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Water-loss methods Aerodynamic and energy balance methods Empirical formulas All of the methods listed in these approaches are weak in one aspect or another. However, a number of studies are underway at the present time that should reduce some of the uncertainties. The limitations or uncertainties in each method will be discussed in the light of present-day knowledge. Water-loss Methods A number of methods have been developed for measuring water loss from vegetated areas. Some are well established while others are still in the experimental stage. A list of these methods includes evapotranspi- ration tank studies, soil moisture studies, water-table fluctuation, inflow-outflow budget, seepage-run, evaporation as an index to evapo- transpiration and the evapotranspiration tent method. Evapotranspiration tank studies have provided the bulk of the usable basic water-loss data. An evapotranspiration tank, also referred to as an evapotranspirometer and as a lysimeter, is a tank filled with soil, and provided with a water supply, in which representative plants are grown. The tanks have varied in size from 2 feet in diameter and 3 feet deep to tanks with 1,000 feet of surface area and 14 feet deep. In evapotranspiration tank studies, two methods have been used. (1) The flow-thru method where evapotranspiration is computed as the difference between the water added and the water leaving the tank and (2) the controlled water level method, in which evapotranspiration is computed as the amount of water added to maintain the water level at a desired depth below the surface of the tank. This method used in phreatophyte studies simulates the relation of phreatophytes to the water table. Since 1959, tank studies of phreatophytes have been made at four loca- tions in the Western States, Arizona, California, Nevada, and New Mexico. At Buckeye, Arizona, where there are 10 tanks of saltcedar, the studies involve different depths to water, and two different qualities of water. At Imperial Dam, California near Yuma, Arizona, there are 9 tanks, 3 planted to arrowweed, 3 to saltbush and 3 to Bermuda grass. Measure- ments of evapotranspiration are made for different water levels at Imperial Dam, as well as at Winnemucca, Nevada, where there are 12 tanks, 2 planted to greasewood, 3 each to rabbitbrush, willow, and wildrose, and one bare soil tank. The studies at these three sites are under the direction of the Geological Survey in cooperation with the Bureau of Reclamation, and additionally at Winnemucca with the State of Nevada. Also at the N"vada si te, the Agricul tural Research Service is studying the evapotranspiration of meadow grass, largely creeping wildrye and saltgrass, both phreatophytes. At Bernardo, New Mexico, the Bureau of Reclamation have 9 tanks planted to saltcedar. B