Xeriscape Conversion for Urban Water Conservation - Southern Nevada

1. 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 Conversion
Study (XCS) aimed at quantifying water savings generated through converting typical single
family 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 the
periods before and after implementation of XCS were used in conjunction with specific end use
data collected from all homes during the summer of 1998 to evaluate the impacts of the
Xeriscape study.
During the baseline year (1995) the average outdoor water use patterns in the Turf and Xeriscape
groups were similar, but not identical: 193 kgal for the Turf group and 166 kgal for the Xeriscape
group. During the post Xeriscape year (1997) while the outdoor use of the Turf group increased
slightly (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 Xeriscape
Conversion 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 50
gallons per square foot (gal/sf). During the baseline year the average application of both groups
was 49 gal/sf of irrigated landscape. In the year following the conversion the Xeriscape group
applied less water to both turf and Xeriscape areas. They applied approximately 85% of ET to
turf and 40% of ET to Xeriscape areas. The combination of these rates yielded the 41% overall
savings in water use measured for the group. There was surprisingly little relationship found
between the area Xeriscaped and the water savings, but there was a stronger relationship between
the intensity of the irrigation at each home during the baseline year and the savings achieved by
the Xeriscape conversion. This implies that a significant portion of the savings observed in this
study derived from elimination of wasteful irrigation on the remaining turf areas as well as the
reduced water use in Xeriscape areas. Continuing studies are underway to determine if these
savings are permanent, or if they will decrease with time.
BACKGROUND
In 1996 the Southern Nevada Water Authority, in co-operation with the US Bureau of
Reclamation began a three year study of the effects of Xeriscape on residential water use. This
project was called the Southern Nevada Xeriscape Conversion Study (XCS). The aim of the
XCS is to provide detailed comparisons of water use between a group of 499 Xeriscape
conversion homes (the Xeriscape group) and 253 traditionally landscaped homes (the Turf
group). To participate in the study, each conversion home agreed to convert at least 500 sf of
turf 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 and
separate sub-meters on their irrigation systems. At the start of 1997 it was decided to obtain
more detailed information on water use from a sample of homes using portable data loggers and
1

2. flow trace analysis. The data loggers were to provide flow trace data, which could be used to
identify individual water use events, including both interior fixture and appliance use, and
operation of individual irrigation zone valves. Data of this type was then used to determine
indoor use, turf irrigation and Xeriscape irrigation at each home. The data logger study was
designed to provide an independent investigation of the Xeriscape water savings, and to
demonstrate 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 the
English word landscape. Pronounced (zeer’ scape) the word Xeriscape was coined in 1982 by
i’
Nancy Leavitt, an environmental planner for Denver Water and then trademarked by Denver
Water later in the 1980s. Xeriscape is a systematic concept for saving water in landscaped areas
and refers to an entire system of landscaping which seeks to maximize both the beauty and water
efficiency of urban landscaping through application of a set of design principals. The seven
primary principles of Xeriscape are:
1. Appropriate planning and design
2. Limiting turf areas
3. Efficient irrigation systems
4. Improve the soil
5. Use of mulches, where appropriate
6. Use of drought tolerant plants in three distinct zones – natural, drought tolerant, and oasis zones
7. Appropriate and timely maintenance
Since the popularizing of the seven principles of Xeriscape in 1982, low-water use landscaping
programs have been started in more than 40 states and numerous foreign countries. While a
large number of Xeriscape demonstration projects have been documented until 1997, few
systematic Xeriscape conversion studies have been undertaken. Only a few studies have
documented the conservation effectiveness of these landscape programs, these include Nelson
and 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 the
XCS in 1996 and early 1997. These 102 homes were selected because each had a full year of
water consumption data available after the conversion. The 50 homes chosen to participate in
the study were selected at random. The Turf group was randomly selected from approximately
253 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 1504
square feet of their landscape from traditional turf grass to Xeriscape landscape. This
represented an average of 47% of their irrigable area. Figure 1 shows a distribution diagram of
the 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 truly
scientific assessment of the impacts of Xeriscape for the general population, but the results
should 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 the
savings through paired comparisons of the same two groups before and after the Xeriscape
intervention.

3. Conversion Rate in Logged Homes
45%
Mean = 47%
40%
Median = 45%
35% St. Dev. = 20%
Relative Frequency
30%
25%
20%
15%
10%
5%
0%
20% 40% 60% 80% 100%
Percent of Irrigable Area Xeriscaped
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 each
customer’ water meters to collect continuous flow measurements from each home for a two
s
week period. The Meter Master loggers record the flow through the meter every ten seconds and
thereby 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 included
in the study. During each visit, with the assistance of Authority staff, Aquacraft technicians
installed 50 data loggers on an even mix of 25 Xeriscape homes and 25 Turf homes. The first
logging session was between May 26th and June 10th 1998, and the second session ran from June
15th to June 30th 1998. The loggers were left in place for 15 days so that a complete set of 14
days of data would be collected.
DISAGGREGATION OF WATER USE
The data analysis component of the study had several objectives. The primary objective
was to break down the recorded flow trace data from each home into the individual water use
components, and then to use this information in conjunction with the historic billing data and
household 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 traces
were disaggregated into individual end uses by means of a signal processing program3, Trace
Wizard© . Trace Wizard is a program which first breaks the flow trace data into separate water
use events and then assigns a fixture designation to each event according to its most probable end
use based on its volume, peak flow, duration, and time of day. The classification is checked and
refined by an analyst, a process which takes between 1 and 3 hours per recorded trace. In some
cases, especially when breaking a large event into several small events such as splitting a long
irrigation event into turf and Xeriscape uses, the analysis process requires the analyst to make
manual adjustments to the data in a database program after the Trace Wizard analysis is
complete.

