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Publication #AE441

LEED for Homes: Explanation of the Landscape Irrigation Budget Calculation for Florida1

Michael D. Dukes2

New ratings systems are being developed for "green building." Green buildings aim to use less energy, water, and other natural resources than typical buildings. These rating systems are becoming popular within the sustainability movement. One such rating system is the "Leadership in Energy and Environmental Design" (LEED) rating system developed by the U.S. Green Building Council http://www.usgbc.org/leed. The LEED system is a scoring system that aims to rate buildings in categories such as water efficiency, energy efficiency, and use of natural resources. In this document, the landscape irrigation budget calculations will be explained for the LEED for Homes Rating System http://www.usgbc.org/Docs/Archive/General/Docs3638.pdf

This document explains the landscape irrigation budget found under "Landscaping" in section SS2 beginning on page 37 under "Method for Calculating Reduction in Irrigation Demand."

Step 1. Calculate the baseline irrigation water usage:

Where ETo is defined as the "Baseline Evapotranspiration Rate" in units of inches/month and Landscaped Area is in square feet. The 0.62 factor is a conversion factor to result in Baseline Usage of gallons/month.

In the scientific and engineering community, ETo is known as the "reference evapotranspiration" and is defined as the evapotranspiration rate from a hypothetical well watered green grass maintained at 0.12 m (5 inches) tall. The reference ETo concept is briefly explained in the EDIS publication Evapotranspiration: Potential or Reference? http://edis.ifas.ufl.edu/ae256. In general, once ETo is calculated, then it must be adjusted to represent a plant type of interest. In agricultural terms, this adjustment factor is known as a crop coefficient, Kc. The ETo and Kc concepts and detailed calculations can be found in "Crop evapotranspiration – Guidelines for computing crop water requirements" also known as "FAO Irrigation and Drainage Paper No. 56" http://www.fao.org/docrep/X0490E/X0490E00.htm. More recently, the American Society of Civil Engineers, Environmental Water Resources Institute (ASCE-EWRI) has developed a standardized ETo methodology that is being adopted by the scientific community because many approaches to calculating ET have been used in the past. These different approaches tend to lead to different answers depending on local weather variables. The ASCE-EWRI committee task report on this method can be found at, http://www.kimberly.uidaho.edu/water/asceewri/, along with links to purchase the manual for the full method.

The daily grass reference ETo calculation is identical for both FAO-56 and the ASCE-EWRI Standardized Method and is appropriate for Florida conditions. ETo can be calculated from weather data that may be found from sources such as the National Climatic Data Center http://www.ncdc.noaa.gov/oa/ncdc.html, which includes public weather stations in Florida. Data can also be downloaded from the Florida Automated Weather Network (FAWN) http://fawn.ifas.ufl.edu/ or FAWN. The FAWN system reports daily ETo; however, this ETo data is often limited historically and is not available for all locations in the state. Finally, FAWN does not use the ASCE-EWRI Standardized Method. The USGS has recently posted ETo data calculated using the ASCE-EWRI Standardized Method at http://fl.water.usgs.gov/et/ [17 December 2012]. This data set includes calculated ETo for a 2 km grid over the entire state on a daily basis. Thus, ETo values can be used for specific sites or for entire counties. According to the LEED for Homes Reference Guide (can be purchased at, http://www.usgbc.org/Store/PublicationsList_New.aspx, the July ETo should be used in the calculations. Although not specified by LEED, it is recommended that the 10-year average be computed from the USGS dataset. A longer dataset would be preferred if available.

In landscape terms, the crop coefficient adjustment factor is a landscape coefficient, KL as shown in the next step.

Step 2. Calculate the design case irrigation water usage:

Where ETL is defined as the landscape evapotranspiration rate (inches/month). IE represents irrigation efficiency and ultimately adjusts the amount of allowed water for non-uniform irrigation system coverage or minor losses that occur in all irrigation systems and can be found in Table 1 for various types of irrigation equipment. Photos of several of these types of equipment can be found in Operation of Residential Irrigation Controllers http://edis.ifas.ufl.edu/ae220.

