The X Factor

In the Mojave Desert of the Southwestern United States, typically 60 to 90 percent of potable water drawn by single-family residences is used for outdoor irrigation. Thus, in this region, conservation measures focus on reducing outdoor water consumption, one way of which is to use xeriscaping in place of traditional turf. Four years of research into the cost and water savings generated by this type of conversion shows some promise.

THE STUDY. The Southern Nevada Water Authority (SNWA) wants to achieve 25 percent water conservation by the year 2010 since southern Nevada receives only about 4 inches of precipitation annually but the local ET turfgrass water requirement is 90 inches. Landscape conversions are a possible way to help reach this goal.

The water savings potential of xeriscaping led the SNWA and the U.S. Bureau of Reclamation – Lower Colorado Regional Authority to study a residential landscape conversion program, starting in 1995.

The Xeriscape Conversion Study drew from participants who dwell in single-family residences within four different southern Nevada water jurisdictions. Three groups make up the study – the Xeriscape (XS) group, the Turf (TS) group and a non-contacted comparison group.

The XS group is made up of residents who converted at least 500 square feet of traditional turfgrass to xeriscape. In this region, xeric landscape is principally composed of a combination of desert-adapted shrubs, trees, ornamental grasses and mulch. Homeowners were required to plant sufficient vegetation so that the new landscape would have a minimum 50 percent canopy coverage. This avoided the creation of unattractive "zero-scapes" composed exclusively of rocks. The average area converted was 2,160 square feet. A total of 499 properties were enrolled.

The TS group represents properties with traditional design where an average of 2,462 square feet of the landscaped area was turf. A total of 253 residences were recruited.

SNWA and partner agencies monitored each site’s water consumption at the main meter and by a submeter, which monitored per unit area irrigation consumption on a portion of their landscape. Both meters were read monthly. All study participants have in-ground irrigation systems and controllers.

THE RESULTS. Four years of data have revealed a number of findings. Converting a portion of traditional turf to xeriscape reduces total water consumption for residences. The average reduction was 33 percent (8,800 gallons per month or 105,600 gallons annually) for residences that converted an average of 2,160 square feet.

Properties with at least 60 percent of their landscapeable area as xeric zones realized, on average, a one-third reduction in landscape maintenance, both in hours (2.2 per month) and direct dollars ($206 per year) as compared to residences with an equivalent relative amount of turf.

On a per unit area basis, annual water consumption was lower for xeriscape than traditional turf. On average, water consumption for turf areas (79.2 gallons per square foot per year) is four to five times as high as for xeriscape (17.3 gallons per square foot per year).

The results of this study support the conclusion that residential xeriscape promotion is an efficient means for obtaining water conservation in areas where total household consumption is dominated by outdoor irrigation.

The authors are Conservation Programs Coordinator and Senior Conservation Programs Analyst, SNWA, Las Vegas, Nev.