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R.M. Gazal et al. /Agricultural and Forest Meteorology 137 (2006) 56-67 Transpiration by cottonwood forests can be a substantial component of the total water budget of riparian systems (Goodrich et al., 2000). However, there remains considerable uncertainty in estimates of transpiration in these systems because of landscape - scale variation in forest structure and the heterogeneity of groundwater availability (Schaeffer et al., 2000). Spatial and temporal heterogeneity in cottonwood forest transpiration is likely to be substantial, but is poorly documented. Yet heterogeneity in water use of vegetation influences ecosystem water balance and key ecological processes. Water use of cottonwood forests, one of the most dominant riparian vegetation types in the Upper San Pedro Basin in southeastern Arizona, USA, has a great influence on the overall productivity and water balance of the riparian corridor (Goodrich et al., 2000; Scott et al., in press). Within this context, it is important to determine the extent to which cotton- wood trees control hydrological processes and the effects of seasonal variability and temporal trends in water resources on the physiology of cottonwood trees (Gurnell et al., 2000). Plants regulate transpiration (E) to maximize photosynthesis, minimize water loss and avoid drought- induced formation of xylem embolisms (Tyree and Sperry, 1989; Whitehead, 1.998; Sparks and .Black, 1999). Plant structural and physiological traits asso- ciated with regulation of transpiration include the amount of leaf and sapwood area (Gholz, 1982; White et al., 1998; Whitehead, 1998; Cinnirella et al., 2002; Schaeffer et al., 2000) and stomatal characteristics and responsiveness to atmospheric moisture (Oren et al., 1999; Medrano et al., 2002; Vose et al., 2003). The hydraulic connection between the roots and the water table also influences transpiration (Cooper et al., 2003), particularly in riparian forests that are sustained by shallow groundwater tables (Tyree et al., 1994; Zhang et al., 1999; Vose et al., 2003). Plants avoid excessive transpiration during periods of high vapor pressure deficit (D) by regulating stomatal conductance. Cottonwood trees along an ephemeral river system in Arizona were found to be more sensitive to high vapor pressure deficit than other riparian species such as the drought tolerant Tamarix chinensis (Horton et al., 2001 a). Stomatal conductance and net photo- synthesis rate of cottonwood trees decline during drought (Horton et al., 2001 a), which may indicate the influence of soil moisture stress on stomatal opening (Unsworth et al., 2004). Reduction in leaf area, accompanied by physiological changes necessary to counterbalance loss in photosynthetic leaf tissue, provides an effective mechanism that can be used 57 with stomatal closure to prevent excessive water loss (Roberts, 2000). In this paper, we present the seasonal patterns of cottonwood transpiration at two contrasting riparian sites in order to better understand how the trees transpire in relation to canopy structure, evaporative demand and groundwater depth. Along the San Pedro River in southeastern Arizona, we quantified cottonwood tran- spiration using sap flow measurements for a cluster of trees located on a perennial section of the river and another located along a reach with intermittent stream flow. Hydrologically, these sites differed in the depth and seasonal fluctuation of the water table. We also computed crop coefficients for cottonwood transpira- tion to understand mechanisms controlling transpiration and to evaluate suitable proxies for cottonwood transpiration in the absence of direct transpiration measurements. 2. Methods 2.1. Study sites Study sites were located on the floodplain of the San Pedro River in the San Pedro Riparian National Conservation Area (SPRNCA) in southeastern Arizona. A cluster of cottonwood trees was monitored at two sites with contrasting groundwater depth levels (Table 1). The Boquillas site (1180 m elevation, 31.6986 °N 110.1808 °W) was located along an inter- mittent reach of the river and the groundwater depth ranged from 3.1 to 3.9 m (mean = 3.3 m) during this study. In contrast, the Lewis Springs site (1250 m elevation, 31.5533 °N, 110.1391 °W) was located along a perennially flowing reach of the San Pedro River and the groundwater depth ranged from 1.1 to 1.8 m (mean = 1.6 m). The climate in the San Pedro Basin is semiarid with large fluctuations in water availability and diurnal and annual air temperature. Approximately 350 mm of precipitation falls annually with about 60% of that occurring during the summer monsoon from July— September (Scott ct al., 2004). Cottonwood trees usually leaf out in early April, about one month before the last nocturnal frost of the spring. The pre- monsoon period of May and June is usually very dry (midday D > 5 kPa) and hot (maximum temperature >35 °C). The frequent afternoon thunderstorms and river flood- ing during the annual summer monsoon relieve the intense drought. Cottonwood trees in the river basin become dormant in early November and transpiration typically ends after the first hard frost of autumn (Goodrich et al., 2000). The cottonwood stands along