Abstract Transpiration rates from apple trees are calculated on the basis of the energy balance equation for single leaves. The average net flux of radiation to the leaves in a tree is estimated from the temperature of non-transpiring (vaseline covered) leaves, the temperature of normal leaves and windspeed. A relationship between net radiation measured above the orchard and that calculated for leaves is established. The way in which the ratio of latent heat to available net radiation, λE/Qnl, varies with the “climatological resistance”, ri, is shown to be linear at fixed leaf resistance values. Using estimated net flux of radiation to the leaves from field measurements, a good straight line relationship was established between λE/Qnl and ri. This was possible because the range of leaf resistance values calculated for apple leaves was small, so the relationship between λE/Qnl and ri was controlled largely by external rather than physiological parameters. From this relationship a simple equation to estimate transpiration rate from net radiation measured over the orchard and vapour pressure deficit of the air is given. The dominant environmental parameter influencing transpiration rate in the orchard was the vapour pressure deficit.
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