Tree to Grass Water Use Ratios; Assessing Turfgrass’ High Water Use in the Urban Landscape

Water demand in the southwestern United States continues to rise. The population of the Las Vegas Valley doubled from 2000-2010 and now more than two million people call it home. The residential sector uses 60% of all water consumed in the valley. Outdoor urban landscape irrigation is responsible for 70% of all residential use. These landscapes are dominated by trees and turf grass. Although the water use of turf grass species is well studied, there are few published results about the water use of landscape trees in the desert southwest USA. To obtain a more complete picture of the tradeoffs between grasses and trees in urban landscapes in Southern Nevada, we conducted a tree to grass water use ratio study focusing on 10 common landscape trees and four turf grass species grown in the valley. We estimated water use by closing hydrologic balances (Evapotranspiration=water input-drainage-change in soil water storage) on mature trees planted in the ground and turf grass grown in lysimeters. We estimated transpiration of trees using Granier probes and estimated conductive tissue with a novel dye injection system. Sapflow was lower than the hydrological balance estimated evapotranspiration (ET) because of significant evaporation rates associated with irrigating trees in a desert environment. The values for sapflow ranged from 10 to 50 cm per year. Trees used less water than grass in nine out of 10 cases with an ET 38-88 cm/year determined by a hydrological balance. The exception was Lagerstroemia indica that used 196 cm year-1 which was similar to the grass ET (106-262 cm year-1 ) again determined by hydrological balance. We also developed models that predicted the tree water use based on reference evapotranspiration (ETref) and morphological characteristics such as tree height, canopy volume, basal canopy area, leaf area index (LAI) and leaf area. Replacing turf grass and planting trees can save water, if the right iv species are selected. However, turf grass serves its purpose in many areas by providing aesthetics and recreational use. Water use values are listed to help assist in making landscape tradeoffs