XERISCAPE CONVERSION FOR URBAN WATER CONSERVATION

W.B. DeOreo, P.W. Mayer, J. Rosales1

ABSTRACT

This paper presents the results from the Southern Nevada Water Authority Xeriscape ConversionStudy (XCS) aimed at quantifying water savings generated through converting typical singlefamily turf and shrub landscapes to Xeriscape™ . The study was conducted in Las Vegas,Nevada using 50 home samples from both Xeriscape and Turf homes. Water use data from theperiods before and after implementation of XCS were used in conjunction with specific end usedata collected from all homes during the summer of 1998 to evaluate the impacts of theXeriscape study.

During the baseline year (1995) the average outdoor water use patterns in the Turf and Xeriscapegroups were similar, but not identical: 193 kgal for the Turf group and 166 kgal for the Xeriscapegroup. During the post Xeriscape year (1997) while the outdoor use of the Turf group increasedslightly (from 193 to 195 kgal) the outdoor use of the Xeriscape group declined significantly(from 166 kgal to 100 kgal). The reduction in outdoor use among participants in the XeriscapeConversion Study amounted to a decrease of 66 kgal (41%) in outdoor water use.

The average net ET for turf grass in Las Vegas is 83 inches per year, which is equivalent to 50gallons per square foot (gal/sf). During the baseline year the average application of both groupswas 49 gal/sf of irrigated landscape. In the year following the conversion the Xeriscape groupapplied less water to both turf and Xeriscape areas. They applied approximately 85% of ET toturf and 40% of ET to Xeriscape areas. The combination of these rates yielded the 41% overallsavings in water use measured for the group. There was surprisingly little relationship foundbetween the area Xeriscaped and the water savings, but there was a stronger relationship betweenthe intensity of the irrigation at each home during the baseline year and the savings achieved bythe Xeriscape conversion. This implies that a significant portion of the savings observed in thisstudy derived from elimination of wasteful irrigation on the remaining turf areas as well as thereduced water use in Xeriscape areas. Continuing studies are underway to determine if thesesavings are permanent, or if they will decrease with time.

BACKGROUND

In 1996 the Southern Nevada Water Authority, in co-operation with the US Bureau ofReclamation began a three year study of the effects of Xeriscape on residential water use. Thisproject was called the Southern Nevada Xeriscape Conversion Study (XCS). The aim of theXCS is to provide detailed comparisons of water use between a group of 499 Xeriscapeconversion homes (the Xeriscape group) and 253 traditionally landscaped homes (the Turfgroup). To participate in the study, each conversion home agreed to convert at least 500 sf ofturf to Xeriscape. The XCS is an ongoing study with a projected completion date in 2001.

All 752 homes in the study will be analyzed using monthly data from the mian water meters andseparate sub-meters on their irrigation systems. At the start of 1997 it was decided to obtainmore detailed information on water use from a sample of homes using portable data loggers and 1

flow trace analysis. The data loggers were to provide flow trace data, which could be used toidentify individual water use events, including both interior fixture and appliance use, andoperation of individual irrigation zone valves. Data of this type was then used to determineindoor use, turf irrigation and Xeriscape irrigation at each home. The data logger study wasdesigned to provide an independent investigation of the Xeriscape water savings, and todemonstrate a way of assessing the impacts of the Xeriscape conversion without relying on sub-meters.

WHAT IS XERISCAPE?

The term Xeriscape comes from a combination of the Greek word “xeros” meaning dry and theEnglish word landscape. Pronounced (zeer’i’scape) the word Xeriscape was coined in 1982 byNancy Leavitt, an environmental planner for Denver Water and then trademarked by DenverWater later in the 1980s. Xeriscape is a systematic concept for saving water in landscaped areasand refers to an entire system of landscaping which seeks to maximize both the beauty and waterefficiency of urban landscaping through application of a set of design principals. The sevenprimary principles of Xeriscape are:1. Appropriate planning and design2. Limiting turf areas3. Efficient irrigation systems4. Improve the soil5. Use of mulches, where appropriate6. Use of drought tolerant plants in three distinct zones – natural, drought tolerant, and oasis zones7. Appropriate and timely maintenance

Since the popularizing of the seven principles of Xeriscape in 1982, low-water use landscapingprograms have been started in more than 40 states and numerous foreign countries. While alarge number of Xeriscape demonstration projects have been documented until 1997, fewsystematic Xeriscape conversion studies have been undertaken. Only a few studies havedocumented the conservation effectiveness of these landscape programs, these include Nelsonand Kruta (1994), and Gregg et. al. (1994).

