The vegetation properties are used to calculate the Actual evapotranspiration from the Reference evapotranspiration defined under Climate.
The primary vegetation properties are Leaf Area Index (LAI) and Root Depth (RD). The LAI and Root Depth can be specified directly as a time series. Or, they can be defined as a crop rotation in the Vegetation Properties Editor (V1 p. 403).
A good source of local information on LAI and root depth is the agronomy department at your local university.
The LAI is defined as the area of leaves per area of ground surface. The LAI values are characteristic of the plant type, season, and plant stress. LAI values are widely available in the literature for most major plant types.
The LAI is a lumped parameter for a cell that defines the average leaf area of the cell. In forests, it includes both the leaf area of the forest canopy and the understory. In more open areas, it is an average for all vegetation types, such as grass, brush and trees. In areas of largely open water the LAI is usually zero. If the LAI is zero, there will be no interception storage and no water will be removed from the unsaturated zone.
Root depth is defined as the depth below ground in millimetres to which roots extend. The root depth is not necessarily the average root depth. In some cases it may be the maximum root depth. The root depth defines the depth at which water can be extracted from the unsaturated zone. If the root depth is deeper than the depth of the capillary zone, then the roots will be able to extract water from the saturated zone.
The thickness of the capillary zone is defined by the soils function in the soil properties for the Richards and Gravity flow methods. In the 2Layer UZ method, the thickness of the capillary zone is defined by the ET Surface Depth (V1 p. 296).
If you are using the Richards or Gravity Flow UZ methods, then you will also be able to use the Root Shape factor (AROOT) for each vegetation type. This allows you, for example, to extract more water from the upper UZ cells than the lower cells, which is typical of grasses in semi-arid climate zones.