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WP3 - Hydrological modelling (water flow + nitrate)

The modelling systems that will be applied are shown in the figure below. The baseline catchment modelling will be based on DAISY/MIKE SHE/MIKE11. This is the approach presently used for nitrate modelling in Denmark. The novelty of NICA as compared to previous studies lies in the four 'model learning processes' shown in the figure below:

  • Data on geology. With the data from field work and the new geophysical tools, MiniSkyTEM and MRS, we will have a much better description of the geological heterogeneity. The uncertainty related to the geological interpretations will be assessed by stochastically generating multiple plausible geological models by use of TProGS.
  • N leaching from root zone. A comparison will be made between three alternative tools: (a) a fully coupled DAISY-MIKE SHE code developed by use of OpenMI techniques and made operational as part of the NICA project; (b) the standard use of DAISY and MIKE SHE as two separate codes used sequentially without feedback from the groundwater table in MIKE SHE to the lower boundary condition (groundwater table) in DAISY; and (c) a simpler method, N-balance, where the annual N leaching is based on existing agricultural management data reported from farmers to the authorities and the annual values subsequently converted to daily concentrations by use of water balances from MIKE SHE. Based on the intercomparison, recommendations will be made for large scale operational use with different levels of required accuracy.
  • Redox interface. Specifically targeted field investigations (see above) will support the assessment of the location of the redox interface. In addition the HydroGeoSphere code will be used to study the hypothesis that some of the heterogeneity of the redox zone can be explained by the variability in flow pattern caused by irregular fractures. HydroGeoSphere is a new, very ambitious model code for integrated surface water/groundwater simulations at catchment scales. Compared to MIKE SHE, HydroGeoSphere has a flexible model grid, more sophisticated efficient numerical algorithms, a full 3D description of unsaturated-saturated subsurface flow, and options for including fractures in the flow description. It is the only one among many new codes that have emerged during the past 15 years that appear to become a real challenge to MIKE SHE as the most generic operational model code for integrated catchment modelling.
  • Flow paths in catchment. The baseline transport simulation of nitrate at catchment scale will be done by the traditional MIKE SHE/MIKE11 advection-dispersion algorithm. In addition the HydroGeoSphere and the efficient particle tracking model code RWHET will be tested and the best solution used in the further study.
WP leader: Jens Christian Refsgaard

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