Technical Documentation

California’s Next Million Acre-Feet: Saving Water, Energy, and Money
Appendix A: Technical Documentation
Appendix A provides detailed technical documentation of the methods and assumptions used in
the Pacific Institute report “California’s Next Million Acre-Feet: Saving Water, Energy, and
Money.” This analysis explores how to capture 1 million acre-feet of potential water savings
(only a fraction of the conservation potential statewide). We divide these savings between
agriculture and urban uses, with approximately 70% of the savings derived from the agricultural
sector and 30% from the urban sector.
Water Savings
Residential Sector
Table 1 shows the water savings for the residential devices. Estimates for toilets and clothes
washers are based upon the California Urban Water Conservation Council (CUWCC) device
savings estimates. These savings are used to determine compliance with the CUWCC Best
Management Practices under the Flex Track Option (for more information about the CUWCC,
see www.cuwcc.org). Water savings for faucet aerators are based upon data from the U.S.
Environmental Protection Agency. Such estimates were not available for showerheads.
For showerheads, we develop the potential water savings using an end-use analysis based upon
device flow rate and frequency of use. For this analysis, we assume the replacement of a 2.5gallon per minute (gpm) showerhead with a model that uses 1.5 gpm. DeOreo et al. (2010) found
that the average person takes 4.9 showers per week and that the average shower duration is 9
minutes. Based on a Brown and Caldwell (1984) study, Vickers (2001) estimates that showers
are rarely opened at 100% but are maintained at an average throttle factor of 67 percent. The
number of persons per household (pph) is based on the 2005 U.S. Census and is estimated at 2.87
pph for California. We assume that households have two showers but that both devices are
upgraded to more efficient models (thus the cost of the measure is doubled). The calculation that
we use to estimate annual water savings is:
(2.5 gpm – 1.5 gpm) x 67% throttle factor x 9 minutes per shower x 4.9 showers per person per
week x 52 weeks per year x 2.87 persons per household = 4,422 gallons per year
1
Table 1. Water savings assumptions for residential toilets and clothes washers.
Device
High-efficiency toilet (single-Family)
High-efficiency toilet (multi-family)
Clothes washer
Showerheads (1.5 gpm)
Faucet aerator (1.5 gpm)
Annual Water
Savings (gallons)
7,700
9,710
10,200
4,422
629
Device Lifetime
(yrs)
25
25
16
8
5
Source(s)
CUWCC 2009
CUWCC 2009
CUWCC 2009
See text
EPA 2007
Note: gpm = gallons per minute; water savings estimates rounded to 3 significant digits
Commercial Sector
For the commercial sector, the potential water and energy savings are based on a review of
studies that have quantified such savings (Table 2), including the U.S. Environmental Protection
Agency’s EnergyStar calculator for food steamers, clothes washers, and dishwashers (EPA
2009a, 2009b, 2009c, 2009d) and studies conducted and compiled by the CUWCC (CUWCC
2005, 2010).
Table 2. Savings estimates for commercial devices.
Device
Pre-rinse spray valve
Estimated annual
savings (gallons)
Source(s)
50,000
CUWCC 2005
160,000
EPA 2009a
Commercial dishwasher
50,000
EPA 2009d
Commercial clothes washer
38,000
EPA 2009b
Commercial urinal
22,500
EPA 2009c
Commercial toilets
13,600
CUWCC 2010
1,300,000
CUWCC 2010
50,000
CUWCC 2010
Connectionless/boilerless food steamer
Cooling tower pH controllers
Pressurized water brooms
Note: Estimates rounded to 3 significant figures.
Agricultural Sector
For the agricultural sector, the potential water savings, as a percent of total water use, are based
on a review of studies that have quantified such savings (Table 3), including research from the
University of California Cooperative Extension. These percent savings were then applied to the
baseline agricultural water use by crop type and by hydrologic region for 2000 (a normal water
year), from the California Department of Water Resources Annual Land and Water Use Data.
2
Table 3. Savings estimates for agricultural water conservation and efficiency measures.
