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We compared transpiration estimates of three common desert landscape tree species using stem-flow gauges and lysimetry. Argentine mesquite (Prosopis alba Grisebach), desert willow [Chilopsis linearis (cav.) Sweet var. linearis], and southern live oak (Quercus virginiana Mill., seedling selection) were subjected to three irrigation regimes. Leaching fractions of +0.25, 0.00, and -0.25 were imposed for 2 years. During the summer of the second year, we conducted a comparative transpiration study. Trees growing in 190-liter plastic containers had a highly linear correlation (r = 0.98, P = 0.001) between transpiration estimated by stem-flow gauges and lysimetry. An average 18% error was measured between paired data (total runs of 14 to 72.5 hours) of stem-flow gauge and lysimetry transpiration estimates. However, a lower error was correlated significantly with longer run times (r = -0.37, P = 0.05). Based on field measurements taken in this experiment, run times would have to be >68 hours to maintain an associated error below 10%. Higher cumulative transpiration also was associated with longer run times (r = 0.80, P = 0.001). These results suggest that the stem-flow gauge can be used to estimate transpiration accurately to schedule irrigation for woody ornamental trees in an arid environment, provided that irrigation predictions are not based on short-term stem-flow gauge estimates (<68 hours).

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Although water conservation programs in the arid southwestern United States have prompted prudent landscaping practices such as planting low water use trees, there is little data on the actual water use of most species. The purpose of this study was to determine the actual water use of two common landscape tree species in Tucson, Ariz., and water use coefficients for two tree species based on the crop coefficient concept. Water use of oak (Quercus virginiana `Heritage') and mesquite (Prosopis alba `Colorado') trees in containers was measured from July to October 1991 using a precision balance. Water-use coefficients for each tree species were calculated as the ratio of measured water use per total leaf area or per projected canopy area to reference evapotranspiration obtained from a modified FAO Penman equation. After accounting for tree growth, water-use coefficients on a total leaf area basis were 0.5 and 1.0 for oak and mesquite, respectively, and on a projected canopy area basis were 1.4 and 1.6 for oaks and mesquites, respectively. These coefficients indicate that mesquites (normally considered xeric trees) use more water than oaks (normally considered mesic trees) under nonlimiting conditions.

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A 2-year study was conducted to quantify the actual evapotranspiration (ETa) of three woody ornamental trees placed under three different leaching fractions (LFs). Argentine mesquite (Prosopis alba Grisebach), desert willow [Chilopsis linearis (Cav.) Sweet var. linearis], and southern live oak (Quercus virginiana Mill.) (nursery seedling selection) were planted as 3.8-, 18.9-, or 56.8-liter container nursery stock outdoors in 190-liter plastic lysimeters in which weekly hydrologic balances were maintained. Weekly storage changes were measured with a portable hoist-load cell apparatus. Irrigations were applied to maintain LFs of +0.25, 0.00, or -0.25 (theoretical) based on the equation irrigation (I) = ETa/(1 - LF). Tree height, trunk diameter, canopy volume, leaf area index, total leaf area (oak only) and dry weight were monitored during the experiment or measured at final harvest. Average yearly ETa was significantly influenced by planting size (oak and willow, P ≤ 0.001) and leaching fraction imposed (P ≤ 0.001). Multiple regressions accounting for the variability in average yearly ETa were comprised of different growth and water management variables depending on the species. LF, trunk diameter, and canopy volume accounted for 92% (P ≤ 0.001) of the variability in the average yearly ETa of oak. Monthly ETa data were also evaluated, with multiple regressions based on data from nonwater-deficit trees, such that LF could be ignored. In the case of desert willow, monthly potential ET and trunk diameter accounted for 88% (P ≤ 0.001) of the variability in the monthly ETa. Results suggest that irrigators could apply water to arid urban landscapes more efficiently if irrigations were scheduled based on such information.

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., 2006 ), argentine mesquite ( Prosopis alba ) ( Levitt et al., 1995 ), old man saltbush ( Atriplex nummularia ) ( Glenn et al., 1998 ), and red river gum ( Eucalyptus camaldulensis ) ( Al-Jamal et al., 2002 ). The “feedback” by nursery participants

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