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64 R.M. Gazal et al. /Agricultural and Forest Meteorology 137 (2006) 56-67 1.6 (a) o 1.4 12 ° ° ° r2 =0.20 0 1.0 Q CO °° ° ° o 0 W 0.8 o o• °1,11 :1 a ° 0 0 00 cp • . . ° 0.2 ' r2 =0.34 ' 1144 ♦ •. 0.0 � 0 1.2 1.0 0.6 W 0.6 w 0.4 0.2 0.0 1 2 3 D (kPa) (b) ° ° ° r2 =0.16 ° ° • , 4• r2 =0.25 0 •t o • 1 2 3 4 ZGw (m) 4 Fig. 9. Relationship of the ratio of measured transpiration and crop coefficient (EIET°; E is the measured transpiration in mm d -1 and ET° is the reference crop evaporation in mm d -1) and (a) vapor pressure deficit, D (kPa) and (b) depth to groundwater, Zaw (m) at the intermittent (closed circle) and perennial (open circle) stream sites. Regression model is significant at P = 0.05. 1.E 1.4 l.i 1.0 01 O.E 0.4 0.: 0.0 i 50 100 150 200 250 300 350 DOY (2003) Fig. 10. Mean crop coefficient (EIETO) at the intermittent (broken line) and perennial stream (solid line) sites throughout the growing season in 2003. (Vose et al., 2003). At the peak of the dry period, there was evidence of stomata] closure in response to high D at the intermittent stream site (Fig. 2). Stomatal midday depression is indicative of stomatal regulation of E as leaf water potential declines through the morning (O'Grady et al., 1999; Horton et al., 2001 a). Soil water limitation at the intermittent stream site may have caused the decline in hydraulic conductivity during the growing season and reduced their stomatal sensitivity to D. In contrast, there was less apparent stomatal closure during the day in response to D at the perennial stream site throughout the season. This is supported by a significant positive linear relationship between E and D at the perennial stream site that indicates low resistance to water transport (Fig. 4). E appeared to be controlled by water transport capacity and amount of foliage in cottonwood trees at the perennial stream site (Cinnirella et al., 2002). The lack of a significant relationship between is and D at the intermittent stream site implies control of stomatal movement to avoid excessive transpiration in the afternoon when D tends to be high (Fig. 3). Cottonwood trees at the intermittent stream site had lower annual water use that was approximately 50% less than the water use at the perennial stream site. Goodrich et al. (2000) estimated cottonwood con- sumptive water use using limited 1997 synoptic -period sap flow observations of cottonwood stands along a larger perennial reach of the San Pedro River. Our 2003 estimates of the annual cottonwood forest E at the perennial stream site was 20% more water on a per canopy area basis than their 1997 estimates. It is likely that the Goodrich et al. (2000) estimates of cottonwood forest transpiration were lower than that found in the current study due to the shorter growing season in 1997 (by about 30 days) and their lack of nighttime sap flow measurements. We found that sap flow, in fact, continued past sunset and gradually declined to zero between 19:00 and 20:00 h. Nocturnal sap flow accounted for approximately 10-25% of the daily totals. Dahm et al. (2002) also provided an estimate of annual evapotranspiration (980 mm) of a mature cottonwood stand with a closed canopy along the Middle Rio Grande in New Mexico which was similar to our estimate of total annual water use (966 mm) of cottonwood forests at the perennial stream site. Seasonal fluctuation in water use of cottonwood trees at the intermittent stream site was closely related to the fluctuations of the groundwater table (Fig. 6). From the onset of transpiration to the peak stress of the pre - monsoon period (April- June), the depth to groundwater increased by 0.8 m at the intermittent stream site and