Measure 4: Swimming Pool Water Use Analysis by Observed Data and Long term Continuous Simulation

Full text

Measure 4: Swimming Pool Water Use
Analysis by Observed Data and Long-
term Continuous Simulation
April 1, 2008
Joong Gwang Lee and James P. Heaney
Conserve Florida Water Clearinghouse
Dept. of Environmental Engineering Sciences,
University of Florida

ii
Table of Contents
Introduction ................................................................................................................... 1
General Statistics of Swimming Pools ........................................................................ 1
Swimming Pool Dimensions and Water Use .............................................................. 2
Water Use Components in a Swimming Pool ............................................................. 6
Water Balance Model for Analyzing Water Use by a Swimming Pool ...................... 8
Water Balance Fundamentals for a Swimming Pool .................................................... 8
Developed Water Balance Model ........................................................................... 10
Application of the Water Balance Model .................................................................... 11
Applied climate data ............................................................................................... 11
Dimension of the pool and initial assumptions ....................................................... 11
Results of the long-term daily water balance modeling .......................................... 12
Swimming Pool Operating Strategy and Makeup Water Use .................................... 15
Makeup water level and operating range ............................................................... 15
Effectiveness of pool covers .................................................................................. 17
Effectiveness of pool screens ................................................................................ 19
Summary and Conclusions ........................................................................................ 20
References ................................................................................................................... 22

iii
Tables
Table 1. Swimming Pool Statistics ............................................................................................. 1
Table 2. Percent of 7,200 single family homes in Florida with swimming pools .......................... 2
Table 3. Average dimensions for a swimming pool based on the observed data ........................ 3
Table 4. Water Use for Backwashing Water Filters in a Swimming Pool .................................... 7
Table 5. Annual Averages of Itemized Water Uses (1999 to 2006) ...........................................15
Table 6. Annual Averages of Itemized Water Uses with Pool Covers (1999 to 2006) ................18
Table 7. Annual Averages of Itemized Water Uses with Pool Screens (1999 to 2006) ..............19
Figures
Figure 1. Average Swimming Pool Length (mean and standard deviation) ................................. 3
Figure 2. Average Swimming Pool Width (mean and standard deviation) .................................. 4
Figure 3. Average Swimming Pool Depth (mean and standard deviation) .................................. 4
Figure 4. Average Daily Water Uses for a Swimming Pool (mean and standard deviation) ........ 5
Figure 5. Estimation of Swimming Pool Water Uses including Pool Decks ................................. 5
Figure 6. Water Balance in a Swimming Pool ............................................................................ 8
Figure 7. Daily Water Balance Model for a Swimming Pool in Tampa, Florida ..........................10
Figure 8. Annual Precipitation and ET in Tampa Bay, Florida (1999-2006) ...............................11
Figure 9. Daily time-series of the Continuous Modeling Results (1999 to 2006) ........................13
Figure 10. Annual Precipitation and Makeup Water Use ...........................................................14
Figure 11. Changes in Annual Makeup Water Use by Adjusting Makeup Water Levels ............15
Figure 12. Changes in Annual Makeup Water Use by Applying Different Operating Ranges.....16
Figure 13. Changes in Annual Makeup Water Use by Adjusting Operating Ranges and Makeup
Ranges ..............................................................................................................................17
Figure 14. Annual Precipitation and Makeup Water Use with and without Pool Covers .............18
Figure 15. Makeup Water Uses based on the Performance of Pool Covers ..............................19
Figure 16. Makeup Water Uses based on the Performance of Pool Screens ............................20

