordbalt-Ecosafe CatchManSS Odense Fjord Catchment

Odense Fjord Catchment

The 1061 km² Odense Fjord Catchment is located on the Island of Funen in Denmark.

  • Geomorphology is characterized by younger clayey moraines from the Weichsel glaciation.
  • Dominated by agricultural land use (68% of the area).
  • Approximately 80% of the agricultural land is tile-drained.
  • Urban areas (City of Odense) and forests cover 16% and 10% of the catchment area, respectively.
  • The average annual precipitation is 825 mm, and the mean temperature is 8.4 °C.

Baseline model

Model setup

Code/software versions used

Code

Version number

Availability

QGIS

3.22.16

QGIS used as a basis for running the QSWAT+ plugin. In this project, the latest stable release was used, which is the version that QSWAT+ aims to be compatible with. This can be downloaded from:https://qgis.org/downloads

SWAT+

(core model)

2025.61.0.2.11-104-gfcb8738

The SWAT+ Fortran code is version controlled through bitbucket. Official code releases are available here: https://bitbucket.org/blacklandgrasslandmodels/modular_swatplus/src/master/

QSWAT+ (interface)

2.4.1

Code and official installer releases are available here: https://bitbucket.org/ChrisWGeorge/qswatplus3/downloads

SWAT+ Editor (interface)

2.3.1

Code and official installer releases are available here: https://bitbucket.org/swatplus/swatplus.editor/downloads/

SWATdoctR (model setup verification)

1.0

R package for SWAT+ model calibration and model diagnostics.

https://git.ufz.de/schuerz/swatdoctr

SWATrunR calibration tool

0.1.3>

R-package developed for hard calibration of the SWAT+ model.

https://chrisschuerz.github.io/SWATrunR/

Catchment delineation

Channel threshold: 3 km2
Stream threshold: 3 km2
Upslope/Floodplain LSUs: Branch Length Method with a Slope Position Threshold of 0.1
Lakes: Shapefile with lakes > 5 ha

To include areas that drain directly to coastal waters in the model setup, the landscape units delineated by QSWAT+ based on the DEM and stream network were edited manually in QGIS and subsequently loaded into the interface as a pre-defined catchment.

The delineated floodplains were compared to the river valley bottom map by Sechu et al. (2021) and a map of lowland soils in Denmark.

Some lakes were not located on the stream network and thus excluded from the setup by QSWAT+.

HRU creation

Land use: Customized land use map created by combining the general DK land use map and a field boundary map from 2020
Soil: National Topsoil Texture Map
Slope classes: 0-9999%
HRU filtering: none

Final configuration

Total watershed area: 1.053 km2
Subbasins and channels: 213
LSUs: 420 (including both upslope and floodplain LSUs)
Number of lakes: 8

Point sources

Recall files with flow data were added for the 10 largest wastewater treatment plants. Each point source was connected to its nearest channel.

Other changes to the model setup and parameterization

Some of the default values used by the SWAT+ Editor did not reflect the specific characteristics of the Odense Fjord Catchment, so several input files were edited manually before model calibration:

  • Subsurface tile drains were implemented in all agricultural HRUs with a mean slope of less than 5%, which resulted in approximately 80% of the agricultural land in the catchment being tile-drained.
  • The values of three parameters that control the runoff and leaching potential of the HRUs (perco, cn3_swf, latq_co) were edited based on recommendations from the model development team.  
  • Management schedules were implemented to define agricultural management operations (sowing, harvest, fertilizer and manure applications, and tillage) for the different crop rotations implemented in the model.
  • An outlet object was added to the model setup, which summarizes the discharge and nutrient loads from all streams draining into Odense Fjord.

More information about the model setup can be found here: https://projects.au.dk/nordbalt-ecosafe/deliverables

Calibration and validation

The calibration and validation of daily discharge and total nitrogen was performed at three gauges for streamflow and one gauge for total nitrogen: Odense Å at Kratholm (discharge and total nitrogen), Stavis Å (discharge), and Lindved Å (discharge).

  • Model warm-up: 1/1/2008 – 31/12/2010
  • Calibration: 1/1/2011 – 31/12/2016  
  • Validation: 1/1/2017 – 31/12/2022

Model evaluation statistics for daily discharge during the calibration (Cal) and validation (Val) periods

Gauge KGE Pbias NSE
Cal Val Cal Val Cal Val
Odense Å at Kratholm 0.77 0.99 -15.7 -6.7 0.81 0.86
Stavis Å 0.83 0.77 11.1 17.7 0.73 0.69
Lindved 0.86 0.74 -4.2 11.9 0.73 0.71
Observed and simulated discharge for Odense Å at Kratholm, Lindved Å and Stavis Å during the calibration period.
Observed and simulated discharge for Odense Å at Kratholm, Lindved Å and Stavis Å during the validation period.

