AA/aterSense
WaterSense Water Budget Approach and Tool
WaterSense® Water Budget Approach and Tool
I. Introduction
On May 22, 2008, EPA released a draft specification for water-efficient single-family new homes
(www.epa.gov/watersense/specs/homes.htm). To meet the Landscape Design Criteria (Section
4.1.1), the builder may choose to comply with one of the following options:
Option 1 - Turf shall not exceed 40 percent of the landscapable area. Turf shall not be
installed on slopes greater than 4:1.
Option 2 - Develop the landscape design using a water budget approach. The
evapotranspiration (ET) limit on the landscapable area shall be no more than 60 percent
of the reference ET (ET0). For purposes of the ET calculation, the available rainfall shall
be no more than 25 percent of the average annual rainfall amount. Turf shall not be
installed on slopes greater than 4:1.
If the builder chooses Option 2, the water budget approach, the water budget calculations must
be documented and submitted as part of the inspection package. To aid in the development of
landscape water budget and make a complex calculation more straightforward, EPA has
developed a tool to guide the builder, landscape professional, or irrigation partner through the
water budget calculations. This document provides an overview of the tool as well as detailed
instructions for using the tool. Definitions of technical terms are provided at the end of the
document.
This tool is based upon accepted methodology developed by the irrigation industry and is laid
out in a simple format for ease of use. The methodology is based upon the Irrigation
Association (IA) publication, "Landscape Irrigation Scheduling and Water Management,"
published in March 2005.
The water budget approach allows the builder/landscape professional to design a sustainable
landscape using a specified amount of water. A water budget is a site-specific method of
calculating an allowable amount of water to be used by the landscape and then designing the
landscape to meet this budget. The budget takes into account plant type and plant water
needs, irrigation system efficiencies, and all applied water that the landscape receives either by
irrigation or by precipitation, as described in detail below.
For the draft specification's Option 2 approach to landscape design, WaterSense has set a
water budget (or allowance) for the designed landscape at 60 percent of the water requirement
for a similarly sized landscape consisting of vegetation at the local reference evapotranspiration
(ET0), or, specifically, the amount of water required by a landscape consisting of cool-season
grass of a uniform height of 12 centimeters, actively growing, completely shading the ground,
and not short of water (FAO 1998 and ASCE 1990 in IA 2005). The water budget approach will
allow landscape designers to plant a mixture of high-, medium-, and low-water-using plants,
lending flexibility in the design of the water-efficient landscape. Budgeting for 60 percent of
local ET0 does not mean that each hydrozone of the designed landscape can only receive 60
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percent of the water the plants need. It means that the designed landscape, as a whole, can
only require 60 percent of the water the same size landscape would require if watered at 100
percent of ET0. This allows the landscape designer to use a combination of plants to meet this
criteria.
II. The Water Budget Tool
The tool (file name: WaterSense Water BudgetTool.xls) is provided in an Excel spreadsheet
format that guides the user through the water budget calculation in three parts. First, the tool
calculates the amount of water the designed landscape is allowed based on EPA criteria (Part 1
- Landscape Water Allowance). Next, the tool calculates how much water the designed
landscape requires based on climate, plant type, and irrigation system efficiency (Part 2 -
Landscape Water Requirement). Last, the tool compares the allowable amount of water from
Part 1 to the required amount of water from Part 2 and determines whether the designed
landscape meets the budgeted amount (Part 3 - Results). In other words, does the landscape
design meet EPA's criteria?
The rest of this document is broken down into "Background on the Calculations" and "Detailed
Instructions." The background section describes the equations and information about the
calculations being used in each part of the tool. The detailed instructions provide the user with
detailed directions, guiding the user through the tool step by step.
General Instructions: Review the "Background on the Calculations" section first and then follow
the directions included in the "Detailed Instructions" section. Please note, the tool only allows
information to be entered in white cells. Note that each part is formatted in an identical fashion:
• The blue box at the top displays the user, builder, and site information. Once the
information is entered for Part 1, it is automatically populated on Part 2 and Part 3 of the
tool.
