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The rate of evaporation from a crop is expressed in terms of weather parameters and a quantity rs derived from profiles of temperature, humidity, and wind. In transfer equations, rs is formally similar to the diffusion resistance of the stomata in a single leaf, and measurements in a field of barley support the hypothesis that rs is the diffusion resistance of the complete crop canopy. The resistance is relatively small when the leaf area index is great, when soil is moist, and when sunlight is bright. It increases as the plants mature, but is independent of wind speed and is therefore unrelated to the rate of diffusion by turbulent mixing. More directly, rs was correlated with stomatal resistance rp measured on individual leaves with a porometer. From seventy-five sets of porometer readings taken when the leaf area index was between 6 and 10, the relation was rp = 1.11+0.87 rs Because rp was measured on upper, sunlit leaves, which were more porous than lower, shaded leaves, the ratio of rp to rs was expected, and was found, to be less than the leaf area index. Finally, stomata were closed and rp was increased by spraying the top of the canopy with NSA (the methyl ester of nonenyl succinic acid). The increase of rs calculated from the decrease of transpiration rate was consistent with the change of rp.
Monteith et al. (Mon,) studied this question.