1
Introduction
Irrigation and recreational water use are the two major domestic outdoor water uses. Water use
from swimming pools should have a clear signature because swimming pools are a significant
user of water and energy. Water use analyses for a swimming pool have not been done with
respect to the entire spectrum of climate conditions, e.g., swimming pool water uses in wet, dry,
or average years. In this report, general information on swimming pool statistics and monitored
data on existing pool dimensions were summarized. Regional swimming pool water use patterns
were investigated using a high-frequency observed data on residential water uses. A long-term
continuous simulation model for analyzing swimming pool water use has been developed in this
study. Eight years of daily precipitation and evapotranspiration (ET) data, which had been
observed in Tampa Bay, Florida, were applied to the developed model to evaluate swimming
water uses with long-term perspectives. There are couples of wet, dry, and average years during
the eight years of simulation. The developed model was also applied in examining the
effectiveness of pool covers and screens.
General Statistics of Swimming Pools
There are more than 8.5 million swimming pools reported in the contiguous 48 States and
Washington, D.C. of the United States (The Association of Pool & Spa Professionals, 2007b) as
shown in Table 1. A total of 4.8 million of these pools are in-ground. In 2006, a total of about
389,000 new swimming pools were installed within the same extent, which is over 2.5 percent
increase in the number of pools compared to the previous year. Florida has about 735,000
swimming pools, which constitute 15 percent of the U.S. market. More details on swimming
pool statistics are shown in Table 1.
Table 1. Swimming Pool Statistics (The Association of Pool & Spa Professionals, 2007b)
In ground
Above ground
Total Units 1)
4,847,380
3,724,950
Units sold in 2006
166,750
222,000
Total change in installed base
2.79%
2.62%
Top 5 States
California
1,089,157
New York
394,840
Florida
734,702
California
350,598
Texas
444,691
New Jersey
338,740
Arizona
290,431
Pennsylvania
240,499
New York
243,111
Illinois
182,173
1) Contiguous 48 States and Washington, D.C.
Whitcomb (2005) estimates that there are about 3.9 million single family homes in Florida.
Assuming that the vast majority of pools are in single family homes, then about 15 to 18 percent
of Florida homes would have pools. Whitcomb (2005) did an extensive study of water use by
7,200 randomly selected single family homes in Florida. An average of nearly 37 percent of
these homes have swimming pools as shown in Table 2. The percentage with swimming pools

2
ranges from as low of 11 percent in Tallahassee to 55 percent in Palm Coast. As expected, the
percentage of homes with swimming pools increases as income increases with 14 percent of the
homes in the lowest quartile of incomes having pools to 60 percent of the homes in the highest
income quartile having pools.
Table 2. Percent of 7,200 single family homes in Florida with swimming pools (adapted from
Whitcomb (2005)
By Utility Location
By Income Level
Utility
Pools
Utility
Pools
Utility
Pools
Income
Pools
Lakeland
22%
Hillsborough
52%
Spring Hill
54%
Quartile 1
14%
Melbourne
45%
Indian River
40%
Toho
15%
Quartile 2
30%
Ocoee
37%
Miami Dade
19%
Quartile 3
43%
St. Petersburg
40%
Palm Beach
35%
Average
36.7%
Quartile 4
60%
Tallahassee
11%
Palm Coast
55%
Maximum
55%
Tampa
28%
Sarasota
41%
Minimum
11%
Escambia
23%
Seminole
49%
Based on the above statistics, it is clear that swimming pools are quite popular in Florida and it is
important to evaluate their water use patterns.
Swimming Pool Dimensions and Water Use
Swimming pool dimensions and water use patterns have been analyzed using an existing high-
frequency residential water use database from the Residential End Uses of Water Study (Mayer
et al. 1999). Mayer et al. (1998) collected detailed data on the end uses of water in single-family
residential settings across the country during late 1990’s. The database includes indoor and
outdoor water uses at a total of 1,188 households from 12 nationwide locations (approximately
100 per study site). A total of 943 swimming pools were reported in this database with their
basic dimensions (length, width, and depth) and monitored daily water uses, which were based
on high frequency monitoring data for the pools. Averages of swimming pool dimensions are
presented in Table 3 and Figures 1 through 3. Variations of the measurements are shown in the
same figures as red lines based on plus and minus of standard deviations from the averages. This
analysis shows that the average dimension of a swimming pool is about 30.1 feet long, 15.7 feet
wide and 5.6 feet deep.

3
Table 3. Average dimensions for a swimming pool based on the observed data
Study site
Pools
Length
Width
Depth
Area
Water use
(#)
(ft)
(ft)
(ft)
(ft2)
(in/ft2/d)
Tampa, FL
59
27.2
15.3
5.2
418
0.61
Phoenix, AZ
161
29.5
15.5
5.6
479
0.34
Scottsdale & Tempe, AZ
267
30.1
15.4
5.5
480
0.87
Walnut Valley WD, CA
107
29.7
16.0
6.1
477
0.44
Las Virgenes MWD, CA
198
32.4
16.2
5.7
545
0.66
All Swimming Pools*
943
30.1
15.7
5.6
489
0.61
* All swimming pools from the REUWS database, which are 4 from Lompoc, CA; 6 from Boulder, CO; 7 from
Denver, CO; 15 from Eugene, OR; 15 from Seattle, WA; 49 Waterloo and Cambridge, AZ; and 55 San Diego, CA
in addition to 792 from the above 5 sites.
010 20 30 40 50
Tampa, FL
Phoenix, AZ
Scottsdale & Tempe, AZ
Walnut Valley WD, CA
Las Virgenes MWD, CA
All Swimming Pools*
Length (ft)
Figure 1. Average Swimming Pool Length (mean and standard deviation)