Measure
Irrigation
scheduling
Regulated deficit
irrigation
Applied to
20% of vegetable,
orchard, and vineyard
acreage
30% of almond and
pistachio acreage
Applied water
savings (%)
Source(s)
13%
Eching 2002
20%
(for almonds
and pistachios)
Goldhamer et al. 2006
Goldhamer and Beede 2004
Goldhamer et al. 2003
Our estimate of the potential water savings associated with conversion to more efficient
irrigation technologies is based on a shift from baseline irrigation methods (Table 4) to an
efficient irrigation technology scenario (Table 5). The efficient irrigation technology scenario
moves only a portion of the acreage currently in flood irrigation to sprinkler irrigation, and a
portion of the acreage currently in sprinkler irrigation to micro or drip irrigation. This scenario
was first developed in Cooley et al. 2008, and modified here by excluding field crops. Ten
percent of orchard and vineyard acreage and 15% of vegetable acreage remains flood irrigated in
the efficient irrigation technology scenario. In addition, the savings are only calculated for three
hydrologic regions (Sacramento River, San Joaquin River, and Tulare Lake). It is therefore a
fairly conservative estimate of potential water savings.
Table 4. Irrigation method by crop type in 2001 (in percentage of irrigated acres).
Vegetables
Orchards
Vineyards
Flood
42.9%
20.3%
20.8%
Sprinkler
36.0%
16.2%
8.7%
Micro/Drip
21.1%
63.5%
70.5%
Source: Orang et al. 2005
Table 5. Irrigation method by crop type in the efficient irrigation technology scenario (in percentage of
irrigated acres).
Vegetables
Orchards
Vineyards
Flood
15%
10%
10%
Sprinkler
35%
20%
10%
Micro/Drip
50%
70%
80%
Source: Cooley et al. 2008
Landscape
Agronomists and hydrologists estimate crop water demand, or theoretical irrigation
requirements, using the concept of evapotranspiration. Evapotranspiration, or ET, is a
combination of evaporation of water from the soil and plant surfaces, and transpiration, which is
3
water lost by the plant through stomata, or openings in its leaves. During daylight hours, plants
open stomata to take in carbon dioxide and, in so doing, lose water vapor, a process referred to as
“transpiration.” Transpiration losses increase under hot and dry conditions such that the plant
must take up more water through its roots in order to survive and grow.
Potential evapotranspiration, or PET, is the evapotranspiration that would occur for a given crop
with an ample supply of water. PET is affected by hydro-climatic factors, including air
temperature, wind speed, humidity, solar radiation, and cloud cover. Actual evapotranspiration
will equal PET in wet conditions, where water is abundantly available. Under drier conditions,
ET will be some fraction of PET. On an annual basis, natural evapotranspiration in California is
usually less than PET, which will only occur when water is abundantly available.
Monthly Irrigation Requirement
We estimate the monthly crop irrigation water requirement using a simple water balance model
that has only two inputs: the long-term average monthly PET and precipitation for areas in
California. For each month, we calculate the net irrigation requirement using the field water
balance method. We follow equation 27.2.32 in the Handbook of Hydrology (Maidment 1993):
(1)
I is the monthly irrigation requirement, ETcrop is the evapotranspiration for a cropped area, P is
the monthly precipitation, G is the groundwater contribution, and W is the stored water at the
beginning of the month. We ignore the terms G and W, assuming that they are negligible for
household landscapes and the relatively long time scale of one month.
We develop an estimate of annual irrigation use that is appropriate in warm climates, where
irrigation may take place year round.
(2)
The application of equation 2 is shown in Figure 1. The plot shows natural moisture demand, and
is patterned after the “water balance charts” that were shown in the California Water Atlas
(Kahrl ed. 1979). In months where precipitation exceeds the PET, the plants’ water needs are
fully met without irrigation and the irrigation requirement is zero. The location shown in Figure
1 (zip code 06111, Pyramid Lake in southern California) is marked by hot, dry summers where
PET is high, and most of the precipitation occurs during the winter months. The height of the
green bars indicates the water deficit that needs to be fulfilled by irrigation water to meet plant
water needs.
