The VSD (Very Simple Dynamic) model is a single layer soil model designed as a simple extension of the Simple Mass Balance (SMB) model for critical loads. It consists of a set of mass balance equations describing the soil input and output relationships, and equations describing the rate-limited and equilibrium soil processes. The soil solution chemistry depends on the net element input from the atmosphere and the geochemical interaction in the soil. Soil interactions are described by simple rate-limited reactions or by equilibrium reactions.
The VSD runs on yearly time-steps. The model requires the same input data as used for critical load calculations plus:
- Carbon pool in topsoil
- Bulk density
- Concentration of organic acids
- Equilibrium and exchange constants
- Cation exchange capacity
- Base saturation
- C/N ratio in toposoil
- Hindcast, present day and future scenarios of sulphur and nitrogen deposition.
A full description of the model can be found in Posch & Reinds (2009). A single site version of the model (VSDStudio) can be downloaded from the CCE website.