Measuring Transpiration of Undisturbed Tamarisk Shrubs.

Scarcity of water suitable for human use has influenced the history of mankind profoundly (4) and is now of major concern in the United States. It is of acute local concern in Phoenix, Ariz. and desert environs, where a rapidly increasing human population depends on one remote watershed (Salt River) for its primary water supply (2). Here, problems of watershed management are not of mere scholarly interest; they are of intense and immediate public concern. One of the major problems is how to reduce evaporative losses from the watershed itself. This is a general problem in watershed management (6), and its importance increases with aridity of a region. On one experimental part of the Salt River watershed, evaporative losses of summer rainfall ranged from 89 to 100 % (12). Evaporative losses from soil and vegetation, usually considered together as evapotranspiration (ET), can be changed by altering vegetation (6). Much more information is needed about what kind, size, age, and arrangement of plant cover is least wasteful for specific situations and there is acute need for a convenient method for direct measurement of ET in undisturbed situations (6). In the present paper is reported the development of one such method and its use on thickets of fivestamen tamarisk (Tamarix pentandra Pall). Tamarisk, also known as saltcedar (7), is an important phreatophyte along the Salt River and throughout much of the Southwest. Replacement of tamarisk by plants that transpire less has been suggested as a means of reducing ET (13). Bermuda grass [Cynodon dactylon (L) Pers] may be a feasible replacement along the lower stretches of the Salt River (8).