4
5
Precipitation (P)
Potential
Evapotranspiration (PET)
4
Water 3
depth
(inches) 2
Irrigation demand (I)
1
Figure 1
DEC
NOV
OCT
SEP
AUG
JUL
JUN
MAY
APR
MAR
FEB
JAN
0
Monthly water deficit as a proxy for irrigation demand
In this simplified model, we assume that for vegetated areas, all of the precipitation infiltrates
into the soil and that there is no runoff. We also assume that no water percolates deep
underground where it is unavailable for uptake by plant roots. In reality, runoff and percolation
can be significant fluxes of water. In practice, ignoring the runoff and percolation means that our
model may slightly overestimate the quantity of rainfall that is available to fulfill plant water
demand and underestimate irrigation requirements. Our simplified model also does not account
for precipitation that falls as snow. Snow will not infiltrate into the soil and may not melt for
several months. This is a further source of inaccuracy in our model.
Using equation 2, we develop an estimate of irrigation demand for each zip code in California.
We perform the analysis in a digital map using Geographic Information Systems (GIS) software
from ESRI. The important input layers are monthly precipitation and evapotranspiration. All
outputs are initially reported by zip code. We obtain zip code boundaries from ESRI Data &
Maps. We create a new GIS point layer of zip codes by converting the point at the centroid of
each zip code polygon to a new feature. Many US zip codes represent post office boxes; these
are not included in the analysis. It should also be noted that new zip codes are created every year
as the population grows and moves. The datasets we used were created in 2006.
Evaporation & Evapotranspiration
We use estimates of reference evapotranspiration from a digital dataset published by the
California Department of Water Resources (Figure 2). This map of evapotranspiration zones is
based on data from the California Irrigation Management System, or CIMIS. There are 18 zones
within the state, which represent areas of similar climate. The agency reports monthly average
reference ET for each zone based on measurements from the network of 120 measurement
stations deployed since 1982 (DWR 2009).
5
We obtain a GIS shapefile of the ET zone boundaries from DWR staff. We determine the ET
zone for each zip code by overlaying the zip code centroids and the ET zone polygons using an
intersect operation in ArcGIS. The resulting database table is exported to Microsoft Excel, and
lookup functions are used to assign monthly ET values.
Monthly Precipitation
Next, we sought monthly precipitation datasets. We find that the highest spatial accuracy among
readily available data layers is from the PRISM project (Oregon State University 2009). The
PRISM researchers created a spatial distribution of point measurements of rainfall for the time
period 1997 to 2004. The rainfall values are distributed using the PRISM model, developed by
Christopher Daly, Director of the Spatial Climate Analysis Service, and documented in a series
of reports and journal articles, e.g. Daly 2008. The resolution of the raster datasets is
approximately 2 km, i.e. each pixel covers about 4 km2, or slightly more than 1 square mile. We
download monthly average precipitation data layers for January through December, and in order
to analyze these layers in the map, we convert them from ASCII Grid to ESRI Grid format using
the ASCII to Raster conversion tool in ArcToolbox.
The zip code centroids are assigned a set of attributes for monthly and annual precipitation in
GIS. We use the free ArcGIS extension Hawth’s Tools, and its Intersect Points tool. Figure
shows the precipitation map for the month of February, with darker shades of blue indicating
greater rainfall depth.
6
Figure 2
Reference evapotranspiration zones in California.
Source: http://www.cimis.water.ca.gov/cimis/images/etomap.jpg
7
Figure 3
Monthly precipitation depth for the conterminous 48 states from the PRISM process.
Crop Coefficients and Water Use
We now have an estimate for every zip code of irrigation water requirements in an average year
for a reference crop. A reference crop is well-watered grass; specifically, reference ET (Erc) is
defined as “the rate of evapotranspiration from an extensive surface of 8 to 15 cm (3.1 to 5.9 in)
tall, green grass cover of uniform height, actively growing, completely shading the ground and
not short of water” (Handbook of Hydrology, page 27.29, quoting Doorenbos and Pruitt, 1977).
To account for differences in water requirements among different crops, a crop coefficient, kc, is
used.
Ecrop = kc · Erc
By the definition above, the crop coefficient for well-watered grass is 1.0.
In choosing crop coefficients, we follow guidelines from EPA as well as the California
Landscape Contractors Association (CLCA), reported in Table 6, below. CLCA developed water
budget standards for the state of California as part of the development of the California Model
Water Efficient Landscape Ordinance (CLCA 2008). The crop coefficients are derived from
Costello and Jones (1994).
