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R.M. Gazal et al. /Agricultural and Forest Meteorology 137 (2006) 56-67 <br />Total water use of the entire cottonwood cluster was <br />divided by projected canopy area (m2) to determine <br />ground -area based transpiration, E (kg m -2 d -1 or <br />mm d -1). The projected canopy area of the clusters was <br />estimated digitally using aerial photographs. <br />2.4. Leaf area index, meteorological and <br />groundwater depth measurements <br />Leaf area index (LAI) was measured using a plant <br />canopy analyzer (LAI 2000, LiCor, Lincoln, NE) in <br />October 2003. LAI readings were taken from the four <br />cardinal directions around the base of each tree within <br />the cluster. Air temperature, relative humidity, solar <br />radiation, wind speed, air pressure and precipitation <br />were measured at nearby meteorological towers located <br />3 km from the Boquillas intermittent stream site and <br />0.3 km from the Lewis Springs perennial stream site. <br />Depth to groundwater (Zc;w) at each site was measured <br />in piezometers located close to the sites. Water levels <br />were measured manually on a weekly basis at the <br />intermittent stream site and every 30 min with a well <br />transducer at the perennial stream site. <br />2.5. Calculation of resistances and crop coefficient <br />To assess the seasonal variations in stand -level <br />canopy resistance, rc (s m -1), the Penman - Monteith <br />equation ( Montieth and Unsworth, 1990) was inverted <br />to become: <br />\ <br />r� = I A I A -E E ra + paC D (4) <br />where A is the slope of the saturation vapor pressure and <br />temperature curve (kPa °C -1), y the psychrometric <br />constant (kPa °C -1), XE the latent heat flux (W m -2), <br />A the available energy (W m -2) , ra the aerodynamic <br />resistance (s m -1), Pa the density of moist air (kg m -3), <br />Cp the specific heat for dry air under constant pressure <br />(1013 J kg -1 °C -1) and D is the vapor pressure deficit <br />(kPa). Parameters such as A, pa, y and D were calcu- <br />lated from air temperature, relative humidity and air <br />pressure measured from nearby meteorological stations. <br />Since canopy available energy measurements were not <br />available, A was estimated by using measured incoming <br />solar radiation and a formula for reference crop avail- <br />able energy developed for southern Arizona (Brown, <br />1989). The canopy aerodynamic resistance was domi- <br />nated by and assumed equal to the boundary layer <br />resistance, which was estimated following the methods <br />described by Goodrich et al. (2000). Since wind speeds <br />and leaf dimensions were nearly equal between sites, <br />59 <br />the magnitude of ra was influenced mainly by the <br />difference in LAI between the sites. <br />The canopy aerodynamic resistance was estimated <br />following the methods described by Goodrich et al. <br />(2000). Stomatal resistance, rs, was determined by: <br />rs = r�LAI (5) <br />where we assume that LAI at the perennial (LAI = 2.75) <br />and intermittent stream sites (LAI = 1.75) were constant <br />throughout the growing season. <br />A crop coefficient, k, was calculated as: <br />E (6) <br />k� = ETO <br />where E (mm d -1) is the transpiration from actual sap <br />flow measurements and ETO is reference crop evapora- <br />tion (mm d -1). k. is simply referred to as EIETO in this <br />paper. ETO is an estimate of evaporation that would <br />occur from short, well- watered grass that fully covers <br />the ground. ETO was calculated using a modified Pen- <br />man- Monteith equation developed for southern Ari- <br />zona (Brown, 1989) and is defined as: <br />ETo =QA yA +Q +y.f(U2)D (7) <br />where A is the same available energy as above except in <br />units of mmd -1. Two wind functions (f(U2); mm <br />h -1 kPa-) are used to compute ETO —one for daytime <br />conditions (A > 0) and one for nighttime conditions <br />(A < 0). These wind functions are as follows: <br />f (U2) = 0.03 + 0.0576(U2); A > 0, Daytime (8) <br />f (U2) = 0.125 + 0.0439(U2); A<0, Nighttime <br />(9) <br />where U2 (ms-1) is the mean hourly wind speed <br />obtained at a height of 2 m. <br />3. Results <br />3.1. Seasonal variation in meteorological <br />conditions <br />Because the meteorological conditions at both the <br />intermittent and perennial stream sites were very <br />similar, they can be adequately summarized graphically <br />by using just one site (Fig. 1). Diurnal temperature <br />ranges were typically 22 °C with the highest tempera- <br />ture (42 °C) recorded in July and lowest temperature <br />( -9 °C) recorded in November. The last hard frost of <br />spring (defined by minimum temperature <3 °C) <br />occurred on DOY 132 and the first hard frost of fall <br />was on DOY 295. Monsoon rains began on DOY 192 <br />