• The yellow box displays the equation that is used in the worksheet.
• The gray box is the area of the worksheet where the user enters the required data.
• The green box displays the output.
When all of the information has been entered into the tool and the water budget is complete,
print the results tab (Part 3) and submit it to the builder to be included in the inspection
documentation. This documentation must be provided to the builder prior to a home being
inspected for compliance with WaterSense criteria.
A. Background on the Calculations
The following sections describe the equations and calculations used by the tool to complete the
water budget and determine if the design landscape meets EPA criteria.
Part 1: Determining the Landscape Water Allowance
The Landscape Water Allowance (LWA) is the amount of irrigation water that WaterSense is
specifying for designing a water-efficient landscape. The LWA is based on the local
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reference evapotranspiration (ET0), a water adjustment factor (Kwa), and the area (A) of the
designed landscape (IA 2005):
Equation A-1: Landscape Water Allowance
EToX Kn.a X A
Where:
LWA = Landscape water allowance (gallons/year)
ET0 = Grass reference evapotranspiration (inches/year), location specific
Kwa = Water adjustment factor (dimensionless), 0.60 (60 percent) for the WaterSense
specification (see equation A-2 below)
A = Area of the landscape (square feet)
Cu = Conversion factor (1.6043 for results in gallons/year)
Further explanation of the water adjustment factor (Kwa): The (Kwa) helps to determine the
amount of water to be allocated to a landscape by considering the landscape coefficient and
overall irrigation system efficiency (IA 2005). The landscape coefficient is the percentage of
ET0 a specific plant or plant type should require. This is usually about 80 percent or 0.8 for
high-water-using plants such as turf, 50 percent or 0.5 for medium-water-using plants, and
20 percent or 0.2 for low-water-using plants.
Equation A-2: Water Adjustment Factor
El
I\. . —
AIE
Where:
Kwa = Water adjustment factor (dimensionless) for a particular type of landscape
KL = Area-weighted average landscape coefficient (dimensionless) for a particular type of
landscape
AIE = Average irrigation efficiency, or irrigation system efficiency (dimensionless)
The calculation supporting the WaterSense 60 percent limit is:
Equation A-3: Water Adjustment Factor (0.60 for WaterSense Specification)
0.43
0.60= ~
Where:
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KL = 0.43 This is the area weighted landscape coefficient designating a mixture of high-,
medium-, and low-water-using plants.
AIE = 0.71 This is the average irrigation efficiency of today's systems as determined by the
California Model Water-Efficient Landscape Ordinance (CA DWR 2008).
Part 2: Determining the Landscape Water Requirement
The Landscape Water Requirement (LWR) is the amount of irrigation water required by the
designed landscape. The LWR is calculated for each hydrozone and summed to determine
the LWR for the site. The LWR is based on the local reference evapotranspiration (ET0), the
landscape coefficient (KL), the hydrozone area, the irrigation efficiency (IE) of the associated
system, and the effective rainfall (Re) (IA 2005):
Equation B-1: Landscape Water Requirement
Where:
LWRh = Landscape water requirement for the hydrozone (gallons/year)
RTM = Run time multiplier, equal to 1/irrigation efficiency
ET0 = Reference evapotranspiration (inches/year), provided locally
KL = Landscape coefficient for the type of plant in that hydrozone (dimensionless)
Re = Effective rainfall, 25 percent of annual precipitation (R) as designated by the
WaterSense specification
A = Area of the hydrozone (square feet)
Cu = Conversion factor (1.6043 for results in gallons/year)
Further explanation of the effective rainfall (Re): As defined by the IA, the effective rainfall
(Re) is "the amount of total rain that is actually stored in the root zone" (IA 2005). IA states
that for planning purposes, no more that 50 percent of monthly historical rainfall should be
considered "effective" toward the future water needs of landscape plants (IA 2005). Due to
patterns of increased drought frequency and no guarantee that annual rainfall will reflect
historical precipitation patterns, WaterSense is limiting Re to 25 percent of annual
precipitation. This is a conservative estimate to allow for a landscape to survive on less
rainfall than expected and still meet the water budget and is currently being used by the
State of California in the development of its Model Water-Efficient Landscape Ordinance
(CA DWR 2008).