4
0 5 10 15 20 25
Tampa, FL
Phoenix, AZ
Scottsdale & Tempe, AZ
Walnut Valley WD, CA
Las Virgenes MWD, CA
All Swimming Pools*
Width (ft)
Figure 2. Average Swimming Pool Width (mean and standard deviation)
0 1 2 3 4 5 6 7 8 9
Tampa, FL
Phoenix, AZ
Scottsdale & Tempe, AZ
Walnut Valley WD, CA
Las Virgenes MWD, CA
All Swimming Pools*
Depth (ft)
Figure 3. Average Swimming Pool Depth (mean and standard deviation)
Average water uses per unit area of a swimming pool are presented in Table 1. The nationwide
average of swimming water use is 0.61 in/ft2 per day. This average water use is just based on the
swimming pool surface area. The average water uses for different locations are also shown in
Figure 4 with standard deviations as red lines. Compared to the pool dimensions, the water use
patterns indicate more variation by location and wider ranges of water uses at the same location.

5
0 0.5 1 1.5 2 2.5 3
Tampa, FL
Phoenix, AZ
Scottsdale & Tempe, AZ
Walnut Valley WD, CA
Las Virgenes MWD, CA
All Swimming Pools*
Water use (in/ft2/d)
Figure 4. Average Daily Water Uses for a Swimming Pool (mean and standard deviation)
In-ground pools come in a wide variety of sizes and geometries. In addition to the pool itself,
there is a deck surrounding the pool. Thus, the overall area is the sum of the pool itself and the
associated decking. If this overall area with decks were applied into the estimation of average
water uses per unit area, the actual swimming pool water use per unit area will be reduced as
shown in Figure 5. Unit area water uses for a swimming pool were estimated by applying
assumed decking widths around the swimming pool. If there are about 10 foot wide decks
around the pool, the actual average water use per unit area is decreased by a quarter of the initial
estimation, which is just based on the pool water surface area.
0
0.1
0.2
0.3
0.4
0.5
0.6
0246810 12
Deck width (ft)
Water use (in/ft 2/d)
Figure 5. Estimation of Swimming Pool Water Uses including Pool Decks

6
Water Use Components in a Swimming Pool
Water use in a swimming pool can be categorized into four major components:
1. initial filling water
2. makeup water for maintaining the water level within an operating range
3. backwash water for cleaning water filters, and
4. cleaning water for pool decks and structures.
The initial filling of the pool is typically a one time process. Pools in Florida are not usually
drained because the weight of the water is needed to compensate for possible pool uplift due to
the local ground water. Makeup water compensates for water loss by evaporation, leakage, and
splash-out. Evaporative loss can be estimated using local climatic data. Leakage can be
estimated from abnormal water use meter readings. Splash out and cleaning water are more
difficult to estimate but should be a minor item in the water budget.
Evaporation loss can be estimated by knowing the local evaporation rate and swimming pool
surface area. In nature, the evaporation rate from a free standing water surface depends on air
temperature, humidity, water temperature, wind velocity, shadowiness, etc. In a swimming pool,
evaporation rate is also affected by pool covering, pool water heating (gas/electric or solar), pool
style (standard or negative edge), and number of water features installed (e.g., fan jets, spa
spillway, sheer descents waterfalls, etc.). If there is a pool cover, evaporation loss could be
reduced by 30 to 95 percent – e.g., 30 percent (Maddaus and Mayer, 2001), almost 50 percent
(The Association of Pool & Spa Professionals, 2007a), or by 90 to 95 percent (Marin Municipal
Water District, 2007). Average evaporation loss from a swimming pool around Tampa Bay,
Florida was reported to range from 1/8 to 1/2 inches per day (Valet Pool Service Corporation,
2007).
Water leaks from a swimming pool depend on many variables with site-specific conditions.
Water leaks may happen in any swimming pool, even a new one, because of poor plumbing
and/or a structural failure. Splash-out water depends on human behavior and swimming pool
arrangement, e.g., diving. There are virtually no data reported on water losses by leakage and
splash-out from a swimming pool except on a case-by-case evaluation.
Backwash water for cleaning water filters depends on the manufacturer of water filters. The
limited amount of reported data are summarized in Table 4. As shown in the table, water use for
backwashing filters varies widely. If the amounts of water uses in Table 3 are normalized by
water use per month, it could be from 70 gallons to 1,300 gallons per month during a swimming
season. The number of months for a swimming season varies regionally. Thus, the total amount
of backwash water usage depends on type/specification of the water filter, number of months
during a swimming season, and maintenance practices.

