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<br />001719 <br /> <br />Preliminary results of the studies at Buckeye are given in papers by <br />T. E. A. van Hylckama (1963) and at Imperial Dam in a papec by G. H. <br />Hughes and C. C. McDonald (1966). Results of the studies of woody <br />phreatophytes in Nevada by T. W. Robinson are contained in a secies of <br />progress reports, published by the Department of Conservation and Natural <br />Resources, State of Nevada (1960-1966) and of the grasses in a report by <br />A. S. Dylla and D. C. Muckel ([964). <br /> <br />In the controlled-level method, dissolved solids in the supply water <br />accumulate in the tank, and eventually reach a concentration that is <br />injurious to the plants. The result is diminshed plant growth and <br />rate of evapotranspiration. This condition was experienced at all of <br />the four sites. In the controlled-level method it is essential to <br />monitor the soil and water, in order to avoid toxic concentrations of <br />alkaline and saline salts and other injurious solutes such as boron. <br />At Winnemucca, concentrations of boron in the root zone above the water <br />table in the greasewood and rabbitbrush tanks was extremely detrimental <br />to the plants. The condition at Winnemucca, and the alkaline and saline <br />accumulationS at the other three sites was corrected by leaching by <br />backwashing after which evapotranspiration returned to normal. <br /> <br />Tank size and environment are important in tank studies. Small diameter <br />tanks are unsuitable for most woody phreatophytes as root development <br />is restricted. Tanks of 100 to 1,000 square feet and greater than 5 <br />feet deep appear to avoid the effect of root restriction. The environ- <br />ment should be of the same species or species association, and sufficietly <br />extensive to avoid the 1I08sis effect"--the phenomenon in which isolated <br />plants use more water than in dense growths. <br /> <br />An example of the oasis effect is furnished by studies that were made at <br />Victorville, California. An isolated tank planted to tules in an area <br />of low-growing saltgrass about 25 feet from the edge of a swamp, used <br />22.64 feet of water during a 12-month period, while a similar tank of <br />tules within the swamp used only 7.04 feet (Blaney and othecs, 1933, <br />p. 74-871. <br /> <br />Soil moisture studies are not suited to the determinations of evapo- <br />transpication by phreatophytes, as the plants depend on their supply <br />from the water table for their needs and there is relatively little <br />use of soil moisture from above the capillary fringe of the water table. <br /> <br />The daily rise and fall of the water table in an extensive area of <br />phreatophytes that results fronl their draft on shallow unconfined <br />ground water is an index to the evapotranspiration during the 24-hour <br />period. These diurnal fluctuations represent the period of draft <br />during daylight hours when the water level falls, and the period of <br />little or no draft at night, when the water level rises. <br /> <br />In 1927, W. N. White (1932) developed a method for analyzing and <br />estimating evapotranspiration from the trace of the fluctuation recorded <br /> <br />9 <br />