Part 3: Results
This worksheet is used to compare the LWA to the LWR to determine if the water budget is
met. If the Landscape Water Requirement is LESS than the Landscape Water Allowance,
then the water budget criterion is met. If the Landscape Water Requirement is GREATER
than the Landscape Water Allowance, then the landscape and/or irrigation system needs to
be redesigned to use less water.
LWRh = RTMx [{ETo x Kl) - Re]
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Additionally, this worksheet will calculate and display the total amount of turfgrass used in
the landscape. This information is necessary for the inspector to verify that the designated
amount of turf was actually installed.
B. Detailed Instructions
The following sections guide the user through the water budget tool, step by step.
Part 1: Determining the Landscape Water Allowance
1. Complete the site information in the blue box at the top of the worksheet. This
information will be automatically populated in the next two parts.
2. In the gray box, complete Step 1A by entering the area (A) of the landscape in square
feet. Note: For purposes of this tool, the landscapable area is defined as "The area of a
site less the building area, driveways, paved walkways, impermeable decks, patios, and
other structures."
3. In the gray box, complete Step 1B by entering the annual grass reference
evapotranspiration (ET0) for the city or region where the landscape is located. Location
specific ET0 can be obtained from local entities such as universities, Cooperative
Extension offices, or local weather station organizations.
4. The results, or output, are displayed in blue cells in the green box. The LWA is
displayed in both gallons per year (gallons/year) and hundred cubic feet (hcf).
5. Move to the next tab, Part 2, to calculate the Landscape Water Requirement.
Part 2: Determining the Landscape Water Requirement
1. In the blue box, confirm that the site information is pre-populated from Step 1.
2. Complete Step 2A by entering the average annual precipitation, or rainfall, (R) in inches
per year. This information can be commonly found online, for example at Web sites such
as the National Climactic Data Center (hurricane, ncdc.noaa.qov/cqi-
bin/HPD/HPDStats.pi) or Weather Underground (www.wunderqround.com/historv/).
3. Complete Step 2B by entering the required information into Table 1. This table, when
completed, should represent the designed landscape (i.e., all of the landscapable area
should be represented in this table). Note that the combined hydrozone/landscape
feature areas should equal the total landscapable area entered in Step 1 (if not, an error
message will appear in Step 3).
a. Hvdrozone/Landscape Feature Area: Enter the hydrozone or landscape feature
area in square feet.
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AA/aterSeriSe WaterSense Water Budget Approach and Tool
b. Plant Type or Landscape Feature: From the dropdown list, choose the plant type
(i.e., ground cover, shrubs, trees, etc.) or landscape feature (i.e., mulch or non-
planted area, or pool/spa or water feature) for the associated
hydrozone/landscape feature area. The landscape coefficient (KL) for the
respective plant type (or landscape feature) will automatically populate in the
adjacent cell. For mulch or non-planted area the KL is assumed to be zero and no
water requirement will be assigned in Table 1. For a pool/spa or water feature,
the associated KL is assumed to be that of a high water using plant such as turf
and is set to be 0.80. The source data for the plant type dropdown list and
associated KL values are displayed in Table 2. These plant types and associated
KL values are based on data provided by the University of California Cooperative
Extension's Leaflet 21943 (Costello 1993), also cited in IA, 2005. Note:
Additional plant types and associated landscape coefficients can be added to
Table 2 in the "[custom]" cells if specific or additional plant types are being
utilized in the landscape. If additional plant types and landscape coefficients are
added to Table 2, then these options will appear in the dropdown list in Table 1
as options. Landscape coefficients for common landscape plants may be
attainable at local Cooperative Extensions or online. For example, The
University of California's Cooperative Extension published a list of KL data for
common landscape plants in California in The Landscape Coefficient Method and
Water Use Classifications of Landscape Species III at
www.owue.water.ca.gov/docs/wucolsOO.pdf. If custom plant types are entered,
designate whether or not the plant type is turf by choosing "Yes" or "No" from the
dropdown list in the column titled, "Turf?."
