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Published online 1 March 1973
Published in Agron J 65:326-328 (1973)
© 1973 American Society of Agronomy
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Estimating Transpiration Resistance1

Lynn J. Brun, Edward T. Kanemasu and William L. Powers2

Previous estimates of transpiration resistance have been based on the stomatal resistance of only a few leaves per plant. Because the lower stomatal resistances observed in upper leaves compared to lower leaves in dense canopies could introduce sampling errors, transpiration resistance was determined in a sorghum [Sorghum bicolor (L.) Moench] canopy by two methods based on the stomatal resistance of all the leaves on a plant and a third method based on the stomatal resistance of only the upper three leaves. Stomatal resistance was measured with the diffusion porometer.

The three methods of determining transpiration resistance included: (i) harmonically averaging the stomatal resistance of all the leaves on a plant; (ii) dividing the crop canopy into layers and weighting each layer resistance by its leaf area index; and (iii) harmonically averaging the stomatal resistances of the upper three leaves of the plant. Methods 1 and Z gave almost identical estimates of transpiration resistance. When methods 1 and 3 were evaluated by the Monteith evapotranspiration model, the evapotranspiration estimate using method 1 gave closer agreement with lysimeter evapotranspiration than when method 3 was used in the model.

Key Words: Evapotranspiration • Stomatal diffusion porometer

1 Contribution no. 1277, Department of Agronomy, Evapotranspiration Laboratory, Kansas Agricultural Experiment Station, Manhattan, Kansas, and contribution no. 97, Kansas Water Resources Research Institute, Manhattan, Kansas.

2 Research Assistant in Soil Physics, Assistant Professor of Microclimatology, and Associate Professor of Soil Physics, Department of Agronomy, Kansas State University, Manhattan, Kansas 66506.

Received for publication May 25, 1972.




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Copyright © 1973 by the American Society of Agronomy.