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<br />on charts of water-level recorders, installed on wells. White theorized <br />that the hourly rate of rise of the water table from midnight to 4 a.m. <br />(when transpiration was essentially nil) times the hours in the day (24) <br />plus or minus a net rise or fall of the water table was an index to <br />the transpiration during the 24-hour period. This index times the <br />specific yield or drainage coefficient of the soil within the range of <br />the water-table fluctuation measured the day's evapotranspiration. <br /> <br />The method is expressed by the formula: <br /> <br /> q = y <24r + s) <br />in which q depth of water withdrawn <br /> y = specific yield or drainage coefficient <br /> r = hourly rate of ri se <br /> s = net rise or fall of the water table <br /> <br />The specific yield or coefficient of drainage is determined from <br />undisturbed samples in which the fluctuations occur. This is done <br />experimentally either by saturation and drainage or by centrifuging <br />in the laboratory. Evapotranspiration values calculated from water- <br />table fluctuations represent the growth around the well. The size of <br />the effective area is uncertain. When the amplitude of the fluctuations <br />are very small, indicating low draft or high permeability in the zone of <br />fluctuation, the method becomes insensitive. The method is not valid <br />when the water level in the well is affected by pumping, by changing <br />stage in a nearby body of water, or when the fluctuations are interrupted <br />by rain. <br /> <br />The water-table fluctuation is a simple inexpensive and readily adapted <br />method under most conditions for determining evapotranspiration by <br />phreatophytes. It has not been widely used, probably due to the dif- <br />ficulty and uncertainties of determining the coefficient of drainage. <br />This aspect of the method deserves intensified study with a view to <br />simplifying the procedure and reducing these uncertainties. <br /> <br />The inflow-outflow budget method, developed in the Safford Valley, <br />Arizona, by Gatewood and others (1950), is adapted to a reach of valley <br />streams between gaging stations. Evapotranspiration is calculated as <br />the difference between the water entering and leaving the reach, plus <br />or minus soil moisture change. The reliability of the method depends <br />upon the accuracy with which the items of inflow and outflow can be <br />measured. The method is not usable where there are numerous unmeasured <br />diversions or surface inflow, ground-water inflow or outflow, or where <br />the water inflow and outflow are relatively large. Currently the <br />inflow-outflow budget is being used to determine evapotranspiration on <br />a reach of the Gila River, above Coolidge Dam in Arizona. The area un- <br />der study involves a l6-mile reach of the river and includes about <br />8,500 acres. <br /> <br />The seepage run method, also developed in the Safford, Arizona, study <br />by Gatewood and others, is a variation of the inflow-outflow budget. It <br /> <br />10 <br />