7
Table 4. Water Use for Backwashing Water Filters in a Swimming Pool
Reference
Water Use
Frequency
APSP 1)
50 to 300 gallons
2 weeks
CUWCC 2)
250 to 1,000 gallons
3 to 4 months
MMWD 3)
500 gallons
n/a
1) The Association of Pool & Spa Professionals
2) California Urban Water Conservation Council, Pool & Spa
3) Marin Municipal Water District
Maddaus and Mayer (2001) estimated that homes with a swimming pool use about 58 percent
more water outdoors than homes without a swimming pool. However, a swimming pool might
use less water than the same size of automatically irrigated turf grass. Analysis by the City of
Sacramento shows that lawn irrigation use equals 49 inches a year while pool use is 20 inches a
year (Bash, 200?; The Association of Pool & Spa Professionals, 2007a). This analysis indicates
that a swimming pool might use substantially less than the same area developed in lawn and/or
landscape. The water use difference between a swimming pool including the deck area and the
same area of lawn/landscape area depends on local climate, the type of irrigation system as well
as how the pool and irrigation system are operated.
A number of factors can affect the amount of water used to fill and maintain the desired water
level in a swimming pool. These factors for swimming pool water usage can be summarized as
follows:
 Size of the pool (surface area and depth)
 Local climate: Precipitation, evaporation (air and water temperatures, wind, humidity,
shadowiness, etc.)
 Condition of the pool: Presence and use of a pool cover, temperature of pool water,
presence of water features like a fountain or waterfall, pH and chemical content of pool
water, and leakage
 Individual maintenance trends: Frequency of backwashing, frequency and method of pool
and pool deck cleaning
 Human behavior: Splashing-out, and swimming habits
Some of the above factors can be controlled to reduce the total water use by a swimming pool.
In order to estimate average and peak water use patterns and water conservation strategies based
on long-term perspectives, a continuous simulation model for a swimming pool has been
developed. Locally monitored long-term climate data can be applied in this model. The
following sections describe the developed model in detail and present long-term water use
analysis results based on ranges of swimming pool operating strategies.

8
Water Balance Model for Analyzing Water Use by a Swimming Pool
Water Balance Fundamentals for a Swimming Pool
In order to perform a long-term water use analysis for a swimming pool, a continuous simulation
model has been developed using a daily time step. This model is based on the conservation of
mass, i.e., water. The water balance for the swimming pool is checked at every time-step to
estimate overall water use throughout the time span of the applied time-series data. There are
two main inputs, which are makeup water and direct precipitation on the swimming pool water
surface, and four main outputs, which are evaporation, backwash, splash-out, and leaks as shown
in Figure 6. The main water balance can be summarized as follows:
∆ Water = Inputs – Outputs (1)
Inputs = Precipitation + Makeup (2)
Outputs = Evaporation + Backwash + Splash-out + Leaks (3)
Where, ∆ Water is change in water level at a swimming pool during a time-step.
Figure 6. Water Balance in a Swimming Pool
Daily precipitation data can be obtained from locally monitored climate time-series data in most
regions. Time series of natural evaporation rate from free standing water are available in some
areas. If time series of typical climate data such as solar radiation, air temperature, humidity,
water temperature, and wind speed are available, the evaporation rate can be estimated using the
data set. Otherwise, the evaporation rate can be estimated using more commonly available
regional evapotranspiration (ET) data. Brouwer and Heibloem (1986) explain how to convert
evaporation rate of free standing water to ET rate for a reference crop as follow:
ET = Kpan (Evaporation) (4)
Length
Width
Depth1
Depth2
Precipitation
Evaporation
Leaks
Backwash
Splash-out
Overflow
Makeup

9
Where, Kpan is pan coefficient.
They explained the above equation based on two types of commonly used evaporation pans:
Class A evaporation pan and Sunken Colorado pan. The average pan coefficient of the Class A
evaporation pan is 0.7; and that of the Sunken Colorado pan is 0.8 (Brouwer and Heibloem,
1986).
Kadlec and Knight (1996) presented a more comprehensive equation for the relation of ET and
evaporation as follows:
ET = 0.755 ∙ (Evaporation) ∙ CT ∙ CW ∙ CH ∙ CS (5)
CT =
2
20
T
041.0
20
T
179.0862.0 