METHODOLOGY

Study Group Selection

The Xeriscape group was selected from the population of 102 homes which participated in theXCS in 1996 and early 1997. These 102 homes were selected because each had a full year ofwater consumption data available after the conversion. The 50 homes chosen to participate inthe study were selected at random. The Turf group was randomly selected from approximately253 accounts which agreed to participate in the ongoing monitoring program as part of the XCS,but did not convert any portion of their landscape to Xeriscape.

The homes selected for logging from the Xeriscape group had converted an average of 1504square feet of their landscape from traditional turf grass to Xeriscape landscape. Thisrepresented an average of 47% of their irrigable area. Figure 1 shows a distribution diagram ofthe percentages of the irrigable areas on each lot that were converted as part of the project.

Because the selection process involved use of volunteers this study can not be considered a trulyscientific assessment of the impacts of Xeriscape for the general population, but the resultsshould be indicative of the water savings achievable from this water conservation technique.The methodology devised for this study sought to obtain the best possible estimates of thesavings through paired comparisons of the same two groups before and after the Xeriscapeintervention.

Conversion Rate in Logged Homes

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Mean = 47%Median = 45%St. Dev. = 20%

Figure 1: Percent of irrigable areas converted to Xeriscape

DATA COLLECTION

A key element of the study was the use of portable data loggers2, which attach to eachcustomer’s water meters to collect continuous flow measurements from each home for a twoweek period. The Meter Master loggers record the flow through the meter every ten seconds andthereby produce what is essentially a continuous record of water use over the logging period,which is referred to as a “flow trace”.

Aquacraft Inc made two visits to Southern Nevada to collect data from the 100 homes includedin the study. During each visit, with the assistance of Authority staff, Aquacraft techniciansinstalled 50 data loggers on an even mix of 25 Xeriscape homes and 25 Turf homes. The firstlogging session was between May 26th and June 10th 1998, and the second session ran from June15th to June 30th 1998. The loggers were left in place for 15 days so that a complete set of 14days of data would be collected.

DISAGGREGATION OF WATER USE

The data analysis component of the study had several objectives. The primary objectivewas to break down the recorded flow trace data from each home into the individual water usecomponents, and then to use this information in conjunction with the historic billing data andhousehold level data to provide an assessment of the impacts of the Xeriscape Conversion Study.The end use analysis provided the foundation for all other analyses in the study.

After downloading each data logger and verifying the accuracy of the flow trace, the flow traceswere disaggregated into individual end uses by means of a signal processing program3, TraceWizard . Trace Wizard is a program which first breaks the flow trace data into separate wateruse events and then assigns a fixture designation to each event according to its most probable enduse based on its volume, peak flow, duration, and time of day. The classification is checked andrefined by an analyst, a process which takes between 1 and 3 hours per recorded trace. In somecases, especially when breaking a large event into several small events such as splitting a longirrigation event into turf and Xeriscape uses, the analysis process requires the analyst to makemanual adjustments to the data in a database program after the Trace Wizard analysis iscomplete.

Figure 2 shows a combination of turf and xeriscape irrigation in a six hour view. The first twoturf irrigation events are the same four zone irrigation system set to run twice consecutively. TheXeriscape irrigation, which follows, is a two zone system which continues for nearly four hours.The analysis was simplified by knowing how the clocks were programmed and which zoneswere turf and which were Xeriscape, which information was obtained as part of the fieldverification process.

Figure 2: Turf irrigation and Xeriscape irrigation and misc. toilets and faucets

The result of the flow trace analysis provided average daily indoor use for each house brokendown in to toilets, shower, dishwasher, clothes washer use etc, and daily outdoor use for turf andXeriscape irrigation over the logging period. The indoor use was assumed to remain constantover the course of the year, which other studies have shown to be true (Mayer and DeOreo1999). The outdoor use was used only to compare application rates during the logging period,since this varies significantly over the year. Knowing indoor use accurately, however, allowedoutdoor use to be calculated from the billing data by subtraction.

