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Resistances to Evapotranspiration from a St. Augustinegrass Turf Canopy¹

A controlled environment study of evapotranspiration from St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] turf showed that, under adequately watered conditions, evapotranspiration was influenced to a greater extent by environmental factors which were external to the plants rather than by disposition of leaf stomata. Using an Ohm's Law analogue to define a resistance network for the turfgrass canopy, magnitudes of resistances to water loss were measured and compared. Internal resistance was defined as aggregate diffusive resistance of the foliage and was determined from porometer measurements of diffusive resistance of leaves. External resistance was defined as the sum of aerodynamic resistance and resistance to air mass exchange within the canopy and was measured by three different methods. Internal resistance was found to be only one-fourth to one-half the external resistance. Under wind speed conditions of 0.6 m/sec, actual evapotranspiration rates of St. Augustinegrass were only slightly lower than potential evapotranspiration rates. It is concluded that chemical or genetic control of stomatal resistance would not result in appreciable savings of irrigation water.

Key Words: Stenotaphrum secundatum (Walt.) Kuntze • Diffusive resistance meter

² Postdoctoral fellow and professors, respectively, Dep. of Soil and Crop Sciences, Texas A&M Univ., College Station, TX 77843. Current address of the senior author is: Lilly Research Laboratories, P.O. Box 708, Greenfield, IN 46140.

Received for publication February 11, 1982.

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