CW =
2
86.1
W
051.0
86.1
W
24.0189.1 













CH =
2
60
H
119.0
60
H
62.0499.0 













CS =
2
80
S
088.0
80
S
008.0904.0 













Where, CT = temperature coefficient; CW = wind coefficient; CH = humidity coefficient;
CS = sunshine coefficient; T = temperature (ºC); W = wind speed (m/s); H = relative
humidity (%); and S = percentage of possible sunshine (%).
Using one of the above two equations, it is possible to estimate the evaporation rate from free
standing water, like a swimming pool, based on regionally available ET rates.
Once the amounts of precipitation and evaporation are known, the makeup water can be
estimated. The allowable operating range of water levels varies between the minimum and
maximum water levels for proper swimming pool operation. When the water level drops below
the minimum level, the makeup water fills the pool up to the pre-specified level. This makeup
level must be between the minimum and maximum levels. When the water level exceeds the
maximum level, the extra water will overflow out of the swimming pool. These relationships
can be summarized as follows:
Level0i = Leveli-1 + Precipitation – Evaporation (6)
Overflow = Level0 – Levelmax (When Level0 > Levelmax) (7)
Makeup = Levelmakeup - Level0 (When Level0 < Levelmin) (8)
Level = Level0 – Overflow + Makeup (9)

10
Where, Level0 = artificial water level before overflow or makeup; Level = swimming
pool water level; Levelmax = maximum allowable water level; Levelmin = minimum
allowable water level; i = current time-step; and i-1 = previous time-step.
Backwash water use is done following a periodic schedule. The amount of water used for
compensating for splash-out and leaks is hard to estimate. For the initial version of this model,
these terms will be considered to be zero.
Developed Water Balance Model
The daily water balance model for a swimming pool that has been developed using MS-Excel
spreadsheets is illustrated in Figure 7. Detailed column-by-column descriptions are presented
next for an illustrative application using climatic data for Tampa Bay, Florida.
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
Date
Prcp
ET
Evaporation
Level0
Overflow
Makeup
Level
(in)
(in)
(in)
(in)
(in)
(in)
(in)
0
1/1/1999
0
0.074
0.098
-0.098
0.000
0.000
-0.098
1/2/1999
0.99
0.101
0.135
0.757
0.000
0.000
0.757
1/3/1999
0.19
0.070
0.094
0.853
0.000
0.000
0.853
1/4/1999
0
0.048
0.064
0.789
0.000
0.000
0.789
1/5/1999
0
0.045
0.060
0.730
0.000
0.000
0.730
1/6/1999
0
0.051
0.068
0.662
0.000
0.000
0.662
1/7/1999
0
0.064
0.085
0.576
0.000
0.000
0.576
1/8/1999
0
0.073
0.097
0.480
0.000
0.000
0.480
∙∙∙
∙∙∙
∙∙∙
∙∙∙
∙∙∙
∙∙∙
∙∙∙
∙∙∙
12/26/2006
0.03
0.048
0.064
-0.024
0.000
0.000
-0.024
12/27/2006
0
0.034
0.046
-0.070
0.000
0.000
-0.070
12/28/2006
0
0.048
0.064
-0.134
0.000
0.000
-0.134
12/29/2006
0
0.058
0.078
-0.211
0.000
0.000
-0.211
12/30/2006
0
0.071
0.095
-0.306
0.000
0.000
-0.306
12/31/2006
0
0.068
0.091
-0.397
0.000
0.000
-0.397
Figure 7. Daily Water Balance Model for a Swimming Pool in Tampa, Florida
Values in columns [1], [2], and [3] were obtained from locally monitored data for the Tampa Bay
area in Florida. Evaporation in column [4] was estimated from ET in column [3] using Equation
(4). Pan coefficient (Kpan) was assumed as 0.75 in this example. Level0 in column [5] is the
water level before accounting for operating adjustments. The values in this column are
represented water levels before either overflow or makeup. Overflow, Makeup, and Level in
columns [6], [7], and [8] were estimated by Equations (7), (8), and (9) respectively. In this
example, operating range was assumed as 6 inches and makeup water fills the pool up to the
middle (50 percent) of the operating range when water level goes below the minimum level.
Values in column [8] represent actual water levels throughout the simulation period. Water
levels in columns [5] and [8] are expressed as relative differences based on the makeup level,
which is expressed as 0 in this model. Thus, positive values mean above the makeup level and

11
negative values mean below the makeup level. The initial water level at the beginning of the
modeling was assumed as 0, which is the exact makeup level.
Application of the Water Balance Model
Applied climate data
Long-term water usage by a swimming pool was analyzed by the developed water balance model.
Monitored daily time-step precipitation and ET data for Tampa Bay, Florida from 1/1/1999 to
12/31/2006 were applied to the model. The applied data were obtained from the Florida
Automated Weather Network (FAWN, http://fawn.ifas.ufl.edu/). Annual precipitation and ET
are presented in Figure 8.
0
10
20
30
40
50
60
70
1999 2000 2001 2002 2003 2004 2005 2006
Year
Annual Precipitation & ET (in)
Precipitation
ET
Figure 8. Annual Precipitation and ET in Tampa Bay, Florida (1999-2006)
As shown in Figure 8, annual ET values are quite stable while precipitation varies considerably
from year by year. While average annual ET is 47.28 inches with 2.48 inches of standard
deviation, average annual precipitation is 45.02 inches with 10.87 inches of standard deviation
during the eight years of monitoring period. Years 2000 and 2001 were dry years; and years
2002 and 2004 were wet years. A number of hurricanes hit this area during those wet years.
Based on the annual precipitation, the applied data set includes couples of typical, dry, and wet
years. The developed model can continuously analyze water use of a swimming pool for the
entire period.
Dimension of the pool and initial assumptions
A typical rectangular swimming pool, which is presented in Figure 6, was considered in this
model. Any types of swimming pools can be modeled as far as the pool dimensions, such as