Table 6. Crop Coefficients for urban landscapes.
Lawn
Shrubs/ Trees
EPA WaterSense
0.8
0.5
California Landscape
Contractors Association
0.8
0.5–0.6
Efficient
Landscaping
None reported
0.3
8
Irrigation Efficiency
Irrigation efficiency is the ratio of water beneficially used divided by the water applied. The
efficiency of an irrigation system depends on the system characteristics and management
practices. Well-designed and maintained systems will have a higher efficiency. For instance, if
an irrigation system is optimized and performing at theoretical 100% efficiency, this means that
all water makes its way to the plants root zone, and the exact amount of water required is
applied. In reality, there are a number ofways that water is lost during irrigation, such as
percolation, runoff, and wind. The Handbook of Hydrology (page 27.33) reports field application
efficiencies range from 0.5 to 0.8. Typical efficiencies for different irrigation methods are shown
in Table 7 (from Brouwer et al. 1989).
Table 7. Typical irrigation efficiencies for different irrigation methods.
Irrigation methods
Surface irrigation (border, furrow, basin)
Field application
efficiency
60%
Sprinkler irrigation
75%
Drip irrigation
90%
We follow the California Model Water-Efficient Landscape Ordinance in selecting an Average
Irrigation System Efficiency equal to 0.71. If a landscape requires 1” of water in a week, then the
irrigation requirement is 1 inch/0.71 = 1.41 inches.
The theoretical efficiency for a given technology reported above assumes a professionallyoperated irrigation system. At the household level, some irrigators may apply more or less than
the optimal amount of water. To describe whether an individual is over- or under-watering,
analysts have defined the application ratio as the actual water applied divided by the theoretical
irrigation water requirement.
Recent evidence indicates that householders apply water in many different ratios, with
approximately equal numbers of households under-watering and over-watering. There is also
some evidence that the mean application ratio is about one, meaning that the average household
applies the amount of water needed to fill the needs of a well-watered grass crop. (DeOreo et al.
2010). By omitting this factor from our analysis, we assume an application ratio of one.
Water Savings
Thus far, precipitation, evapotranspiration, and irrigation water requirement are expressed as
depths in inches. Our method for calculating irrigation water savings is very similar to guidance
recently developed by EPA’s WaterSense program for certifying landscape water use. We
assume that the irrigation requirements can be lowered by replacing lawn with low-water-use
plants. Water savings are calculated by replacing a portion of the original landscape with a lower
crop coefficient, 0.3, based on California Landscape Contractors Association’s estimate (Table
9
6). This permits the irrigator to use a lower application ratio while still maintaining healthy
plants.
We calculate the potential water savings as the difference between the average theoretical
application depth for lawn, and the depth that would be applied to low-water-use plants. The
potential irrigation water savings (Isavings) is the difference in irrigation depth:
Isavings = Igrass – Iefficient
Isavings = 0.8Erc – 0.3Erc
Isavings = 0.5 Erc
We calculate the potential irrigation water savings in inches for converted landscape areas for
each zip code.
To convert depths to a volume of water saved per year per square foot of lawn replaced, we
converted as follows:
For example, in zip code 90012 (Los Angeles), the annual irrigation requirement is 36.5 inches,
and the annual savings for replacing one square foot of lawn is 11.4 gallons. Replacing one acre
of lawn (43,560 ft²) in Los Angeles with water-efficient landscaping would yield a savings of
500,000 gallons per year, or the equivalent or 1.5 acre feet per year.
Results
We estimate the amount of water that could be saved by converting one square foot of grass to
low-water use vegetation ranges from 4.7 gal/ft²·yr in Crescent City to 30 gal/ft²·yr in the
Imperial Valley (Table 6). A map showing the average annual savings is shown in Figure 4. The
figure also shows water deficit plots for select locations in the state.
In the northern California city of Eureka, the total average annual precipitation exceeds the total
annual PET. However, PET is highest during summer months when precipitation is at its lowest,
creating a soil moisture deficit and a modest irrigation demand of 17 inches per year. Replacing
one square foot of irrigated grass with low-water use plants saves 4.7 gallons per year. At the
opposite extreme is the city of El Centro in the Colorado Desert in Southern California, also
known as the Imperial Valley. The city receives scant rainfall, less than 3 inches on average, but
has high PET year-round, ranging from 2” in January to over 9” in summer months. In total, the
year-round irrigation demand is 69 inches, or 5.8 feet. A lawn replacement program in this
location can save 30 gallons per square foot per year.
