Tree to Grass Water Use Ratios; Tradeoffs in the Urban Landscape.

Water demand in the southwestern United States continues to rise, especially in the Las Vegas Valley, where the population now exceeds 2 million people. It is estimated that 60 percent of all the water used in the valley is used in the residential sector, with 70% of that water used outdoors to irrigate urban landscapes. These landscapes are dominated by trees and turfgrass and although much is known about the water use of turfgrass species, little is known about the water use of landscape trees and therefore little is known about the tradeoffs between grasses and trees in urban landscapes. We are conducting a tree to grass water use ratio study focusing on ten common landscape tree species grown in the valley (mesquite, ash (2), desert willow, oak, palo verde, vitex, locust, elm and crepe myrtle) and four turfgrass species (bermudagrass, bentgrass, tall fescue and ryegrass). We are estimating water use by closing hydrologic balances on the trees (basins) and turfgrass (lysimeters). We are also estimating transpiration of trees using Granier probes and estimating conductive tissue with a novel dye injection system. We will compare water use of all ten tree species with the four turfgrass species and develop models that incorporate reference ET and morphological characteristics such as tree height, canopy volume, basal canopy area, LAI and leaf area. Observations are ongoing.