The model, initially developed to simulate conditions in forest soils, has recently been generalised to elucidate water and heat processes in any soil independent of plant cover. This was possible since the model is based on well-known physical equations. The fundamental nature of these physical equations allows the model to be adapted to many different types of ecosystems providing that we have quantitative knowledge of the governing properties of these systems. Recently nitrogen and carbon cycles have also been included in the model. This has enabled a dynamic interaction between the abiotic environment and the plant, and subsequently plant growth can be simulated. It is possible to include several plants that compete for water, nitrogen and radiation.
The basic structure of the model is a depth profile of the soil. Processes such as snow-melt, interception of precipitation and evapotranspiration are examples of important interfaces between soil and atmosphere. Two coupled differential equations for water and heat flow represent the central part of the model. These equations are solved with an explicit numerical method. The basic assumptions behind these equations are very simple.
1) The law of conservation of mass and energy
2) Flows occur as a result of gradients in water potential (Darcy’s Law) or temperature (Fourier’s law).