10
Landscape replacement programs will save the most water in areas with high landscape water
demand. While the report only includes the results of landscape replacement programs in six
counties, we have developed estimates of water demand and potential savings for every zip code
in the state (Figure 4). To develop an average for each county in the state (Table 8), we have
taken a weighted average based on the population of each zip code. This approach gives more
influence to areas with greater numbers of residents.
Figure 4
Average annual theoretical irrigation requirement for lawn by zip code for California
11
Table 8. Potential average annual water savings from converting lawn to water-efficient landscaping,
by county.
County
Alameda
Alpine
Amador
Butte
Calaveras
Colusa
Contra Costa
Gallons per
square foot
13.4
14.8
16.7
16.5
15.7
18.6
15.5
Del Norte
El Dorado
Fresno
Glenn
7.2
16.8
19.9
18.9
Humboldt
Imperial
Inyo
Kern
Kings
Lake
Lassen
Los Angeles
Madera
Marin
Mariposa
Mendocino
Merced
Modoc
Mono
Monterey
Napa
Nevada
7.9
30.2
22.5
22.5
23.4
15.2
17.0
16.5
19.1
12.6
16.0
12.7
20.1
14.2
20.3
14.5
15.1
15.7
County
Orange
Placer
Plumas
Riverside
Sacramento
San Benito
San
Bernardino
San Diego
San Francisco
San Joaquin
San Luis
Obispo
San Mateo
Santa Barbara
Santa Clara
Santa Cruz
Shasta
Sierra
Siskiyou
Solano
Sonoma
Stanislaus
Sutter
Tehama
Trinity
Tulare
Tuolumne
Ventura
Yolo
Yuba
Gallons per
square foot
15.7
17.7
15.9
19.5
19.0
16.4
19.4
15.8
11.2
19.0
15.4
13.2
14.7
16.4
11.0
16.2
15.0
15.2
16.3
13.6
19.5
18.4
18.2
16.1
19.4
15.5
16.3
19.2
18.3
12
Discussion and Limitations
One shortcoming of our analysis is that it is based on a theoretical average year, rather than any
actual year in the climatic record. Actual irrigation needs in a given year may be lower or higher
due to changes in precipitation, temperature, cloud cover, or other climate variables. We
recommend future work to repeat this analysis using actual monthly data from the climate record
to back-cast actual irrigation demands for the past. This would give a better estimate of the
variability of water demand, and how it responds to dry and wet years. Further, a more
sophisticated analysis might look at how urban outdoor demand will respond to climate change
in the future.
The input datasets are of limited spatial resolution and accuracy. As climate researchers and
meteorologists produce more detailed datasets in the future, this analysis should be expanded and
refined. Inaccuracy also comes from the limitations of our modeling technique. Our simplified
monthly water balance model does not include the complexities of snowfall, runoff, deep
percolation, or irrigation system management (e.g., distribution uniformity, pump efficiency).
The advantages of this technique are its ease of use, and that it does not require calibration. A
more sophisticated model could more explicitly account for soil moisture, perhaps relying on
GIS soils datasets for input data.
Energy Savings
Many of the water conservation and efficiency devices reduce the amount of water that requires
heating in homes and businesses, thereby providing substantial end use energy savings.
Additionally, capturing, treating, and conveying water also requires energy, referred to as
embedded energy. Thus, saving water produces embedded energy savings as well. We calculate
the end use and embedded energy savings from the water conservation and efficiency measures
identified in this analysis. Below, we describe our methodology for each calculation.
