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This article in AJ

  1. Vol. 65 No. 2, p. 326-328
     
    Received: May 25, 1972
    Published: Mar, 1973


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doi:10.2134/agronj1973.00021962006500020041x

Estimating Transpiration Resistance1

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

Abstract

Abstract

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.

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