RESULTS

Table 1 shows that in 1995 the outdoor use of the Turf Group averaged 193 kgal. and in 1997 theTurf group increased their average annual outdoor use by 2,000 gallons to 195 kgal. We knowfrom the customer information data collected by the Authority the average irrigable area on theTurf group lots averaged 3731 sf . This implies that the Turf application rate of the Turf groupaveraged 52 gallons per square foot (gpsf) in 1995.

Table 1: Turf group outdoor water use in 1995 and 1997, t-test results1995 Baseline Year 1997 Post Xeriscape

Mean annual use (kgal.) 193 195Sample size/observations 49 49Hypothesized mean difference 0Degrees of freedom 48P(T<=t) two-tail 0.73

Table 2 shows the outdoor use for the Xeriscape group during 1995 and 1997. In 1995 theXeriscape group used 166 kgal for outdoor purposes while in 1997, after the conversion ofsignificant portions of each homes’ landscape to Xeriscape, their average outdoor use decreasedby 66 kgal to 100 kgal. The average irrigable area on the Xeriscape lots was 3631 square feet, sothe application rate of the Xeriscape group was 46 gallons per square foot during the baselineyear, and 28 gallons per square foot after the conversion.

Table 2: Xeriscape group outdoor water use in 1995 and 1997, t-test results1995 Baseline Year 1997 Post Xeriscape

Mean annual use (kgal.) 166 100Sample size/observations 47 47Hypothesized mean difference 0Degrees of freedom 46P(T<=t) two-tail 0.0000

Figure 3 shows distribution diagrams of the outdoor use of the Xeriscape group before and afterthe conversion. The dark line shows the baseline distribution while the light line shows thedistribution after the Xeriscape project (1997). The change in the average use is -66 kgal peryear.

Outdoor Use Distributions 1995 and 1997 Xeriscape Group

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Figure 3: Outdoor use distributions, Xeriscape group

An intriguing aspect of this study was the almost total lack of correlation between reduction inwater use and percent conversion to Xeriscape. If the water savings were due to a strictlymechanical switch from high to low water use plant material then there should have been astrong correlation between the percent of the landscape converted and the savings. As shown inFigure 4 this did not occur. Clearly, there were other factors involved with the reduction inwater use observed in this group. An indication of one of these factors is shown in Figure 5,which shows that there is a clear correlation between the amount of water saved and the degreeto which the customers were over-irrigating their landscapes in the baseline year. Thosecustomers who were farthers above the ET line (shown as a vertical line at 82” in Figure 5)showed the largest savings, while the customers who were applying at or below the ET lineshowed less savings.

Percent Converted to Xeriscape vs. Water ReductionRegression Analysis

y = 0.0608x + 0.383R2 = 0.0029

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Figure 4: Reduction in outdoor use vs percent conversion to Xeriscape

Savings vs. Previous Water UseRegression Analysis

y = 0.4234x + 27.294R2 = 0.2864

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Figure 5: Water savings vs. Pre-Xeriscape application rate

CONCLUSIONS

1. In this group of single family customers, who converted an average of 1500 sf of turflandscape to low water use Xeriscape, a significant reduction in outdoor water use wasobserved. Outdoor use declined from 166 to 100 kgal between the baseline and postconversion year. This is a 41 reduction in total outdoor use. During the same period of timethe outdoor use of the turf group remained unchanged.

2. The water savings associated with the Xeriscape conversion program appear to derive fromboth the low water demand of the Xeriscape and the fact that the customers appear to reducethe amount of over-irrigation of their standard landscapes as well.

3. On average the single family customers in both the Xeriscape and Turf groups were irrigatingat the net ET level prior to the intervention. Their average application was 49 gal/sf,compared to the net ET of 50 gal/sf.

4. There is a wide distribution in the use patterns, and some customers use far less than 50 gal/sf

participating in the project the best results came from the largest users, and these would bethe best group to target.

5. Since we know that a significant amount of the savings were due to changes in irrigationpractice induced through participation in the project, it is of great interest to track the wateruse over time to determine the extent to which the savings decline as people either revert totheir old habits.

6. As the plants in the Xeriscape areas mature (and grow) their water requirements would beexpected to increase as well. Tracking applications to the Xeriscape areas will shed light onhow water use increases due to plant growth.