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 plants need at various times of the year. The crop coefficient used for drought tolerant plants is 0.6 while the crop-coefficient for warm season turf is 0.65. The potable outdoor budget uses a combination of drought tolerant (40%) and warm season turf (60%) crop-coefficients while the recycled water outdoor budget uses 100% of the crop-coefficient for warm season turf.

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