4. Figure 2 shows a combination of turf and xeriscape irrigation in a six hour view. The first two
turf irrigation events are the same four zone irrigation system set to run twice consecutively. The
Xeriscape 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 zones
were turf and which were Xeriscape, which information was obtained as part of the field
verification 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 broken
down in to toilets, shower, dishwasher, clothes washer use etc, and daily outdoor use for turf and
Xeriscape irrigation over the logging period. The indoor use was assumed to remain constant
over the course of the year, which other studies have shown to be true (Mayer and DeOreo
1999). 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, allowed
outdoor 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 the
Turf group increased their average annual outdoor use by 2,000 gallons to 195 kgal. We know
from the customer information data collected by the Authority the average irrigable area on the
Turf group lots averaged 3731 sf . This implies that the Turf application rate of the Turf group
averaged 52 gallons per square foot (gpsf) in 1995.
Table 1: Turf group outdoor water use in 1995 and 1997, t-test results
1995 Baseline Year 1997 Post Xeriscape
Mean annual use (kgal.) 193 195
Sample size/observations 49 49
Hypothesized mean difference 0
Degrees of freedom 48
P(T<=t) two-tail 0.73
Table 2 shows the outdoor use for the Xeriscape group during 1995 and 1997. In 1995 the
Xeriscape group used 166 kgal for outdoor purposes while in 1997, after the conversion of
significant portions of each homes’ landscape to Xeriscape, their average outdoor use decreased
by 66 kgal to 100 kgal. The average irrigable area on the Xeriscape lots was 3631 square feet, so
the application rate of the Xeriscape group was 46 gallons per square foot during the baseline
year, and 28 gallons per square foot after the conversion.

5. Table 2: Xeriscape group outdoor water use in 1995 and 1997, t-test results
1995 Baseline Year 1997 Post Xeriscape
Mean annual use (kgal.) 166 100
Sample size/observations 47 47
Hypothesized mean difference 0
Degrees of freedom 46
P(T<=t) two-tail 0.0000
Figure 3 shows distribution diagrams of the outdoor use of the Xeriscape group before and after
the conversion. The dark line shows the baseline distribution while the light line shows the
distribution after the Xeriscape project (1997). The change in the average use is -66 kgal per
year.
Outdoor Use Distributions 1995 and 1997
Xeriscape Group
45%
40%
35%
Relative Frequency
30%
1995 Baseline
25%
1997 Post-
20% Xeriscape
15%
10% 66 kgal.
5%
0%
50 100 150 200 250 300 350
Outdoor Use (kgal.)
Figure 3: Outdoor use distributions, Xeriscape group
An intriguing aspect of this study was the almost total lack of correlation between reduction in
water use and percent conversion to Xeriscape. If the water savings were due to a strictly
mechanical switch from high to low water use plant material then there should have been a
strong correlation between the percent of the landscape converted and the savings. As shown in
Figure 4 this did not occur. Clearly, there were other factors involved with the reduction in
water 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 degree
to which the customers were over-irrigating their landscapes in the baseline year. Those
customers 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 line
showed less savings.

6. Percent Converted to Xeriscape vs. Water Reduction
Regression Analysis
100%
y = 0.0608x + 0.383
R2 = 0.0029
80%
Percent Outdoor Water Reduction
60%
40%
20%
0%
0% 20% 40% 60% 80% 100% 120%
-20%
Percent of Irrigable Area Converted to Xeriscape
Figure 4: Reduction in outdoor use vs percent conversion to Xeriscape
Savings vs. Previous Water Use
Regression Analysis
200
Xeriscape Water Savings (kgal)
150
100
50
y = 0.4234x + 27.294
2
R = 0.2864
0
Average Turf ET=82.6"
-50
0 50 100 150 200 250 300
Irrigation Application in 1995 (inches)
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 turf
landscape to low water use Xeriscape, a significant reduction in outdoor water use was
observed. Outdoor use declined from 166 to 100 kgal between the baseline and post
conversion year. This is a 41 reduction in total outdoor use. During the same period of time
the outdoor use of the turf group remained unchanged.
2. The water savings associated with the Xeriscape conversion program appear to derive from
both the low water demand of the Xeriscape and the fact that the customers appear to reduce
the 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 irrigating
at 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

7. participating in the project the best results came from the largest users, and these would be
the best group to target.
5. Since we know that a significant amount of the savings were due to changes in irrigation
practice induced through participation in the project, it is of great interest to track the water
use over time to determine the extent to which the savings decline as people either revert to
their old habits.
6. As the plants in the Xeriscape areas mature (and grow) their water requirements would be
expected to increase as well. Tracking applications to the Xeriscape areas will shed light on
how water use increases due to plant growth.