12
surface area and depth(s), are known. The dimensions of the pool in this example are presented
below (check with Figure 6):
 Width: 18 feet
 Length: 36 feet
 Depth1: 3 feet
 Depth2: 5 feet
 Surface area: 648 ft2
 Volume: 2,592 ft3 (19,390 gallons)
Other modeling parameters were assumed as follows:
 Swimming season: 12 months/year
 Kpan (pan coefficient): 0.75
 Backwash: 750 gallons/month
 Operating range: 6 inches
 Makeup range: 50 percent of the operation range, i.e., ± 3 inches of operating range
Backwashing filters may happen during a swimming season only. However, Florida pools
would operate year round. Pool filters need to run all year to maintain acceptable water quality.
Thus, the swimming season is assumed 12 months. Splash-out and leaks were ignored in this
example because no reference data are available as well as it totally depends on site-specific
conditions.
Results of the long-term daily water balance modeling
The entire time-series of long-term daily modeling results are presented in Figure 9. The
modeling results are based on a typical rectangular swimming pool in Tampa Bay, Florida. The
figure shows precipitation, evaporation, overflow, makeup, and swimming pool water levels for
the entire simulation years from 1999 to 2006. We can differentiate dry, average, or wet years
from the precipitation time series. However, the evaporation time series shows very similar
seasonal patterns throughout the years. ET rates are relatively high during the dry season such as
in May, thereby exacerbating the use of water during stress periods. The overflow time series
confirms that overflow may occur in the condition of abundant precipitation but limited
evaporation. On the other hand, the makeup water time series confirms that makeup water needs
to be added when there is not enough precipitation to offset high evaporation losses. As a result,
high frequency of makeup is shown in dry years. Swimming pool water levels in dry years are
mostly lower than the makeup level, but those in wet years are mostly higher than the makeup
level, which is in the middle of the operating range in this example.

13
0
2
4
6
8
1/1/1999 1/2/2000 1/2/2001 1/3/2002 1/4/2003 1/5/2004 1/5/2005 1/6/2006
Precipitation (in)
0.0
0.1
0.2
0.3
0.4
0.5
1/1/1999 1/2/2000 1/2/2001 1/3/2002 1/4/2003 1/5/2004 1/5/2005 1/6/2006
Evaporation (in)
0
1
2
3
4
5
1/1/1999 1/2/2000 1/2/2001 1/3/2002 1/4/2003 1/5/2004 1/5/2005 1/6/2006
Overflow (in)
0
1
2
3
4
1/1/1999 1/2/2000 1/2/2001 1/3/2002 1/4/2003 1/5/2004 1/5/2005 1/6/2006
Makeup (in)
-3
-2
-1
0
1
2
3
1/1/1999 1/2/2000 1/2/2001 1/3/2002 1/4/2003 1/5/2004 1/5/2005 1/6/2006
Pool Level (in)
Figure 9. Daily time-series of the Continuous Modeling Results (1999 to 2006)

14
A summary of annual makeup water use for the swimming pool is presented in Figure 10.
Annual patterns of precipitation and makeup water use show opposite signals as shown in Figure
10. As expected, more makeup water was used in dry years than wet years. Makeup water use
in dry years is about four times larger than in wet years. Average annual makeup water is about
22.5 inches per year for the entire eight years of period. The required makeup water varies
widely ranging from 9.3 to 37.2 inches per year. The demand for makeup water is largest during
drought periods such as 2000 and 2001. Thus, demand for pool water increases when the supply
decreases.
39.4
31.6
30.7
53.4
49.3
61.6
51.5
42.7
24.8
36.9
37.2
24.9
9.3
15.8
9.4
21.7
0
10
20
30
40
50
60
70
1999 2000 2001 2002 2003 2004 2005 2006
Year
Precipitation and Makeup (in)
Precipitation
Makeup Water
Figure 10. Annual Precipitation and Makeup Water Use
Averages of itemized water uses are summarized in Table 5. The sum of precipitation and
makeup water (Inputs) is well matched with the sum of evaporation and overflow (Outputs). As
mentioned earlier, splash-out and leaks were ignored in this example. Thus, the total water use
in this example is equal to be the sum of the makeup water and the backwash water. If any data
are available for estimating splash-out and leaks, those data can be added as either monthly- or
yearly-based fixed water use. Splash-out would happen only during the swimming season while
leaks might happen throughout the entire year. In this example, the average annual water use for
the swimming pool is about 44.8 inches per year based on the eight years of continuous
simulation from 1999 to 2006. However, the actual annual water use varies significantly based
on climatic conditions and maintenance habits as shown in Figures 9 and 10.