The CF, or "control factor" represents conservation potential from an irrigation controller such as an ET controller, rain sensor or soil moisture based controller. Under Florida conditions a CF value of 0.6 (40% average savings) is recommended as a conservative value for soil moisture sensor controllers and ET controllers. The ET controller must have a rain sensor to bypass irrigation due to on-site rainfall measurement. If an ET controller does not have an on-site rainfall sensor, then a CF value of 0.80 should be used. If a rain sensor is used alone, a CF factor of 0.85 (15% average savings) should be used. These numbers are based on published and ongoing Smart Controller testing at the University of Florida (see What Makes an Irrigation Controller Smart? http://edis.ifas.ufl.edu/ae442, for more information on Smart Controllers). For detailed information on this testing and on current results go to, http://abe.ufl.edu/mdukes/.

As seen above, ETL is computed from ETo and a "species factor", KS, as well a "microclimate" factor, KMC. The species factor is meant to represent the inherent water demand of a given species category. Values are given for low water using plants to high water using plants in Table 2. Use an average value if low or high water need for a particular plant type is unknown. Ideally, plants with similar water requirements are grouped together in the landscape within individual irrigation zones to allow separate application of water to those plants. If this is not done in the landscape and irrigation design and installation, the adjustment factors should correspond to the plant type with the highest water requirement within an irrigation zone.

It is important to understand that for both the Baseline Usage and the Design Case Usage, the usage is calculated by a weighted average across plant types depending on the given amount of area they comprise in the overall landscape. As an example, consider a site as follows: 5,000 ft2 turfgrass, 2,000 ft2 shrubs, 2,500% ft2 groundcover, and 3,000 ft2 trees. The site is in Alachua County, Florida. From FAWN, the July ETo is 5.27 inches/month averaged from 2000 through 2008. Note that a longer period of record and use of the ASCE-EWRI ETo estimation method is also preferred.

Table 3 shows a summary of the calculations for the Design Case Usage. The Baseline Usage for this scenario is as follows:

Next, the Percent Reduction needs to be calculated,

Step 3. Calculate percentage reduction

For this example based on Table 3,

Since the Design Case Usage was higher than the Baseline Usage, the Percentage Reduction is negative. This result is largely due to the inefficient irrigation used for the turfgrass area. If an irrigation system consisting of rotary sprinklers with average efficiency were used on the turfgrass area, IE would become 0.7 and would change the calculations according to Table 4. Now the Percent Reduction becomes,

This example highlights the importance, not only of plant material needs, but also of a well designed and installed irrigation system. Replacing the rain sensor with a proven soil moisture sensor irrigation controller should increase the Percentage Reduction to 42% due to this type of controller being more than twice as efficient as a rain sensor and time clock at efficiently scheduling irrigation (i.e. CF changes from 0.85 to 0.60).

Tables

Table 1. 

Irrigation Efficiency

Irrigation type

Irrigation efficiency (IE)

Low

High

Fixed spray

0.4

0.6

Impact and microspray

0.5

0.7

Rotors

0.6

0.8

Multistream rotators

0.6

0.8

Low volume and point source (e.g., drip)

0.7

0.9

Source: LEED® for Homes Rating System. 2008. Retrieved February 5, 2015 from US Green Building Council: http://www.usgbc.org/Docs/Archive/General/Docs3638.pdf

Table 2. 

Species Factor

Vegetation type

Species factor (KS)

Low

Average

High

Trees

0.2

0.5

0.9

Shrubs

0.2

0.5

0.7

Groundcover

0.2

0.5

0.7

Turf

0.6

0.7

0.8

Source: LEED® for Homes Rating System. 2008. Retrieved February 5, 2015 from US Green Building Council: http://www.usgbc.org/Docs/Archive/General/Docs3638.pdf

Table 3. 

Example calculations for LEED Homes landscape irrigation budget using relatively inefficient sprinklers for turfgrass irrigation.

