How are Residential Water Budget Rates Calculated?

There are two components to residential water budgets: the indoor budget (Tier 1) and the outdoor budget (Tier 2).

Indoor Water Budget

The indoor budget is calculated using three factors:

  • The average amount of water an efficient person uses daily
  • The number of people in the household
  • The number of days in the billing cycle
Average Amount of Water Used
Studies show that, on average, a typical person uses less than 60 gallons of water each day indoors. This amount includes all indoor water use, such as showers and washing clothes, and is based on the use of common water-efficient devices, including low-flow toilets and shower heads. Based on this data, and a review of our customers’ historical water use, the District adopted an indoor water budget of 60 gallons per person per day.

People per Household
The most recent census data show there is an average of approximately three people per household in Western’s service area, so each single-family home receives a default water budget for three people. Condominiums and apartments receive a default budget for two people per dwelling unit. If you have more than three people in your household, call us at 951.571.7104 to request a change.

Days in the Billing Cycle

The number of days that you are being billed for service. This information can be located on your water bill and comes from the actual dates your meter is read. It may differ from bill to bill, but will usually be between 28 and 31 days.

Indoor Budget Formula
(60 gallons per day x number of people x number of days in the billing cycle) / 748 gallons per billing unit
• 3 people in the household, 30-day billing cycle
• 60 x 3 x 30 = 5,400 gallons of water = 7.2 billing units (1 billing unit =748 gallons)

Outdoor Budget
The outdoor budget is calculated using three factors:

  • Daily localized weather data
  • Irrigated area
  • Landscape factor 
Localized Weather Data (Evapotranspiration or ET)
The amount of water that is lost each day from your landscaping due to evaporation and plant transpiration is known as evapotranspiration (ET), and it varies daily due to factors such as wind, humidity and temperature. Western measures ET daily using a system that calculates precise weather data for more than 460 distinct microclimate zones within our retail service area. ET values change with the weather, so the water budget for landscapes is higher in hot weather and lower in cool weather.

Irrigated Area
The irrigated area is the amount of landscaped area on your property that receives regular watering. Pools and spas are also included in the irrigated acreage because they use approximately the same amount of water (due to evaporation and refilling) as a lawn does. County Assessor parcel data and the District’s Geographic Information System (GIS) can be used to determine the default irrigated acreage for your home.
Landscape Factor
The landscape factor measures the specific amount of irrigation water required by each type of plant in your yard. The rate structure is based on the assumption that your landscaped area is a mix of grass, trees, shrubs and ground cover, which combine for a landscape factor of 0.8. If your landscaping is mostly grass and you water it wisely, you can stay within your outdoor water budget. If your yard includes drought-tolerant landscaping and you water it wisely, you’ll have an even easier time staying within your outdoor water budget. Customers requesting new water service (installing a new water meter) from Western after January 1, 2012, receive a landscape factor of 0.7. For a list of water wise climate appropriate plants, click here.

Outdoor Water Budget Formula
(Irrigated acreage) x (ET) x (Landscape factor) x (.62 [conversion from inches to gallons of water])
• 5,000 square feet of irrigated acreage, ET for May 2010 of 5.72 inches
• 5,000 x 5.72 x 0.8 x .62 = 14,186 gallons of water =19 billing units (1 billing unit = 748 gallons)