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4 E 3 E LU 2 R.M. Gazal et al. /Agricultural and Forest Meteorology 137 (2006) 56-67 °� r2 =0.18 o o CID 000 0 ° o ° r2=0.12 o Q� Zcw (m) Fig. 7. Relationship of measured transpiration, E (mm d— 1) and depth to groundwater, &w (m) at the intermittent (closed circle) and perennial (open circle) stream sites. Regression model is significant at P = 0.05. At the intermittent stream site, Zoe, increased from 3.1 m during the early part of the spring season to 3.9 m during the peak of the drought period. At the perennial stream site, Zc;w had a gradual but much smaller decline during the pre - monsoon drought, and the largest increases were the result of groundwater recharge from river flood events during the monsoon season. The depth at the beginning of the spring season was 1.5 m and increased to only 1.8 m at the peak of the drought period (a decline which was 0.5 m less than at the intermittent site). At the perennial stream site, the small fluctuations in groundwater depth did not appear to strongly influence transpiration (Fig. 7a). Perhaps unexpectedly, there was a general tendency for increased E with increased Zoe,, but this tendency was likely not casual as pre- monsoon period with the greatest evaporative demand coincided with the maximum groundwater depths. At the inter- mittent stream site, there was a stronger relationship between E and Zc;w; lower rates of transpiration corresponded with greater depths to groundwater (Fig. 7). The inverted Penman — Monteith equation was used to assess the seasonal variation in stomatal resistance (rs) of cottonwood forest. rs at the intermittent stream site was higher than at the perennial stream site with maximum rs attained at the peak of the drought period (Fig. 8). Leaf defoliation (DOY 102 -112) at the perennial stream site also caused rs to increase. It was also evident that even the perennial stream site also exhibited some water stress during the pre - monsoon season as rs decreased following monsoon rains and flood flows. 3.5. Modeling transpiration In order to determine if a simple modeling approach would adequately predict E, we examined the functional 5000 4000 3000 E 2000 1000 0 50 I 9 I �II jIII I; I I I I J 1 1 II� Intermittgnt stream site Perennial stream site l I it l I' I, t I III 4 I I' II II II III II III 'V I IIA t tlti ul4 /�ii'Iwi II f 100 150 200 250 300 DOY (2003) 63 350 Fig. 8. Stomatal resistance (rs, s m I) at the intermittent (broken line) and perennial stream (solid line) sites throughout the growing season in 2003. dependence of EIETo on D and ZGw at both sites (Fig. 9). Larger daily average D, which occurred during the pre- monsoon season, had the effect of decreasing El ETo (Fig. 9). Likewise, as depth to groundwater increased, the crop coefficient was reduced. In general, variations in D accounted for more of the variability in EIETo than did variations in ZGw. Nevertheless, there was a well - defined decrease in EIETo for ZGw greater than —3.5 m. Fig. 10 shows the average EIETo throughout the growing season. EIETo is higher at the perennial than the intermittent stream site. The average crop coefficient at the intermittent stream site was 0.43 f 0.14 and 0.74 f 0.24 at the perennial stream site. The ratio between intermittent and the perennial mean crop coefficient (0.58) was slightly less than the ratio of the site LAI's (0.64), likely due to the greater water stress exhibited at the intermittent stream site. 4. Discussion Spatial and temporal heterogeneity of water avail- ability in riparian ecosystems along the San Pedro River in southeastern Arizona has marked influences on the physiology of the cottonwood forests. Canopy structure, atmospheric demand and depth to groundwater played significant roles in the seasonal fluctuations in transpiration of cottonwood trees in this riparian ecosystem. Understanding the mechanisms by which trees respond to these factors is important to accurately assess and manage water budgets of this riparian system. Climatic factors such as temperature, vapor pressure deficit and solar radiation are important regulators of transpiration which interact with water availability and the physiological and phenological status of plants