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 |
Landscape Area (LA) |
Base Water Budget |
Base Water Budget |
Total Water Budget |
|
Residential |
4 |
1300 sq. ft |
# Residents x 50 gpd |
ET x Kc x 1.25 x LA |
(Indoor x #days in bill service period) + Outdoor |
|
Residential |
3 |
435 sq. ft |
# 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 |
|
Site specific based on irrigated acreage |
N/A |
ET x Kc x 1.25 x LA |
Outdoor based on bill service period |
|
Recycled |
|
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 0.5 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