15
Table 5. Annual Averages of Itemized Water Uses (1999 to 2006)
(inches/yr)
(gallons/yr)
Precipitation
45.03
18,191
Evaporation
63.07
25,475
Overflow
4.52
1,827
Makeup
22.50
9,091
Backwash
22.28
9,000
Total Water Use
44.78
18,091
Swimming Pool Operating Strategy and Makeup Water Use
Makeup water level and operating range
The amount of makeup water use also depends on how the pool is operated. The developed
swimming pool water budget model can be used for this type of analysis with a long-term
perspective. Changes in makeup water use by applying different makeup water levels are
presented in Figure 11. The makeup water level is an upper limit of water level when the
makeup water fills the swimming pool. The operating range is 6 inches in this example. Thus,
this figure shows the entire spectrum of changes in makeup water use by adjusting makeup water
levels. As shown in the figure, there might be up to about 5 inches of annual water use
differences by adjusting makeup water levels in the scenario of a 6-inch operating range.
15
20
25
30
-3 -2 -1 0 1 2 3
Makeup Water Level (in)
Annual Makeup Water (in/yr)
Figure 11. Changes in Annual Makeup Water Use by Adjusting Makeup Water Levels
Annual makeup water use can also be varied by adjusting the swimming pool operating range.
The operating range is the difference between allowable minimum and maximum water levels

16
for the swimming pool. Changes in makeup water use by applying different operating ranges are
presented in Figure 12. Makeup water levels for this example are in the middle of the applied
operating ranges. Annual makeup water uses are presented based on zero to 12 inches of
swimming pool operating ranges. If a wider operating range is applied, less makeup water needs
to be added in general. However, annual makeup water uses between 8 and 12 inches of
operating ranges do not show much difference in this example.
10
20
30
40
50
60
0 2 4 6 8 10 12
Operating Range (in)
Annual Makeup Water (in/yr)
Figure 12. Changes in Annual Makeup Water Use by Applying Different Operating Ranges
Changes in makeup water use by adjusting both operating and makeup water ranges are
presented in Figure 13. The makeup ranges are proportional to makeup water levels within the
applied operating range in this example. Zero percent means that makeup water is just added to
meet the minimum water level, which is zero percent of the operating range. Vice versa, 100
percent means that makeup water is added up to the maximum water level, which is 100 percent
of the operating range. The figure shows that we need to apply a wider operating range with a
smaller makeup range to minimize makeup water use.

17
Operating Range
15
20
25
30
35
40
45
50
55
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Makeup Range (%)
Annual Makeup Water (in/yr)
0 in.
1 in.
2 in.
4 in.
6 in.
9 in.
12 in.
Figure 13. Changes in Annual Makeup Water Use by Adjusting Operating Ranges and Makeup
Ranges
Effectiveness of pool covers
Swimming pool covers are initially considered to prevent heat loss from a pool, but the covers
can be applied for water conservation. Actually, evaporation is the largest source of energy loss
because it requires tremendous amounts of energy. If there is a pool cover, evaporation loss
could be reduced significantly. The developed swimming pool water budget model was adjusted
to estimate the effectiveness of pool covers with a long-term perspective as follows:
 No rain water can be collected if pool covers are installed.
 If a pool cannot collect rain water, overflow may not happen from the pool throughout
the period.
 The performance of pool covers to prevent evaporation is assumed 70 percent.
 All of the other conditions will remain the same.
In this analysis, the average annual water use for the swimming pool is estimated about 40.9
inches per year during the 8 years of long-term simulation as summarized in Table 6. This
means the pool cover can save about 3.9 inches of water use per year, which are about 9 percent
of the total water use and about 17 percent of the makeup water use.

