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284 D.C. Goodrich et al. /Agricultural and Forest Meteorology 105 (2000) 281 -309 <br />ready interpretation in terms of a physically realistic <br />equivalent ". <br />More recently, several additional methods have been <br />employed to estimate riparian ET. Gay and Fritschen <br />(1979) and Weeks et al. (1987) employed micromete- <br />orological and energy balance techniques to estimate <br />ET from large, dense salt cedar riparian stands. Un- <br />land et al. (1998) employed similar techniques over <br />a dense mesquite bosque located in the Santa Cruz <br />riparian system. Comparable methods were used in <br />this study to measure ET over relatively large uniform <br />stands of mesquite and sacaton grasses (Scott et al., <br />2000). However, as noted in Section 1, the necessary <br />fetch conditions for these techniques are typically not <br />met in semi -arid riparian forest galleries. <br />3. Experiment and measurements <br />3.1. Site description <br />The USPB, located in the semi -arid borderland <br />of southeastern Arizona and northeastern Sonora, is <br />a broad, high -desert valley bordered by mountain <br />ranges and bisected by a narrow riparian corridor <br />(Fig. 1). The vibrant San Pedro riparian ecosystem is <br />primarily made up of three vegetation communities <br />dominated by mesquite, grasses, and C/W forests. <br />Portions of the river contain some of the healthiest <br />desert riparian ecosystems remaining in the south- <br />western US (Grantham, 1996; Stromberg, 1998). This <br />riparian forest supports a great diversity of species <br />— some endangered with extinction — and is widely <br />recognized as a regionally and globally important <br />ecosystem (World Rivers Review, 1997; Kingsolver, <br />2000). In 1988, the United States Congress estab- <br />lished the San Pedro Riparian National Conservation <br />Area (SPRNCA), the first of its kind in the nation, to <br />protect riparian resources along 60 km of river north <br />of the US– Mexico border. <br />However, there is serious concern that nearby <br />groundwater withdrawals have affected the quantity <br />and timing of groundwater reaching the San Pedro <br />River (Pool and Coes, 1999) as groundwater sustains <br />perennial flow in large reaches of the river (Richter <br />and Richter, 1992; CEC, 1999). The close coupling <br />10 =11..1 <br />walnut <br />wou <br />Tombstone <br />Gaye • Tomb <br />0 50 km <br />Fig. I. Location of the San Pedro River Basin and pertinent geographic and measurement locations. The box corresponds to the boundaries <br />of the groundwater model of Corell et al. (1996). <br />a <br />ARIZONA <br />Charleston <br />R <br />o <br />Coge <br />Ettapule <br />o <br />Phoeft <br />San Pedro <br />Wash <br />V o <br />• <br />Glo R. <br />Riv <br />Groundwater <br />tiG Model Domain <br />Sierca mste <br />5awngs <br />Tucson• <br />tipper <br />9 <br />�yG <br />San Pedro <br />Satin <br />�p <br />Herel <br />Palo <br />SONORA <br />Gage <br />Cage <br />• <br />USA <br />Hermosillo <br />L <br />MEXICO <br />0 50 km <br />Fig. I. Location of the San Pedro River Basin and pertinent geographic and measurement locations. The box corresponds to the boundaries <br />of the groundwater model of Corell et al. (1996). <br />