Simulation models have been constructed at three scales; drop, leaf and canopy scales.

Drop evaporation models have been produced by at least three research groups (Leclerc et al. 1985, Butler, 1985, Barr and Gillespie 1987, Butler 1990, Zhang and Gillespie 1990, Hoppmann and Wittisch 1997). The main problem with drop scale models is the complex geometry of a drying drop. Evaporation of water from a droplet depends on its size and shape (Butler 1985). The shape of droplets is largely determined by the wetting angle of the leaf (Leclerc et al. 1985). The most obvious shape is a hemisphere (Leclerc et al. 1985) but other models have used oblate-hemispheres (Barr and Gillespie 1987) and truncated oblate hemispheroids (Butler, 1985) or a combination of shapes (Butler 1990). Droplet shape may also change as it evaporates. Droplets that are initially hemispherical in shape maintain a constant base area as they dry (Leclerc et al. 1985). In general, the complexity of drop models means that a simpler approach is more desirable.

Pedro and Gillespie (1982a,b) present a model for calculating dew duration on individual corn and apple leaves using: i) micro-meteorological data; and ii) standard weather station data. The micro-meteorological version of the model had an accuracy of half an hour for shaded leaves and an hour for unshaded leaves. Using standard weather station data the model estimates were half an hour less accurate for both leaf types. The model has been adapted for other crops and situations (Gillespie and Barr 1984, Weihong and Goudriaan 1991, Scherm and van Bruggen 1993, and Giesler et al. 1996).

Several models have been built for fruiting bodies. Fruits, because of their greater heat capacity, accumulate and lose dew more slowly than leaves. They also have different water holding capacities and other physical properties from leaves. Models have been developed for cocoa pods (Monteith and Butler 1979, and Butler 1980) grape clusters (Huber et al. 1991) and sunflowers (Payen 1983).

In practice, it is more convenient to consider a whole canopy than individual drops or leaves. The basis for many canopy models is the Penman-Monteith equation (1.2), from which the flux of latent heat is calculated. The Penman-Monteith model has been adapted for calculating evaporation rates from full canopies (Shuttleworth 1975, 1976, 1977, 1978, 1979) and from sparse canopies (Shuttleworth and Wallace 1985). The Shuttleworth models rely upon the complicated principle of canopy resistance, which has been criticized by Tanner and Fuchs (1968) because it poorly represents a canopy.

A more sophisticated approach is to consider the evaporation of water from a multi-layered canopy using a surface energy balance model (Thompson 1982, Norman and Campbell 1983 and Huber and Itier 1990). In a multi-layer model, the surface energy balance and SWD are estimated for each canopy layer. The basic approach is to calculate the water storage capacity of the canopy after rainfall and then consider SWD until this water has evaporated. For this reason leaf area estimates of the canopy are essential. Some canopy models are complex, especially the Cupid model (Norman and Campbell 1983).