18
Table 6. Annual Averages of Itemized Water Uses with Pool Covers (1999 to 2006)
(inches/yr)
(gallons/yr)
Evaporation
18.92
7,643
Makeup
18.58
7,506
Backwash
22.28
9,000
Total Water Use
40.86
16,506
Annual swimming pool water uses are relatively constant if pool covers are installed as shown in
Figure 14. This figure indicates that pool covers can save swimming pool water use during dry
years, but may use more water during wet years because of losing chances of rain water
collection.
0
10
20
30
40
50
60
70
1999 2000 2001 2002 2003 2004 2005 2006
Year
Precipitation and Makeup (in)
Precipitation
Makeup w/o covers
Makeup with covers
Figure 14. Annual Precipitation and Makeup Water Use with and without Pool Covers
The potential water use savings by pool covers may vary by the performance levels of the cover.
Figure 15 shows the variation of makeup water uses based on pool cover performance over
ranges from 50 to 90 percent. For direct comparison, the figure also shows the makeup water
use without a pool cover. If pool covers work poorly, i.e., low performance, it may cause more
water use. Thus, pool covers with high performance could be applied as an effective drought
control strategy.

19
0
5
10
15
20
25
30
35
40% 50% 60% 70% 80% 90% 100%
Performance of Pool Covers
Makeup Water (in)
Makeup w/o covers
Figure 15. Makeup Water Uses based on the Performance of Pool Covers
Effectiveness of pool screens
Swimming pool screens are initially considered to protect bugs and sunlight for a pool, but the
screens can also work for water conservation. Pool screens may abstract some of precipitation
water and could prevent some of evaporation loss. The developed swimming pool water budget
model was adjusted to estimate the effectiveness of pool screens with a long-term perspective as
follows:
 Precipitation water abstraction by screen is assumed 0.1 inches per day.
 The performance of pool screens to prevent evaporation is assumed 20 percent.
 All of the other conditions will remain the same.
In this analysis, the average annual water use for the swimming pool is estimated about 40.0
inches per year during the 8 years of long-term simulation as summarized in Table 7. This
means the pool screen can save about 4.8 inches of water use per year, which are about 11
percent of the total water use and about 21 percent of the makeup water use.
Table 7. Annual Averages of Itemized Water Uses with Pool Screens (1999 to 2006)
(inches/yr)
(gallons/yr)
Precipitation
45.03
18,191
Evaporation
50.45
20,380
Overflow
3.93
1,589
Makeup
17.71
7,156
Backwash
22.28
9,000
Total Water Use
40.00
16,156

20
The potential water use savings by pool screens may vary by the performance levels of the
screen. Figure 16 shows the variation of makeup water uses based on pool screen performance
over ranges from 5 to 30 percent. For direct comparison, the figure also shows the makeup water
use without a pool screen.
0
5
10
15
20
25
30
0% 10% 20% 30% 40%
Performance of Pool Screens
Makeup Water (in)
Makeup w/o screens
Figure 16. Makeup Water Uses based on the Performance of Pool Screens
Summary and Conclusions
Actual swimming pool dimensions and water use patterns were analyzed using a countrywide
monitoring database on single-family residential water use. This analysis shows that an average
size of a pool is about 30.1 feet long, 15.7 feet wide and 5.6 feet deep. The nationwide average
of swimming water use is 0.61 in/ft2 per day, which is just based on the swimming pool water
surface area. However, if 10 feet wide decking areas around the pool are included as pool areas,
the actual average water use per unit area is decreased by about a quarter of the above pool-
surface-only estimation, i.e., about 0.15 in/ft2 per day during a swimming season.
Development and application of a continuous water budget model for analyzing swimming pool
water use have been presented in this report. Overflow, makeup water, and water level in a
swimming pool can be estimated for every time-step by applying time-series of locally
monitored climate data into the model. The long-term simulation results show the whole
spectrum of swimming pool water use patterns in dry, average, and wet periods. Swimming pool
water use differences between dry and wet years are significant. Eight years of long-term
continuous simulation results show that about four times of more water was used in dry years
compared with water use in wet years. This simulation was performed for a typical rectangular
swimming pool in the Tampa Bay area of Florida. The model was also applied in analyzing
potential changes in water use by adjusting swimming pool operating strategies. Two operating
variables, which are operating range and makeup level, were considered in this study. The
results indicate that about 6 to 8 inches of operating range with small makeup range could be
reasonable for water conservation. Pool covers may reduce water uses, but not in wet years

21
because the covers prevent rain water collection. Thus, pool covers could be considered as an
effective drought control strategy. Swimming screens to protect bugs and sunlight can work for
water conservation because the screens can also prevent evaporation loss.
The developed model can be used for other locations as long as long-term climate data are
available around the areas. Instead of applying fixed water use data, the developed model
provides a better understanding of swimming pool water use patterns over several years. The
fixed water use data are most likely to be assumed values independently from regional and
temporal climate conditions. The developed model can also arrange more practical alternatives
on swimming pool operating strategies for water conservation purposes. Energy balance and
cost analysis can be added onto the developed model for performing more comprehensive
analysis.

22
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