Plant

Type

Area

(ft2)

Irrigation

Type

IE[a]

KS[b]

Conditions

KMC[c]

ETL[d]

(inch/month)

Design

Case Usage

Each Area[e]

(inch/month)

Design

Case

Usage[f]

(gal/month)

Turfgrass

5,000

Fixed spray

0.5

0.7

Full sun

1.5

5.53

9.41

29,162

Shrubs

2,000

Drip

0.8

0.5

Some shade

0.8

2.11

2.24

2,777

Ground-

cover

2,500

Drip

0.8

0.5

Avg sun

1.2

3.16

3.36

5,207

Trees

3,000

Drip

0.8

0.5

Wind protection

0.9

2.37

2.52

4,687

Total

12,500

 

41,833

 

Data Origin

Site

plan

Site

plan

Table

8

Table

6

Site plan

Table

7

Eqn. 4

Eqn. 2

Footnote

[a]IE averaged across "high" and "low" values.

[b]KS uses "average" conditions from Table 6 (Note that Tables referenced here can be found in the LEED for Homes Rating System, http://www.usgbc.org/DisplayPage.aspx?CMSPageID=147).

[c]KMC uses factors matching conditions described above from values in Table 7. Note that for the KMC, the adjustments refer to water use as influenced by a microclimate. Thus, higher water demand conditions such as full sun will result in a larger KMC.

[d]ETo used in ETL calculation is 8 years of average July ETo from Alachua County as reported by FAWN as 5.27 inches/month.

[e]Design Case Usage Each Area is the depth of irrigation budgeted for each plant area and includes a CF of 0.85 in Eqn. 2 assuming that a rain sensor is used on the irrigation system.

[f]Design Case Usage Converts Design Case Usage Each Area by using 27,152 gallons/ac-inch of water and 43,560 ft2/acre = 0.62 conversion factor.

Table 4. 

Example calculations for LEED Homes landscape irrigation budget using relatively efficient sprinklers for turfgrass irrigation.

Plant

Type

Area

(ft2)

Irrigation

Type

IE[a]

KS[b]

Conditions

KMC[c]

ETL[d]

(inch/month)

Design

Case Usage

Each Area[e]

(inch/month)

Design

Case

Usage[f]

(gal/month)

Turfgrass

5,000

Rotor

0.7

0.7

Full sun

1.5

5.53

6.72

20,945

Shrubs

2,000

Drip

0.8

0.5

Some shade

0.8

2.11

2.24

2,777

Ground-

cover

2,500

Drip

0.8

0.5

Avg sun

1.2

3.16

3.36

5,207

Trees

3,000

Drip

0.8

0.5

Wind protection

0.9

2.37

2.52

4,687

Total

12,500

 

33,616

 

Data Origin

Site

plan

Site

plan

Table

8

Table

6

Site plan

Table

7

Eqn. 4

Eqn. 2

Footnote

[a]IE averaged across "high" and "low" values.

[b]KS uses "average" conditions from Table 6 (Note that Tables referenced here can be found in the LEED for Homes Rating System, http://www.usgbc.org/DisplayPage.aspx?CMSPageID=147).

[c]KMC uses factors matching conditions described above from values in Table 7. Note that for the KMC, the adjustments refer to water use as influenced by a microclimate. Thus, higher water demand conditions such as full sun will result in a larger KMC.

[d]ETo used in ETL calculation is 8 years of average July ETo from Alachua County as reported by FAWN as 5.27 inches/month.

[e] Design Case Usage Each Area is the depth of irrigation budgeted for each plant area and includes a CF of 0.85 in Eqn. 2 assuming that a rain sensor is used on the irrigation system.

[f]Design Case Usage Converts Design Case Usage Each Area by using 27,152 gallons/ac-inch of water and 43,560 ft2/acre = 0.62 conversion factor.

Footnotes

1.

This document is AE441, one of a series of the Agricultural and Biological Engineering Department, UF/IFAS Extension. Original publication date November 2008. Revised February 2015. Reviewed June 2018. Visit the EDIS website at http://edis.ifas.ufl.edu.

2.

Michael D. Dukes, associate professor, Department of Agricultural and Biological Engineering, UF/IFAS Extension, Gainesville, FL 32611.


The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

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