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60 <br />E <br />40 <br />30 <br />20 <br />10 <br />a <br />E <br />9 -10 <br />400 <br />350 <br />N <br />E 300 <br />C: 250 <br />0 <br />AR 200 <br />150 <br />0 <br />0) 100 <br />50 <br />R.M. Gazal et A /Agricultural and Forest Meteorology 137 (2006) 56-67 <br />(a) F,rl lay? �4,��1 !j � vw , f's r <br />ti4,�d1 ±r !1411 I f iA <br />1 l., < <br />1 4` hh <br />3 <br />.._ i <br />100 150 200 250 300 <br />DOY (2003) <br />site (2.0 mm event) and at DOY 193 at the perennial <br />stream site (1.5 mm event). <br />3.2. Vapor pressure deficit and transpiration <br />During the peak of the pre - monsoon drought (around <br />DOY 185), mean maximum D (Dmax) was 6 kPa at the <br />intermittent and perennial stream sites (Fig. 2). D and E <br />followed the same trend throughout the days. At the <br />intermittent stream site, however, declines in E <br />coincided with the peaks in D during the day. Separation <br />a of 30 min is values into morning (07:00 -12:00 h) and <br />afternoon (12:30 -17:00 h) periods at both sites during <br />5 the peak of the drought period shows unique responses <br />of cottonwood trees to D (Fig. 3). In the morning, a <br />4 ri positive, linear relationship between D and JS exists at <br />2 <br />0 <br />Fig. 1. Seasonal variations in environmental conditions: (a) maximum <br />(solid line) and minimum (broken line) air temperature ( °C) and <br />precipitation (mm; bar); (b) vapor pressure deficit (D, kPa; solid line) <br />and solar radiation (W m-2 ; broken line); during the 2003 growing <br />season at the intermittent stream site. <br />and resulted in reduced D. The end of the monsoon is <br />harder to define as rainy periods with dry interstorm <br />periods continued throughout July —October. Maximum <br />D was often greater than 5 kPa during the pre- monsoon <br />and was reduced to less than 5 kPa during the peak of <br />the monsoon. Average daily solar radiation peaked in <br />June and was reduced for the rest of the season due to <br />the reduction in extraterrestrial forcing and greater <br />percentage of cloudy days. <br />Monthly distribution of rain events was not similar <br />between sites due to localized and patchy occurrence of <br />the summer convective storms (data not shown). At the <br />Boquillas site (intermittent stream site), the highest <br />monthly amount of rainfall (52 mm) was observed in <br />August while it was in July (80 mm) for the Lewis <br />Springs site (perennial stream site). The total amount of <br />rainfall, however, was almost the same for both sites <br />throughout the duration of sap flow measurements from <br />April to October 2003 (intermittent, 186 mm and <br />perennial, 188 mm). Approximately 80% of the total <br />growing season rainfall occurred in the months of July, <br />August and September. The first precipitation event of <br />the monsoon was on DOY 192 at the intermittent stream <br />the perennial stream site. However, during the same <br />morning period, there was already evidence of stomatal <br />closure at D greater than 3 kPa at the intermittent stream <br />site. In the afternoon, JS at the perennial stream site was <br />still linearly related to the even greater values of D, but <br />JS was not correlated to any change in D at the <br />intermittent stream site. <br />The trees responded differently after the summer <br />monsoon rainfall had recharged soil moisture and the <br />alluvial aquifer (Fig. 2) than before the rainy period. <br />From July 29 to August 13, after a total of 31 and 32 mm <br />of precipitation had fallen at the intermittent and <br />perennial stream sites, respectively, Dmax ( =5 kPa) was <br />similar to that during the pre- monsoon season in July. JS <br />increased at both sites during the monsoon season with a <br />much greater increase at the intermittent than at the <br />perennial stream site. There was still evidence of stomata] <br />limitation at the intermittent stream site in response to <br />high D during the course of the day during the rainy <br />period (Fig. 3c and d). On a daily basis, E increased with <br />D at the perennial stream site (r2 = 0.61; Fig. 4a). <br />However, at the intermittent stream site, E was not <br />correlated to D. Leaf - area - normalized E (E /LAI) at the <br />perennial stream site was higher than the intermittent <br />stream site when D was greater than 3 kPa (Fig. 4b). <br />3.3. Total growing season transpiration <br />Daily total transpiration of the cottonwood cluster at <br />the perennial stream site was higher than that at the <br />intermittent stream site throughout the growing season <br />(Fig. 5). A marked decline in E at the intermittent <br />stream site was observed during the peak of the pre - <br />monsoon drought. Leaves of cottonwood trees fully <br />leafed out by DOY 91 and completely senesced by DOY <br />309. In order to determine total growing season <br />