Terrestrial Vegetation Model (TVM)

The Terrestrial Vegetation Model (TVM) simulates the potential distribution of natural vegetation and crops on the basis of climate conditions and soil characteristics on a spatial resolution of 0.5 degree latitude by 0.5 degree longitude.The TVM, part of the Terrestrial Environment System (TES), is driven by CO2 concentration, as calculated by the Atmospheric Chemistry Model (ACM) and (changes in) climate, as simulated by the climate model. The output of the TVM is one of the inputs to the Land-Cover Model (LCM).

Model input	Temperature
	Precipitation
	Soil
	CO2 concentration
	Crop requirements related climate conditions
	Harvest index (crop-specific)
Model output	Length and temperature of growing season
	Moisture availability
	Potential vegetation
	Potential productivity of crops
	Reduced potential productivity of crops

TVM consists of three components:

1. The first component calculates relevant climate indices. These indices integrate important aspects of climate, such as frost occurrence and severity, characteristics of the growing season and moisture availability. The actual and potential evapotranspiration (AET and PET, respectively) as simulated by the climate model are calculated using a simple water bucket model. Subsequently, the growing season is defined as the period with adequate temperature (> 5°C) and sufficient moisture (precipitation > 0.5*PET). Finally, the mean temperature is calculated over the growing season.
2. The second component, a modified version of the BIOME model (Prentice at al., 1992), computes potential vegetation, which is used in the land-cover model. The climate indices from the first model component are used to determine the potential distribution of major plant types (e.g. needle-leafed vs. broad-leafed, deciduous vs. evergreen, trees vs. shrubs and grasses). The plant types are then combined into biomes (large-scale vegetation complexes).
3. The third component, the FAO Agro-ecological Zones approach (FAO, 1981), determines the potential distribution of different crops. If the length of the growing season is adequate for the crop group considered, a simple photosynthesis/respiration model is used to estimate potential crop productivity. This climate-related 'constraint-free rain-fed crop yield', accounts for light attenuation through the crop canopy using local climate. The constraint-free rain-fed crop yield is adjusted for grid-specific soil conditions using a 'soil reduction factor' (ranging from 0.1 to 1.0). This factor is based on the methodology defined by the land evaluation computer system (LECS) (Wood and Dent, 1983) using the FAO soil map of the world (FAO, 1991). This reduction factor takes into account three soil-quality indicators: (i) nutrient retention and availability; (ii) level of salinity, alkalinity and toxicity; (iii) and rooting conditions for plants (related to soil texture). These indicators are assumed to be constant in time. The resulting crop productivity, called 'reduced potential productivity of crops', is used in the land-cover model.

For a detailed description of the terrestrial vegetation model please refer to Leemans and van den Born (1994) and Alcamo et al. (1998).