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13The Residential Urban Landscapeas a Frontier for WaterConservationR o l s t o n St . HilaireN e w Me x i c o St a t e UniversityIn the United States, growing populations inevery community will face different water supply anddemand issues. And the urban residential landscapemight be the frontier where water conservation issuescollide. Irrigation is the largest user of freshwater in theUnited States (Hutson et al. 2004). Irrigation used foragricultural and horticultural practices averages 2.48acre-feet per acre (Hutson) and accounts for 81 percentof the consumptive use of water (Solley et al. 1998)in the United States. Traditionally, this consumptiveuse estimate includes freshwater irrigation on crops,parks, golf courses, and other recreational areas. But,increasing urbanization could mean the water used toirrigate the urban landscape will become an increasinglyimportant factor in consumptive water use estimates.Urban landscape irrigation may not necessarily causean increase in fresh water withdrawals from existingsources, but the consumptive water of the urban landscapeas a proportion of the total consumptive use mayincrease. For example, in Arizona, agricultural wateruse is projected to decline, whilst municipal use (eg.residential water use) will double over the next 50 years(Unruh and Liverman 2008).On average, yearly residential water use ranged froma low of 55 gallons per day per person in the temperatemesic state of Wisconsin to a high of 207 gallons perday per person in the arid state of Nevada (Emrath2000). This difference in water use indicates thatclimate-related differences in outdoor water utilizationcontribute significantly to the high water use in aridwestern states (Emrath, 2000). However, landscapeirrigation can average 40 to 70 percent of residentialwater use in the United States (Ferguson 1987), makingit clear the urban landscape commands a significantportion of residential water use.Although total fresh water withdrawals have flatlinedsince 1950 and remained at about 80 percentsurface water and 20 percent ground water (Hutsonet al. 2004), the drivers that are tending to shiftconsumptive water use to the urban landscape areincreasing. Urban and suburban population growth hasdramatically altered the balance between consumptivewater demand and available supply. This is evident inportions of the arid and semiarid regions of the westernUnited States where rapid expansion of urban areasand increases in population have occurred during thelast few decades. Between 2000 and 2008, the westernstates of Nevada, Arizona, and Utah experiencedpercentage population increases of 30, 27, and 22percent, respectively. Although California’s populationgrowth ranked eighteenth among the states, it is themost populous state (US Census Bureau 2008). Thispopulation growth demands creative strategies to satisfythe increased demand for landscape irrigation fromexisting sources. Some of those existing water sourcesare already fully allocated or are rapidly becoming fullyalloted. For example, two decades ago the entire 7.5million acre-foot of water of the lower Colorado Riverbasin (Arizona, California, and Nevada) became fullyallotted for the first time (Unruh and Liverman 2008).That the urban landscape might be a frontier forwater conservation is clearly illustrated with the caseof Nevada. In addition to Nevada having the highestpopulation growth in the United States, the LasVegas Valley in Nevada is one of the fastest growingmetropolitan areas in the United States (Sovocool et al.2006). Together with population growth, the increasingnumber of people who visit Las Vegas because of itstourism and gaming industries has mandated a seriouscommitment to managing the regions water supplyto ensure adequate future supplies (Sovocool 2006).Rolston St. Hilaire is a professor in the Department of Plant and Environmental Sciences, at New Mexico State Universityin Las Cruces, New Mexico. He teaches landscape horticulture and his research focuses on plant stress physiologyand the efficient use of water in urban landscapes.