Although single leaf level analysis can provide many insights into plant adaptation to the environment, integration with canopy level processes is essential to analysing crop and predicting community productivity (Norman, 1980). To evaluate the significance of changes inferred at the leaf level to the canopy level, WIMOVAC offers three separate models of canopy microclimate.

1). A simplified model which treats the canopy as two populations of leaves, sunlit and shaded (Forseth and Norman, 1991). By dynamically calculating the leaf areas of sunlit and shaded leaves and the mean irradiance of these two populations, the mean assimilation rate for each leaf population and consequently the total canopy photosynthesis can be obtained. The division of leaves into sunlit and shaded classes was shown by Norman (1980) to provide a substantial improvement in prediction over models which simply assumed an exponential decline in light through homogeneously lit canopy layers. Detailed analysis of Norman's equations and the parameters used by WIMOVAC are as given in Long. (1991) and Long & Drake. (1992).

2).The need for layering of diffuse light absorption, however, was documented by Reynolds et al. (1992) with a suggested error of up to 15% ascribed to the single layer model proposed by Norman (1980). WIMOVAC provides a multiple layered approach to calculating the direct and diffuse components of the light microclimate. The canopy is considered as 2-n discrete layers, each layer containing an equal fraction of the total canopy leaf area index (LAI) where n is selectable by the user. Within each layer WIMOVAC calculates the proportion of sunlit and shaded leaves and the direct and diffuse radiation within the layer. In addition to this WIMOVAC calculates the leaf temperature and vapour pressure deficit (VPD) according to Monteith (1973), leaf nitrogen concentration (either manually set or optimised), stomatal conductance according to Ball et al. (1987), wind speed according to Reynolds (1992), and transpiration according to Monteith (1973) for each of the leaf classes (figure 2).

3). In agricultural and planted forest systems, plants are commonly in rows. The preceding model options assume a random distribution of foliar elements. Before canopy closure this assumption can lead to serious overestimation of light interception in row crops where spacing and orientation have an important influence on the light microclimate (Boote and Pickering, 1994; Boote et al., 1989). Within WIMOVAC row spacing, width and orientation may be specified, and light microclimate predicted following the equations of Boote and Pickering (1994) to in turn allow calculation of canopy photosynthesis.

In order to facilitate investigation of elevated Ca and concomitant temperature effects on possible canopy water usage, an expression has been introduced into the canopy models for the instantaneous transpiration of water vapour from the canopy, according to Penman (1948) and Monteith (1965 and 1973). This expression has been combined with a boundary layer conductance model which describes the transfer of water vapour from the evaporating surface to the bulk air stream in terms of the aerodynamics of the turbulent air above the canopy (Campbell, 1977; Thornley and Johnson, 1990). Transpiration rates at both the sunlit and shaded leaves within the canopy are considered according to the appropriate light and temperature microclimate conditions within the canopy, through the effects of radiation both on stomatal conductance via photosynthesis, and on leaf temperature. A derivation of the Penman (1948) and Monteith (1965 and 1973) equation in which transpiration is eliminated, is used to predict the difference between canopy leaf temperature and the ambient air temperature outside the canopy. Default parameter settings for both the transpiration and leaf temperature modules were as for Campbell (1977). The expression relating apparent sink momentum to canopy height, given by Campbell (1977), however was corrected here and has the form, d=0.77h where d is the apparent sink momentum and h is the canopy height in metres. Leaf transpiration and leaf temperature are not independent quantities, and so an iterative procedure is used here to establish their respective equilibrium values.

WIMOVAC contains a database of standard soil types and the ability to input characteristics of other ‘user defined’ soils. The soil database contains a description of the soils appearance, volumetric field holding capacity, volumetric wilting point and critical threshold value. The field holding capacity is taken to be the maximum amount of water that a given soil is able to hold before run-off occurs. The wilting point is taken to be the soil water content at which plants growing on the soil are unable to abstract further water. The critical threshold value is the soil water content at which soil dry down processes due to plant uptake within the model are switched. The model assumes that soil dry down results from run off and evaporation of water at the soil surface, percolation to lower layers and uptake by the plant canopy. If the soil water content is greater than the critical threshold value the canopy uptake of water is assumed to equal the canopy potential transpiration rate, assuming no stomatal resistance to leaf water loss. If the soil water content is less than the critical threshold value and greater than the wilting point value canopy water uptake is assumed to equal the actual canopy transpiration rate, which is limited by stomatal resistance. At a soil water content less than the wilting point value the canopy is assumed to be unable to extract further water from the soil. A multiple layered approach to soil dry down is adopted here according to Johnson (1993).