The soil profile is divided into a number of layers, and for each layer and each boundary between layers, the two basic principles are considered. The number of layers and the thickness of each layer can be varied depending on accuracy requirements.
The calculations of water and heat flows are based on soil properties such as:
• the water retention curve
• functions for unsaturated and saturated hydraulic conductivity
• the heat capacity including the latent heat at thawing/melting
• functions for the thermal conductivity
The most important plant properties are:
• development of vertical root distributions
• the surface resistance for water flow between plant and atmosphere during periods with a non limiting water storage in the soil
• how the plants regulate water uptake from the soil and transpiration when stress occurs
• how the plant cover influences both aerodynamic conditions in the atmosphere and the radiation balance at the soil surface.
• how different plant canopies cover each other in space and therefore compete for radiation
If the nitrogen and carbon cycles are included in the model, the following soil and plant properties are of major importance:
• characteristics gowerning the plant life-cycle such as allocation patterns of assimilates and nitrogen
• plant activities such as assimilation, respiration and nutrient uptake
• external inputs of carbon and nitrogen to the soil
• microbial activity i.e. decomposition
• redistribution between different decomposition products such as humus or litter in the whole soil profile
All properties are represented as parameter values. Numerical values are assigned to a number of different parameters representing properties of the soil-plant-atmosphere system. For each parameter a certain range reflects differences between different types of crops, forests, soils or the range reflects a typical variation found within a certain area.
Meteorological data are the driving variables to the model, but in contrast to parameters the numerical values of driving variables vary with time.
The driving variables govern the flows at the boundaries between atmosphere and soil and between plant and atmosphere. Precipitation and air temperature are the most important driving variables, but air humidity, wind speed and cloudiness are also of great interest due to their influence on evaporation.
The required information on soil properties is large compared to what is normally available from standard field investigations. To determine these properties by independent measurements in each application with the model would be time-consuming and very labour intensive, especially since some of these properties (e.g. hydraulic conductivity) show substantial spatial heterogeneity. The use of the database enables the user to estimate a reasonable range for such soil properties from commonly available information such as soil texture and organic matter content. Most of the material in the database originates from investigations in arable land in Sweden but the material is continuously updated with new sites including forest soils.