Modeling Residential Landscape Water and Energy Use to Evaluate Water Conservation Policies

Summertime temperatures have increased 1°F to 2°F per decade in Tucson and Phoenix, Arizona, during the past forty years. At the same time, vegetated land cover in Tucson has diminished by 9 percent, as lush landscapes were replaced with gravel and fewer plants to reduce water and maintenance costs. Water conservation policies and landscape ordinances can exacerbate urban warming by encouraging this replacement of vegetated surfaces with rock. Data derived from computer simulations using three full-sized landscapes around identical buildings in Phoenix and Tucson indicated that these “zeroscapes” were uneconomical compared with “xeriscapes,” which use low-water-use plants located for summer shade. Projected total energy-water savings for the xeriscapes ranged from $117 to $124 (15 to 22 percent) in Phoenix and $38 to $59 (8 to 15 percent) in Tucson, compared with the zeroscapes. “Mesiscapes” in Tucson, which contain large lawns and high-water-use plants, were $106 to $128 (26 to 34 percent) more costly than zeroscapes. However, in Phoenix, evapotranspirational cooling from strategically located turf areas and high-water-use plants could be cost-effective at current utility prices. Landscape ordinance modifications are presented to promote energy conservation and urban heat-island mitigation, as well as water savings.