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Abstract. High resolution thermal imagery of crops in field can be obtained at high frequency with field traversing phenotyping platforms. A simple field setup replicates thermal camera and weather data that can be acquired with high throughput phenotyping systems. Temperature of leaf tissue in each image is extracted automatically and combined with weather data to model the stomatal conductance required to satisfy an energy balance constructed from first principles. Lack of water leads to a major inhibition of transpiration which results in increased canopy temperature, lowered stomatal conductance, and reduced growth. Stomatal conductance modelled from the energy balance applied to infield measurements in late summer and early spring showed consistent trends in diurnal variation, strongly influenced by solar irradiance. Comparisons between nominally unstressed and water-deprived stressed groups of wheat plants showed significant difference in modelled stomatal conductance. Assessment of stress development indicated traditional measurements of leaf-air temperature difference would indicate the onset of stress sooner than comparisons of stomatal conductance, but the results are clouded as temperature swings could be due to minute changes in environment. The response of wheat to the onset of drought stress differs between cultivars because genetics play a significant role in the activation of processes within the plant. Comparisons of field-level stomatal conductance to a baseline unstressed crop must be done under two conditions – similar time of day and cloudless skies.