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<br />unit volume of transpi ring fol iage. In this method, information is <br />needed of the growth conditions in the study area as well as in the <br />area of natural growth. The method presumes that transpiration, by <br />plant species, is proportional to the total transpiring leaf area, and <br />so, proportional to the volume of foliage. The presumption was tested <br />and found to be valid in studies by Gatewood and others (1950) of <br />bottomland vegetation, phreatophytes, in the Safford Valley. Arizona. <br /> <br />The transpiring leaf area per unit volume as well as the transpiration <br />rate per unit leaf area differ from one species to another. Among <br />three phreatophytes--saltcedar, willow, and cottonwood--the transpira- <br />tion rate per unit of leaf area was found to be least for saltcedar <br />and greatest for willow, while the rate per unit of foliage volume was <br />least for cottonwood and greatest for saltcedar (Tomanek and Ziegler, <br />1960) . <br /> <br />The volume of foliage method requires detailed measurements of cover <br />density and thickness or depth of canopy of the plants for computation <br />of the foliage volume. Foliage volume is the product of the area <br />cover density expressed in percent and thickness of foliage or the <br />crown depth of the green canopy. In order that the measurements may <br />be comparable in the two areas, standards and techniques for the volume <br />determination have been prepared by the Phreatophyte Subcommittee <br />and set forth in the publication "Guide for surveying phreatophyte <br />vegetation" (Horton, RobInson, and McDonald, 1964). Evapotranspiration <br />may be conveniently expressed as acre-feet of water per acre-foot of <br />foliage. <br /> <br />For natural growth areas of low growth density, the method may have <br />limitations due to the oasis effect. Here also, information is needed <br />on the relation of water use to units of foliage volume in growths <br />subject to the oasis effect. <br /> <br />16 <br />