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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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