SMHI develops and maintains a model system for water quality calculations in coastal zone waters around Sweden. It is called the Swedish Coastal zone Model (SCM) and has previously been presented in Sahlberg (2009). Since that report was published the model has been further developed and it is now also used in scientific research. This now calls for an updated report.
The SCM is a coupled 1-dimensional physical and biogeochemical model. The model calculates the vertical profiles of all its variables and assumes that they are horizontally homogeneous in the studied area. In order to resolve horizontal variations, a region is divided into several smaller sub-regions, called basins, connected by sounds. Through these sound connections both water and mass of different constituents are exchanged. The basins in SCM are identical to the national water bodies defined in accordance with the Water Framework Directive (WFD). The vertical resolution is half a metre in the uppermost layers, one metre in the 4-70 m interval, and two metres between 70-100 m. Below 100 m the layer thickness increases to 5 m and to 10 m below 250 m.
The physical part of SCM consists of the equation solver Program for Boundary Layers in the Environment (PROBE, Svensson (1998)), but also several subroutines which calculates, e.g., insolation, ice-cover, and the exchanges between basins. The exchanges that connect the modelled basins are assumed to be governed by baroclinic and barotropic pressure gradient between the coupled basins.
The biogeochemical model is the Swedish Coastal Ocean BIogeochemical model (SCOBI, Marmefelt et al. (2000)). SCOBI is a process-oriented model that includes marine nitrogen, phosphorous and oxygen dynamics, as well as a simple representation of plankton dynamics typical for the Baltic Sea. It calculates 11 variables: zooplankton, three functional phytoplankton groups, detritus, nitrate, ammonium, phosphate, oxygen, benthic nitrogen and benthic phosphorus. SCOBI uses the O2 variable to also, indirectly, model H2S. H2S is represented as a negative oxygen concentration, i.e. the oxygen needed to oxidize a certain accumulated H2S concentration, which can also be considered as an oxygen debt.
The mixing and advection of the nine pelagic biogeochemical variables are calculated by PROBE, while SCOBI calculates the process rates which decide how matter is exchanged between the 11 biogeochemical variables, and also the vertical transfers between the SCM’s grid cells due to the sinking of phytoplankton and detritus, i.e. sedimentation.
SCM needs input data from the atmosphere (weather variables and deposition on nitrogen and phosphorus), from land (land run-off and point sources, e.g. sewage treatment plant and industries) and also from the open ocean.
The model is part of the Swedish water management, but it is also used within research project which results in peer reviewed scientific publications.