Landscapes are vital to the quality of life. They provide recreation areas and enhance the surroundings by cleaning the air and water. Likewise, landscapes prevent erosion, offer fire protection, and replace ecosystems lost to development. Sustainable and healthy landscapes are achievable when using a standard formula to create a water budget.
Most sites rely on urban water for about half of their irrigation, so saving water is a critical factor in the design and maintenance of landscapes. Landscape professionals must identify and implement practical strategies that increase water use efficiency. An essential component of water conservation is to match the water supply to the plant’s needs. In other words, providing only the water needed to maintain landscape health and appearance. Thus, professionals can avoid unwanted applications that exceed plant needs.
Wind, evaporation, and runoff cause the loss of as much as 50 percent of the water we use outdoors. This waste is the result of inefficient irrigation methods and systems. — U.S. Environmental Protection Agency
What Is the Water Budget?
also referred to as “Maximum Allowed Water Allowance (MAWA)“
The Maximum Allowed Water Allowance, or MAWA, is especially important in California where the California Code of Regulations specifies the elements that cities should review before approving landscape projects; one of them is the MAWA calculation.
The MAWA calculation is a method of determining the upper limit of the annual allowed water for an established landscape area.
This estimation includes the following elements:
- Local reference evapotranspiration, or ETo (in inches per year)
- ET Adjustment Factor, or ETAF
- Total Landscape Area, or LA (in square feet)
- Special Landscape Area, or SLA (in square feet)
Local reference evapotranspiration (ETo) refers to a standard measurement of environmental parameters that affect plants’ water use. (This site provides the ETo for any United States zip code.)
ET Adjustment Factor (ETAF) refers to a factor that, when applied to the reference ETo, adjusts for plant water demands and irrigation efficiency. ETAF is 0.55 for residential areas and 0.45 for nonresidential areas.
Special Landscape Area (SLA) refers to the area of the landscape dedicated to:
- edible plants
- areas irrigated with recycled water
- water features using recycled water
- areas dedicated to active play, such as parks, sports fields, and golf courses
- areas where turf provides a playing surface
Landscape Area (LA) refers to all the planting areas, turf areas, and water features in a landscape design plan subject to the water calculation. The landscape area does not include:
- footprints of buildings or structures
- parking lots
- gravel or stone walks
- other permeable or nonpermeable hardscape
- other nonirrigated areas designated for nondevelopment
The Landscape Area includes any Special Landscape Area.
In a landscape or other planted area, water from the soil moves to the atmosphere due to evaporation from the soil surface and transpiration from plants. Evapotranspiration is the name of this process. — University of California, Division of Agriculture and Natural Resources
How Is the Water Budget Calculated?
The water budget can be calculated using the water budget formula:
ETo x 0.62 x [(ETAF x LA) + (1 – ETAF) x SLA]
where 0.62 is a conversion factor (from inches/year to gallons/year)
The following example shows how this water budget formula is done.
A landscape project in Sacramento, California, has an ETo value of 94.9 inches and a total landscape area of 30.000 square feet. Within the 30,000-square-foot project, a 3.000-square-foot area is planted with edible plants. This 3.000-square-foot area is a Special Landscape Area.
ETAF value is 0.45 for nonresidential areas.
ETo x 0.62 x [(ETAF x LA) + (1 – ETAF) x SLA]
94.9 inches x 0.62 gallons/year x [(0.45 x 30.000 square feet) + (1 – 0.45) x 3.000 square feet)]
58.84 gallons/year x [13.500 + 0.55 x 3.000]
58.84 gallons/year x [13.500 + 1.650]
58.84 x 15.150 gallons/year
The landscape will require about 890 gallons of water to maintain healthy plants.
Why Is Water Budget Important?
Many significant benefits emerge when professionals irrigate a landscape based on the water budget formula. Benefits include:
- An understanding of consumption patterns
- Information that helps determine how to best control irrigation
- Facts that can help manage demand during a drought
- Reduced energy costs of treating and pumping water
- Criteria to achieve water conservation
The water budget method works as a design tool, enabling professionals to create a sustainable landscape based on a quantity of water. In other words, this site-precise process determines the allowable amount of water used by the landscape and then helps the designer plan the landscape to fit this budget. The method includes plant variety, plant water demands, irrigation system design, and the amount of water that the landscape uses from either irrigation or precipitation. And the method must be associated with a particular period, such as a week, month, or year.