Water Use of Two Landscape Tree Species in Tucson, Arizona

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. There is little information on the water use and requirements of isolated landscape trees. Most of the information on tree water use in the southwest is based on low-water-use trees recommended by various organizations (e. g., Arizona Municipal Water Users Association, Arizona Native Plant Society, Southern Arizona Water Resources Association) and reported in some literature (Desai, 198 1; McGinnies and Arnold, 1939). Lists of these trees are usually based on empirical observations and the plant’s native habitat, due to the lack of data on actual tree water use. They are also largely based on observations of minimum water requirements and drought survivability, rather than actual water use. Therefore, some drought-resistant tree species on these lists may actually be moderate or high water users when water is nonlimiting. Mesquite (Prosopis var.), for example, is known to be drought resistant, but its actual water use is not known. Methods for estimating water use of isolated trees consist of direct measurement, such as by lysimeter (Fritschen et al., 1977); soil water balance (Grip et al., 1979); chamber (Leuning and Foster, 1990); and heat-pulse or heat-balance (Steinberg et al., 1989) methods, Some models have also been developed for the indirect measure of tree water use based on reference crop evapotranspiration and the tree energy balance (Worthington et al., 1984). Most of the models developed for predicting water use of an isolated tree are based on the Penman–Monteith (PM) equation and require surface and aerodynamic resistance terms, which are often difficult to obtain (Monteith, 1965). There are many studies that use the PM equation to predict isolated tree water use, with varying degrees of success (Denmead, 1984; Landsberg and McMurtie, 1984). Although these methods may accurately estimate water use of r publication 16 June 1994, Accepted for publication 28 Nov. 1994. The lishing this paper was defrayed in part by the payment of page charges, al regulations, this paper therefore must be hereby marked advertiseto indicate this fact. cist. st researcher. r. OC. HORT. SCI. 120(3):409-416. 1995. isolated trees, their complexity and practicality varies considerably. A simple index based on tree species and related to climatological conditions would probably be more useful for homeowners, nurseries, landscape professionals, and the layman, in general. This type of an index has been used for quantifying crop water use. The dimensionless crop coefficient (KC) (Doorenbos and Pruitt, 1977) is calculated as the ratio of actual water use (ETa) to reference evapotranspiration (ETo), where water use is expressed as a depth of water evaporated per time for a well-watered, nonstressed crop: [1] where ETa and ETo have units of mm·day. ETo estimates are meant to approximate the evapotranspiration from an extensive surface of 8 to 15 cm tall green grass cover of uniform height, actively growing, completely shading the ground and not short of water (Doorenbos and Pruitt, 1977). ETo is calculated using either a class A evaporation pan or one of the ETo equations, such as the FAO Penman equation (Doorenbos and Pruitt, 1977). The concept of the dimensionless crop coefficient has been applied to orchards, where tree water use is defined as water use per projected canopy area and the orchard canopy is considered to be a uniform surface. Worthington et al. (1984) reported crop coefficients of 0.71 for peach trees, where water use was measured by weighing lysimeter, and ETo was based on class A evaporation pan data. Tan and Layne ( 198 1) reported coefficients of 0.85 for peach trees, where water use was based on soil moisture measurements and ETo was based on a simplified Priestley Taylor equation. Hoffman et al. ( 1982) reported crop coefficients of 0.75 to 0.85 for mature Valencia orange trees in an arid climate, where water use was based on leaching fraction and ETo was based on class A pan evaporation and several ETo equations. Middleton et al. (1979) reported crop coefficients of 0.02 to 0.2 for apple trees, where water use was measured by weighing lysimeter and ETo was based on class A pan evaporation. Snyder et al. (1989) reported crop