Technical Water Budget Formula

Technical Water Budget Formula

Water budgets are based on number of residents, landscape square footage and actual daily weather and evapotranspiration (ET) data for your area. ET is the amount of water required for landscape to be healthy and attractive. Turfgrass is the highest water-using plant in a landscape. Trees and shrubs use far less water than turfgrass. IRWD's water budget system assumes that 40% of your landscape is planted with drought tolerant varieties irrigated with a water efficient system such as drip irrigation. If you continue to maintain a lawn on 60% or more of your property, you will likely be over your monthly water budget.

For those customers that want to understand the details more fully, below is the actual formula for calculating your water budget. Monthly allocation includes fixed component for indoor usage and variable component based on evapotranspiration (ET) rate for landscape irrigation. 

Account Type

Base # of
Residents

Landscape Area (LA)

Base Water Budget
Indoor

Base Water Budget
Outdoor

Total Water Budget

Residential
Detached

4

1300 sq. ft
(0.03 acres)

# Residents x 50 gpd

ET x Kc x 1.25 x LA

(Indoor x #days  in bill service period) + Outdoor

Residential
Condo
Attached/
Detached*

3

435 sq. ft
(0.01 acres)

# Residents x 50 gpd

ET x Kc x 1.25 x LA

(Indoor x #days in bill service period) + Outdoor

Apartments*

2

N/A

# Residents x 50 gpd

 

Indoor x #days in bill service period

Potable
Irrigation

 

Site specific based on irrigated acreage

N/A

ET x Kc x 1.25 x LA

Outdoor based on bill service period

Recycled
Irrigation

 

Site specific based on irrigated acreage

 N/A

ET x Kc x 1.33 x LA

Outdoor based on bill service period

Commercial, Industrial, Institutional

 

 

Site specific, adjusted for # of days in a bill service period

Site specific, based on irrigation needs

Site specific, adjusted for # days in bill service period

*For master-metered apartments and condominiums, the base water budget is multiplied by the number of dwelling units.

gpd = gallons per day

CCF = 100 cubic feet. 1 CCF = 1 billing unit = 748 gallons

ET (evapotranspiration) – from IRWD weather stations located in coastal, central or foothill zones.

Kc (crop co-efficient) – relative amount of water drought tolerant plants need at various times of the year. Crop-coefficient of 0.6 for drought tolerant landscapes is applied for potable irrigation, and a crop co-efficient averaging 0.65 for warm-season turf is applied to recycled water irrigation.

1.25 or 1.33 irrigation efficiency –  extra water to make up for inefficiencies in the irrigation system. An irrigation efficiency of 80% is applied to potable water and 75% to recycled water. In the formula this is calculated as = 1/0.8 = 1.25 is applied to potable customers and 1/0.75 = 1.33 is applied to recycled water.

LA  = irrigated landscape acreage 

 

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