End-Use Energy Savings
Table 6 provides estimates of the end-use energy savings for each measure. For the residential
measures that save hot water, electricity and natural gas savings were estimated using the
following equations:
electricity savings (kWh per gallon per degree F) = ((1 kWh/3,412 BTUs) x (8.34 lbs per
gallon) x 1 BTU/lb ºF))/(90% efficiency) = 0.002707 kWh per gallon per ºF
natural gas savings (therms per gallon per degree F) = ((1 therm/105 BTUs) x (8.34 lbs per
gallon) x 1 BTU/lb ºF))/(55% efficiency) = 0.000152 therms per gallon per ºF
For showerheads, we assume that temperatures are raised from 60ºF to 105ºF. For faucets we
assume temperatures are raised 60ºF to 80ºF. For clothes washers, we assume that 40% of the
water savings are from hot water with temperatures that were raised from 60ºF to 130ºF. Energy
13
savings for the measures from the commercial and industrial sectors were based on various
reports, as indicated in Table 9.
Table 9. Device end-use energy savings.
Measure
Residential toilet (1.28 gpf)
Showerhead (1.5 gpm)
Residential front-loading clothes
washer
Faucet aerator (1.5 gpm)
Pre-rinse spray valve (1.0 gpm)
Connectionless food steamer
Commercial dishwasher
Commercial front-loading clothes
washer
Commercial urinal (0.5 gpf)
Commercial toilet (1.28 gpf)
Cooling tower pH controller
Pressurized water broom
Replace lawn with low-water-use
plants
Annual End-Use Energy Savings
(per device)
If Water Heated by If Water Heated
Electricity7
by Natural Gas8
(kWh)
(therms)
Notes
539
774
30
37
1
2
34
7,600
4,419
13,950
2,880
2
330
334
608
138
3
4
5
6
2
-
-
Notes/Sources:
(1)
Calculated. Assume raising temperature from 60ºF to 105ºF.
(2)
Calculated. Assume 40% of water savings is hot water and that the temperature of this water is raised from 60ºF
to 130 ºF.
(3)
Calculated. Assume raising temperature from 60ºF to 80ºF.
(4)
Energy savings based on estimates provided in CUWCC 2005.
(5)
Energy savings based on estimates provided in EPA 2009a.
(6)
EPA 2010
(7)
We assume water heating uses 0.00271 kWh per gallon per ºF for an electric water heater with a 90% efficiency
level.
(8)
We assume heating requires 0.000152 therms per gallon per ºF for a natural gas heater with a 55% efficiency
level.
Embedded Energy Savings
Energy requirements for capturing, treating, and conveying water are referred to as embedded
energy. Water conservation and efficiency reduces the volume of water that must be pumped and
14
treated, thereby providing significant embedded energy savings. In order to quantify these
savings, we multiplied the volume of water conserved by the energy intensity of water. Energy
intensity is defined as the total energy requirements for a given volume of water or wastewater
and is often expressed in units of kWh per million gallons or, for natural gas, in units of therms
per million gallons.
Table 10 provides energy intensity estimates for various segments of the water and wastewater
cycle in Northern and Southern California. Energy intensity is higher for water in Southern
California because much of this water is imported across long distances and over steep terrain.
Note that the energy intensity of water used indoors is higher than that used outdoor because it is
subject to wastewater treatment. For this analysis, we assume that water used indoors has an
energy intensity of 5,400 kWh per million gallons in Northern California and 13,000 kWh per
million gallons in Southern California. Water used outdoors has an energy intensity of 3,500
kWh per million gallons in Northern California and 11,100 kWh per million gallons in Southern
California.
Table 10. Energy intensity estimates (in kilowatt-hours per million gallons) for Northern and Southern
California.
Water Supply and
Conveyance
Water Treatment
Water Distribution
Wastewater
Treatment
Regional Total
Indoor Uses (kWh/MG)
Northern
Southern
California
California
2,117
9,727
Outdoor Uses (kWh/MG)
Northern
Southern
California
California
2,117
9,727
111
1,272
1,911
111
1,272
1,911
111
1,272
-
111
1,272
-
5,411
13,022
3,500
11,111
Source: Navigant Consulting, Inc. 2006.
We use 2008 regional population estimates for Northern and Southern California to produce a
population weighted statewide energy intensity estimate. Based on this calculation, we estimate
that the average energy intensity of water used outdoors is 8,100 kWh per million gallons, while
that used outdoors is 10,100 kWh per million gallons (Table 11). To determine the embedded
energy savings, we multiply the indoor and outdoor water savings by the appropriate statewide
energy intensity estimates (Table 12).