c. Irrigation Type and Irrigation Efficiency (IE): Choose the type of irrigation (i.e.,
drip, rotor, etc) that will be installed on that hydrozone. The associated irrigation
efficiency will automatically populate in the adjacent cell. The source data for the
irrigation type and IE are displayed in Table 3 (IA 2005). Note: If the
hydrozone/landscape feature area is designated as "Mulch or Other Non-Planted
Area," then leave the irrigation type blank. If the hydrozone/landscape feature
area is designated as "Pool/Spa or Water Feature," then set the irrigation type to
fixed spray.
d. LWRh: The LWR for the hydrozone, in gallons per year, will be displayed in this
column.
4. The result, or output, is displayed in the blue cell in the green box. The LWR is
displayed in gallons per year (gallons/year).
5. Move to the next tab, Part 3, to review the results of the water budget tool.
Part 3: Results
1. In the blue box, confirm that the site information is pre-populated from Step 1.
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a. Note, If the total area of the hydrozone/landscape feature areas entered in Table
1 of Part 2 does not equal the total landscapable area entered in Part 1, then an
error message in red text will appear requesting that Table 1 be completed.
2. In the gray box, complete Step 3A by reviewing the total area of turfgrass in the
designed landscape (populated from Table 1 in Step 2B).
3. Complete Step 3B by reviewing the LWA and LWR calculated in Part 1 and Part 2
respectively. The results, or output, are displayed in the blue cells in the green box. The
LWR is displayed in gallons per year (gallons/year). The percentage of turfgrass in the
designed landscape is also displayed.
a. If the blue cell displays "YES" then the water budget criterion is met (i.e.,
LWR
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Landscape Water Requirement (LWR): A volume of water that is necessary for the landscape to
be healthy and functional (Water Management Committee 2001 in IA 2005).
Landscapable Area: The area of a site less the building area, driveways, paved walkways, and
other structures, such as impermeable decks and patios (WaterSense 2008).
Reference Evapotranspiration (Grass Reference Evapotranspiration) (ET0): Rate of
evapotranspiration from an extensive surface of cool-season grass cover of uniform height of 12
cm, actively growing, completely shading the ground, and not short of water (FAO 1998 and
ASCE 1990 in IA 2005).
Run Time Multiplier (RTM): Factor used to increase zone run time to account for lack of
distribution uniformity within the root zone (Water Management Committee 2001 in IA 2005).
Water Adjustment Factor (K™): A factor used to compute a landscape water allowance. This
factor is based on a landscape coefficient and an expected irrigation system efficiency (Water
Management Committee 2001 in IA 2005). Also see landscape water allowance.
Water Budget: A water budget is used to calculate the amount of water a landscape needs
taking into account the inputs and outputs of water to and from the root zone. Inputs, such as
precipitation, are subtracted from outputs, such as evapotranspiration, to calculate the water
needs of the landscape. Many factors are taken into consideration when calculating a water
budget, such as plant type and irrigation system efficiencies (WaterSense 2008).
D. References
Costello, et al. 1993. Leaflet 21945. University of California Cooperative Extension.
California Department of Water Resources. 2008. Model Water-Efficient Landscape Ordinance.
Irrigation Association (IA). 2005. Landscape Irrigation Scheduling and Water Management.
[Currently out for review]
WaterSense. 2008. Draft Water-Efficient Single-Family New Home Specification.
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