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Richard C. Beeson Jr.

). Early work by Burger et al. (1987 ) and Regan (1997) calculated crop coefficients (Kc) to relate woody plant actual ET A to Penman-Montieth-based reference evapotranspiration. However, these Kcs were based on a fixed value of container surface area

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Xuewen Gong, Shunsheng Wang, Cundong Xu, Hao Zhang, and Jiankun Ge

, 1965 ): E T o = 0.408 Δ ( R n − G ) + γ [ 608 V P D / ( T a + 273 ) ] Δ + 1.23 γ [1], where ET o is the crop reference evapotranspiration (mm·d −1 ); R n is the net radiation (MJ·m −2 ·d −1 ); G is the soil heat flux (MJ·m −2 ·d −1 ); Δ is the

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Roger Kjelgren*, Thayne Montague, and Richard Beeson

We investigated water use and a water needs index multiplier relative to reference evapotranspiration for a sweetgum cultivar (Liquidambar styraciflua `Moraine') in Logan, Utah, Lubbock, Texas, and Orlando Fla. Three individual trees with ≈80-mm trunk diameter, were potted in to large containers with organic media at each location. Sweetgum water use (Tsw) was measured over the season at each location with load cells and dataloggers, concurrent with measurement of reference evapotranspiration (ETo) from adjacent weather stations. Dawn-to-dusk stomatal conductance (Gs) was measured several times during the season at all locations. Trees were watered daily. At the end of the season, total tree leaf area was collected and used to normalize volumetric water use data to depth units. Tsw was highest in Florida, up to 4 mm/day, as was maximum daily Gs. Tsw only reached 2.5 mm/day in Texas and Utah due in part to stomatal sensitivity to high vapor pressure deficits that moderated transpiration. There was no relationship between Tsw and ETo at ETo levels above 4 mm/day in Texas and Utah, resulting in substantial scatter in the water needs index multiplier relative to ETo that centered on 0.3 in Texas and 0.4 in Utah. Tsw in Florida showed an upper boundary relationship with ETo, under which it varied considerably, resulting in a values relative to ETo centered on 0.6. During a partial dry down in Utah, morning Gs was unaffected while afternoon Gs declined progressively under mild water stress, resulting multiplier values of 0.15-2. The study shows that regional climate affects tree water use independent of effects measured in ETo, increasing the uncertainty of sweetgum water use estimated as a function of ETo.

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Thayne Montague and Lindsey Fox

Recent droughts and depleted water tables across many regions have elevated the necessity to irrigate field-grown (FG) nursery trees. At the same time, ordinances restricting nursery irrigation volume (often without regard to plant water requirements) have been implemented. This research investigated gas exchange and growth of two FG maple tree species (Acer × freemanii `Autumn Blaze' and A. truncatum) subjected to three reference evapotranspiration (ETo) irrigation regimes (100%, 60%, and 30% of ETo) in a semi-arid climate. During Spring 2002, nine containerized (11.3 L) trees of each species were field planted in a randomized block design. Each year trees were irrigated through a drip irrigation system. During the first growing season, all trees were irrigated at 100% ETo. Irrigation treatments began Spring of 2003. Gas exchange data (pre-dawn leaf water potential and midday stomatal conductance) were collected during the 2003 and 2004 growing seasons and growth data (shoot elongation, caliper increase, and leaf area) were collected at the end of each growing season. For each species, yearly data indicates irrigation regime influenced gas exchange and growth of these FG trees. However, it is interesting to note gas exchange and growth of these FG maple trees were not necessarily associated with trees receiving the high irrigation treatment. In addition, it appears the influence of irrigation volume on the growth of these FG trees is plant structure and species specific. Our data suggests irrigation of FG trees based upon local ETo measurements and soil surface root area may be a means to conserve irrigation water and produce FG trees with adequate growth. However, continued research on the influence of reduced irrigation on FG tree species is needed.

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Bernd Leinauer, Matteo Serena, and Devesh Singh

irrigation [98% reference evapotranspiration (ET o )] and at moderate irrigation (56% ET o ). Irrigation was applied through a subsurface drip system (DL2000™; Toro, Riverside, CA) operated at 30 psi and installed at a soil depth of 10 cm. Drip lines were

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Ursula K. Schuch and David W. Burger

Twelve species of woody ornamentals were grown in containers in Riverside and Davis, Calif., to determine plant water use and compare crop coefficients (Kc) calculated with reference evapotranspiration (ET) from local weather stations (ETcim) or atmometers (ETatm). Water use, Kcatm, and Kccim differed by species, location, and month of the year. Raphiolepis indica (L.) Lindl., Pittosporum tobira (Thunb.) Ait., Juniperus sabina L., and Photinia ×fraseri Dress. were the highest water users in Riverside and Arctostaphylos densiflora M.S. Bak., Juniperus, Cercis occidentalis Torr., and Pittosporum used the highest amount of water in Davis, when averaged over the 20-month study period. Rhamnus californica Eschsch., Prunus ilicifolia (Nutt.) Walp., and Cercocarpus minutiflorus Abrams. were among the lowest water users in both locations. Although plant water use fluctuated considerably between individual sampling dates, the relative ranking of species water use in each location changed very little over the study period. During periods of high winds, ETcim may not provide an accurate reference for container crops. Kc values fluctuated seasonally from as much as 1 to 4.7 for high water users, while values were stable for low water users and also for Buxus microphylla japonica Rehd. & E.H. Wils., an intermediate water user. During periods of low ET, especially in fall and winter, Kc values were artificially high and failed to correspond to the plants' low water use. Kc values for low water users seem to be useful to estimate water requirements over an extended period of time, whereas general Kc values seem to have limited value for plants with high water demand and need to be modified for different growth stages and growing locations.

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M.J. Hattendorf and J.R. Davenport

Cranberry (Vaccinium macrocarpon Ait.) evapotranspiration (ET) has not been documented. Micrometeorological techniques based on canopy temperature minus air temperature were used to estimate ET on `Stevens' and `Crowley' cranberry at Long Beach (lat. ≈46°20′N, long. 124°W) and Grayland (lat. ≈46°47′N, long. 124°W), Wash., in 1991 and 1992, respectively. Cranberry ET was 55% of Priestley–Taylor reference ET and ranged from <0.5 to >4 mm·d–1. The Priestley–Taylor reference ET was a very good predictor of cranberry ET (r 2 = 0.795). Running 7-day cumulative ET ranged from 7 to 17 mm·week–1.

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Dennis R. Pittenger, A. James Downer, Donald R. Hodel, and Maren Mochizuki

reference plant surface, known as reference evapotranspiration (ET o ), can be used to calculate climate-based estimates of plant water use or need expressed in depth of water per unit of time ( Allen et al., 1998 ). ET o is calculated from prevailing

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Mike Caron and Roger Kjelgren

–2010) daily probability of precipitation and 30-year average daily reference evapotranspiration using the Hargreaves max/min temperature equation ( Hargreaves and Allen, 2003 ). We measured weather during the study period (1996–97) with an automated weather

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Rhuanito S. Ferrarezi, Alan L. Wright, Brian J. Boman, Arnold W. Schumann, Fred G. Gmitter, and Jude W. Grosser

reference evapotranspiration [ET o (B)] inside screen houses and open-air plots. The purpose of screen houses was the exclusion of asian citrus psyllid. Measurements were recorded from 22 Feb. to 31 July 2014 every 30 min and summed for each day, but only