How are Commercial Water Budgets Calculated?

There are two types of commercial water budgets: Mixed-use accounts for customers that only use water indoors or may have both indoor and outdoor water use on the same meter; and dedicated irrigation accounts that only use water for landscape irrigation.

Mixed-Use Accounts
Water budgets for mixed-use accounts are calculated based on an average of their past use for each month. For new mixed-use commercial customers, a monthly average is calculated following the first full year’s water use.

Dedicated Irrigation Accounts
Water budgets for dedicated irrigation accounts are calculated using three factors:

  • Daily localized weather
  • Irrigated area
  • Landscape factor Localized Weather Data (Evapotranspiration or ET) 
The amount of water that is lost each day from 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 each day 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. County Assessor parcel data, the District’s Geographic Information System (GIS) and, in some cases, physical site measurements are used to determine the irrigated acreage of your landscape.
Landscape Factor
The landscape factor measures the specific amount of irrigation water required by your landscape. Water services installed in the District’s retail service area on or before December 31, 2011, are given a landscape factor of 0.8, which should be sufficient if your landscaping is mostly grass with a mix of shrubs or flowers. Water services installed on or after January 1, 2012, are given a landscape factor of 0.7 because new construction is required to use the latest water- efficient irrigation systems. Landscape sites that qualify under the 2010 State Model Landscape Ordinance as a “special landscape” are given a landscape factor of 1.0. It is the account holder’s responsibility to notify the District if a site qualifies for “special landscape” considerations. The special landscape factor is applied at the sole discretion of the District.

Dedicated Irrigation Water Budget Formula

(Irrigated acreage) x (ET) x (Landscape factor) x (.62 [conversion from inches to gallons of water])
• 43,560 square feet of irrigated acreage, ET for May 2010 of 5.72 inches
• 43,560 x 5.72 x 0.8 x .62 = 123,585 gallons of water =165 billing units (1 billing unit = 748 gallons)