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282 D. C. Goodrich et AlAgricultural and Forest Meteorology 105 (2000) 281 -309 independently estimated. The closure of the water balance was roughly 5% of total inflows. The ET models were then used to provide riparian ET estimates over the entire corridor for the growing season. These estimates were approximately 14% less than those obtained from the most recent groundwater model of the basin for a comparable river reach. Published by Elsevier Science B.V. Keywords: Riparian evapotranspiration; Penman — Monteith model; Cottonwood/willow transpiration; SALSA Program; Interdisciplinary; Water balance 1. Introduction In many semi -arid basins, groundwater resources sustain human habitation, agriculture and riparian ecosystems. Utilizing regional groundwater models to aid in water resources management requires accu- rate estimates of basin boundary conditions. A crit- ical groundwater boundary condition that is closely coupled to atmospheric processes and is poorly quan- tified is the seasonal riparian evapotranspiration (ET) (Maddock et al., 1998). Improved estimates of riparian ET derived from groundwater and its seasonal distribution are neces- sary to improve regional groundwater models so that they can be used more reliably as near -term manage- ment tools versus their typical use for long -range plan- ning. Typical groundwater modeling studies are done at an annual time scale. This ignores the seasonal vari- ations inherent in the normal growing cycle and peri- ods of runoff (Maddock et al., 1998). With a better un- derstanding of the riparian ET processes, we intend to develop a better groundwater model ET module based on potential functions that can be directly coupled to similar potential -based flux functions for aquifer and streamflow. This will alleviate the problems noted in Maddock et al. (1998) of the inconsistent treatment of ET processes related to precipitation and runoff versus those originating from groundwater. To properly couple riparian ET with groundwater models, it is also critical to identify plant water sources (groundwater, surface runoff, precipitation, or vadose zone). In many cases, water from multiple sources are used by the plants. The mixture of the plant water sources may also vary seasonally with surface water availability and the depth to groundwater (Snyder and Williams, 2000). Another difficulty in estimating riparian ET in semi -arid rivers is the complicated geometry of a typical riparian corridor forest gallery. These corridor forest galleries are often relatively high (^ -5 -20 m), narrow (^-20 -200 m), long, and sin- uous, as they typically follow the alluvial floodplain. This geometry precludes the use of classical microm- eteorological flux measurements (e.g. Bowen ratio, eddy covariance) as the required fetch conditions are not satisfied (see Hipps et al., 1998 for further detail). Overcoming these difficulties to provide a better un- derstanding and quantification of large -area riparian ET is one of the primary objectives of the SALSA Program (see overview by Goodrich et al., 2000). To address this objective, a number of integrated inter- disciplinary experimental field campaigns were con- ducted in the Upper San Pedro Basin (USPB) riparian corridor in southeastern Arizona during 1997. Isotopic and plant physiology measurements were carried out to identify plant water sources over a range of hy- drologic regimes. A scanning Raman LIDAR (LIght Detection And Ranging) laser system was deployed (Cooper et al., 2000; Eichinger et al., 2000) and sap flux measurements were made (Schaeffer et al., 2000) to estimate C/W ET. Simultaneous high - resolution re- mote sensing images were also acquired to remotely estimate ET. None of these measurements provide ET estimates over the entire growing season for the en- tire riparian corridor. However, our intent in this pa- per is to utilize combinations of these measurements and methods with a stepwise scaling approach, and a calibrated Penman— Monteith model to address the following primary objectives. 1. Scale riparian ET estimates whose water source is derived from groundwater both spatially and tem- porally over the entire growing season and the en- tire riparian corridor. 2. Assess the validity of the scaling relationships by carrying out a water balance on a portion of the riparian system during a pre- monsoon period. The presentation is organized as follows. Section 2 contains background information and an overview of prior methods used to estimate riparian ET. Section 3, on materials and methods, contains the ex- perimental site description as well as a description of