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<br />001718 <br /> <br />EVAPOTRANSPIRATION LOSSES <br />THE STATUS OF RESEARCH AND PROBLEMS OF MEASUREMENT <br /> <br />by <br /> <br />T. W. Robinson <br />Geological Survey, Menlo Park, California <br /> <br />Evapotranspiration is a coined word, combining the terms evaporation <br />and transpiration to describe water losses from moist soil by evapo- <br />ration and transpiration by pLants. The term which is now firmLy <br />entrenched in hydroLogic literature was first used by A. L. Sonderegger <br />in L929 in his discussion of water supply from rainfall on a valley <br />floor. Earlier in 1926, C. H. Lee had used the longer expression <br />"evaporation-transpirationll in discussing losses from reclamation <br />projects. <br /> <br />Transpiration by plahts occurs in response to the same energy sources <br />as does evaporation from a water surface. The response, however, is <br />modified by physical characteristics of the soil and physiological <br />characteristics of the plant, the effects of which are only imperfectly <br />understood. For a given climatic condition, with water nonlimiting, <br />the rate of transpiration depends on the species, its cover density <br />and plant size, stage of maturity, and its tolerance to mineral salts <br />in the soil and water. For a given plant species, the rate is effected <br />by climatic conditions such as temperature, wind movement, humidity, <br />solar radiation, and growing season period. <br /> <br />This discussion will be concerned with the means for determining evapo- <br />transpiration where water is nonlimiting, as in an irrigated field, but <br />primarily for phreatophytes--plants whose roots reach to the water <br />table for a lasting supply of water. <br /> <br />In the management of water and land resources, phreatophytes are of <br />particular concern in the arid parts of the Western States, for they <br />Cover some 16 million acres, and may dissipate 25 million acre-feet of <br />water each year by evapotranspiration (Robinson, 1957). Most phreato- <br />phytes have a low economic value. One very aggressive high water <br />consuming phreatophyte of low economic value (saltcedar) occurring <br />largely in the southwestern States, now covers about one million acres <br />in the flood plains of rivers and the deltas of reservoirs and continues <br />to spread. As it spreads, evapotranspiration increases. <br /> <br />Accurate measurement of evapotranspiration from crops or from native <br />vegetation is difficult, complex and costly. A number of approaches <br />have developed either on experimental plots, or on whole fields, or <br />drainage basins, or segments of drainage basins. These approaches fall <br />into three classes: <br /> <br />7 <br />