Model evaluation statistics for daily nitrogen loads during the calibration (Cal) and validation (Val) periods

Gauge KGE Pbias NSE
Cal Val Cal Val Cal Val
Odense Å at Kratholm 0.75 0.90 -14.9 -4.1 0.58 0.83
Observed and simulated nitrogen loads for Odense Å at Kratholm during the calibration period.
Observed and simulated nitrogen loads for Odense Å at Kratholm during the simulation period.

More information about the model setup can be found here: https://projects.au.dk/nordbalt-ecosafe/deliverables

Critical source areas

Spatial distribution of nitrate nitrogen (NO3–N, kg/ha) losses across the Odense Fjord Catchment through surface runoff, lateral flow, and tile flow. The color scale represents NO3–N loading rates, ranging from 0-0.01 kg/ha (light beige) to >97.6 kg/ha (dark red). Lighter shades indicate areas with minimal nitrogen loss, while darker shades highlight critical source areas with elevated NO3–N export. Blue lines represent the stream network.
Spatial distribution of nitrate nitrogen (NO3–N, kg/ha) groundwater flow across the Odense Fjord Catchment. The color scale represents NO3–N loading rates, ranging from 0.05 kg/ha (light beige) to 6.34 kg/ha (dark red). Lighter shades indicate areas with minimal nitrogen transport, while darker shades highlight critical source areas with elevated NO3–N transport. Blue lines represent the stream network.

Scenario results

Buffer Strips

Spatial distribution of floodplain and upslope within the Odense Fjord Catchment. The buffer strip scenario was applied throughout the entire agricultural landscape.
Time series of simulated total nitrogen (Total N, mg/L) concentrations in the Odense Fjord Catchment under baseline conditions (black line) and with buffer strip implementation (red dashed line). The blue dashed line represents the water quality threshold (2.3 kg/day), indicating the target limit for nitrogen loading.
Time series of simulated total nitrogen (Total N, mg/L) loads in the Odense Fjord Catchment under baseline conditions (black line) and with buffer strip implementation (red dashed line).

Fertilizer reduction

Spatial distribution of floodplain and upslope within the Odense Fjord Catchment. The fertilizer reduction scenario was applied throughout the entire agricultural landscape.
Time series of simulated total nitrogen (Total N, mg/L) concentrations in the Odense Fjord Catchment under baseline conditions (black line) and with fertilizer reduction implementation (red dashed line). The blue dashed line represents the water quality threshold (2.3 kg/day), indicating the target limit for nitrogen loading.
Time series of simulated total nitrogen (Total N, mg/L) loads in the Odense Fjord Catchment under baseline conditions (black line) and with fertilizer reduction implementation (red dashed line).

Catch crops

Spatial distribution of agricultural land in the Odense Fjord Catchment that receives catch crops. Baseline (green) shows the areas where a catch crop already exists in the rotation. The added catch crop (pink) is the new area that received a catch crop during the rotation. The “baseline + added catch crops” is representative of areas that had catch crops implemented during the baseline and received additional catch crops during the rotation years.
Time series of simulated total nitrogen (Total N, mg/L) concentrations in the Odense Fjord Catchment under baseline conditions (black line) and with catch crops implementation (red dashed line). The blue dashed line represents the water quality threshold (2.3 kg/day), indicating the target limit for nitrogen loading.
Time series of simulated total nitrogen (Total N, mg/L) loads in the Odense Fjord Catchment under baseline conditions (black line) and with catch crop implementation (red dashed line).

Wetland restoration

Spatial distribution of the floodplain within the Odense Fjord Catchment. The wetland restoration scenario was applied throughout the entire floodplain.
Time series of simulated total nitrogen (Total N, mg/L) concentrations in the Odense Fjord Catchment under baseline conditions (black line) and with wetland restoration(red dashed line). The blue dashed line represents the water quality threshold (2.3 kg/day), indicating the target limit for nitrogen loading.
Time series of simulated total nitrogen (Total N, mg/L) loads in the Odense Fjord Catchment under baseline conditions (black line) and with wetland restoration (red dashed line).
Revised 28.08.2025
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Else Vihlborg Staalsen