15
Table 11. Population weighted average energy intensity estimates (in kilowatt-hours per million
gallons) for California.
Population
Northern California
Southern California
State
14,334,052
22,422,614
36,756,666
Indoor Water
(kWh/MG)
5,411
13,022
10,054
Outdoor Water
(kWh/MG)
3,500
11,111
8,143
Source: Population estimates for July 1, 2008 from U.S. Census Bureau 2009.
Table 12. Embedded energy savings (in million kWh per year).
Measure
Residential Toilet (1.28 gpf)
Showerhead (1.5 gpm)
Residential front-loading clothes washer
Faucet aerator (1.5 gpm)
Pre-rinse spray valves
Connectionless food steamer
Commercial dishwasher
Commercial clothes washer
Commercial urinal (0.5 gpf)
Commercial toilet (1.28 gpf)
Cooling tower pH controllers
Pressurized water brooms
Replace lawn with low-water-use plants
Indoor Water
Savings (AF)
Outdoor Water
Savings (AF)
93,500
47,500
13,300
6,750
3,070
3,440
1,300
10,500
51,800
31,300
21,900
7,670
28,400
Embedded Energy
Savings (million
kWh per year)
306
156
43.6
22.1
10.1
11.3
4.27
34.3
170
103
71.8
20.3
75.4
Note: All numbers rounded to three significant figures.
Total Energy Savings
Table 13 summarizes the embedded and end-use energy savings. Based on US Census Bureau
(2007), we assume that 44% of water heaters are electric and the remaining 56% are natural gas.
We estimate that the water conservation and efficiency measures described in this analysis would
save 2,300 million kWh and 86.8 million therms of natural gas each year (Table 10). This is
equivalent to the annual electricity requirements of 309,000 average California households.
Nearly 55% of these savings are a result of end use savings and the remaining 45% are a result of
reductions in embedded energy.
16
Table 13. Embedded and end-use energy savings.
Measure
Residential Toilet (1.28 gpf)
Showerhead (1.5 gpm)
Residential front-loading clothes
washer
Faucet aerator (1.5 gpm)
Pre-rinse spray valves
Connectionless food steamer
Commercial dishwasher
Commercial clothes washer
Commercial urinal (0.5 gpf)
Commercial toilet (1.28 gpf)
Cooling tower pH controllers
Pressurized water brooms
Replace lawn with low-water-use
plants
Total Savings
Embedded Energy
Savings
(million kWh per
year)
306
156
43.6
End Use Energy
Savings
(million kWh
per year)
830
145
End Use Energy
Savings
(million therms
per year)
59.3
8.86
22.1
10.1
11.3
4.27
34.3
170
103
71.8
20.3
75.4
52.4
66.8
13.6
52.1
114
-
3.75
3.70
1.31
2.90
6.98
-
1,030
1,270
86.8
Note: All numbers rounded to three significant figures. We assume 44% of water heaters are electric and the
remaining 56% are natural gas based on U.S. Census Bureau 2007.
Cost Effectiveness Analysis
Economists use cost-effectiveness analysis to compare the unit cost of alternatives, for example,
in dollars spent to obtain an additional acre-foot of water supply. Because each water
conservation measure is an alternative to new or expanded water supply, conservation measures
are considered cost-effective when their unit cost – called the cost of conserved water – is less
than the unit cost of the lowest-cost option for new or expanded water supply.
Our cost-effectiveness analysis is done from a combined utility and customer perspective. We
calculate the cost of conserved water based on the total investment required and any changes in
operation and maintenance costs resulting from the investment.1 We adopted this approach
because it captures both the costs and benefits to the water supplier, which are eventually passed
on to customers, as well as costs and benefits customers experience aside from what they pay for
water service. This approach thus takes a broader view of the potential costs and benefits of
water conservation and efficiency improvements than the agency perspective alone.
1
Savings on water bills are not included as the volume of water conserved is the denominator for the cost of
conserved water calculation.
17
The cost parameters that affect our estimates of the cost of conserved water are the cost of the
device, nominal and real interest rates, useful lifetime, changes in operation and maintenance
(O&M) costs, and the average annual quantity of water conserved. For water conservation
devices that reduce indoor water use, changes in O&M costs are related to reductions in waterrelated heating requirements and reductions in wastewater flows.2 Ultimately, these reductions
save the customer money through lower wastewater and energy bills. Changes in energy and
wastewater costs are shown in Table 14. Note that energy savings are shown for customers with
a gas or an electric water heater. To determine the average energy savings, we calculate a
weighted average based upon the fraction of the population with gas or electric water heater.
Table 14. Changes in customer energy and wastewater costs per device.
Efficiency Measure
Residential Toilet (1.28 gpf)
Showerhead (1.5 gpm)
Residential front-loading
clothes washer
Faucet aerator (1.5 gpm)
Pre-rinse spray valves
Connectionless food
steamer
Commercial dishwasher
Commercial clothes washer
Commercial urinal (0.5 gpf)
Commercial toilet (1.28 gpf)
Cooling tower pH
controllers
Pressurized water brooms
Replace lawn with lowwater-use plants
Changes in O&M Costs (per device)
Wastewater
Energy
($/yr)
If Water Heated
If Water
by Electricity
Heated by
($/yr)
Natural Gas
($/year)
$0.66
$0.76
$14.27
$6.43
$6.44
$74.92
$28.93
Device
Lifetime
(years)
25
8
16
$0.62
$39.60
$158.40
$5.17
$998.26
$621.31
$2.33
$370.92
$375.22
5
5
12
$49.50
$15.70
$2.32
$13.48
$1,284.60
$1,961.37
$48.09
0
0
0
$683.39
$16.86
0
0
0
11
25
25
5
0
0
0
0
0
0
5
15
Note: For residential customers, we assume a price of $1.22 per therm for natural gas (EIA 2010a) and $0.15 per
kWh for electricity (EIA 2010b). For commercial customers, we assume a price of $1.12 per therm for natural gas
(EIA 2010a) and $0.14 per kWh for electricity (EIA 2010b). We assume an average wastewater rate in California of
$0.99 per thousand gallons (Fisher et al. 2008).
2
See Chapter 5 of Gleick et al. 2003 for a detailed discussion of the economics of water conservation and efficiency
improvements.
18
For most devices, we assume that the customer was in the market for a new device, and thus the
cost is the cost difference between a new standard and new efficient device. For some devices,
including faucet aerators, cooling tower pH controllers, water brooms, replacing lawn with lowwater-use plants, and all of the agricultural measures, however, we assume that the customer
would not have made the investment otherwise, and thus the cost is the full cost of the device.
We conducted a literature review to estimate the device lifetime and cost. We also include the
administrative cost for running a rebate program, which typically varies from about 10% to 30%
of the rebate cost, depending on the measure under consideration (Table 15).
Table 15. Cost data for selected urban water conservation and efficiency measures
Conservation Measure
Device Cost ($/device)
Incremental
Incremental
Cost
Plus
Administrative
Efficient
Standard
Cost
Residential toilet (1.28 gpf)
$ 200
$ 150
$ 50
$ 63
Showerhead (1.5 gpm)
$ 40
$ 20
$ 20
$ 25
Residential front-loading clothes
$ 750
$ 492
$ 258
$ 323
washer
Faucet aerator (1.5 gpm)
$ 8
$ $ 8
$ 10
Restaurant pre-rinse spray valve
$ 70
$ 50
$ 20
$ 25
(1.0 gpm)
Connectionless food steamer
$ 6,000
$2,500 (elec.);
$ 3,228
$ 4,035
$3,800
(natural gas)
Commercial dishwasher
$ 9,000
$ 6,950
$ 2,050
$ 2,563
Commercial front-loading
$ 750
$ 492
$ 258
$ 323
clothes washer
Commercial urinal (0.5 gpf)
$ 550
$ 540
$ 10
$ 13
Commercial toilet (1.28 gpf)
$ 200
$ 150
$ 50
$ 63
Cooling tower pH controller
$ 2,250
$ $ 2,250
$ 2,813
Pressurized water broom
$ 250
$ $ 250
$ 313
Replace 1 acre of lawn with low$ 43,560
$ $ 43,560
$ 54,450
water-use plants
Note: Costs shown for showerheads and faucet aerators are based upon replacing all devices within a
single home. We assume that there are two devices per household.
19
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