Water Budgets

Our water budget equation is based on the Landscape Coefficient Method that is the standard of the Green Industry. It is endorsed by the Irrigation Association, California Landscape Contractors Association, California Urban Water Conservation Council, and over 300 water agencies as the best methodology for identifying and maximizing landscape irrigation efficiency.

Water Budget Equation

BUDGET = Volume of water budgeted for a given hydrozone area for a given period. The overall water use budget for a site is the sum of budgets over all hydrozones.
AREA = Landscape area irrigated in hydrozone (square feet).
KL = KS * KD * KMC
KS = Species or plant factor relating a specific plant type’s water requirements as a fraction of ETo.
KD = Density factor accounting for differences in vegetation density or collective leaf area among landscape plantings. The default value is 1.0. For sparsely planted areas, the density factor is less than 1.0. For areas with multiple tiers of foliage canopies, the density factor is greater than 1.0.
KMC = Microclimate factor adjusting ETo for variations in exposure. The default value is 1.0. For areas with abundance of shade (e.g., the north side of buildings and under mature trees), for example, the microclimate factor is less than 1.0. For plantings surrounded by heat-absorbing surfaces or reflective surfaces, the microclimate factor is greater than 1.0.
ETo = Reference evapotranspiration (inches) equals the depth of water evaporated and transpired from a reference crop (4 to 7 inch tall fescue grass) with an abundant water supply. ETo is the “standard” measure of water needs from which other plant types are compared via KS.
ERain = Effective rainfall (inches) equals the depth of rain effective in offsetting ETo for each hydrozone. Effective rainfall varies widely with rainfall frequency, magnitude, time of year, and root zone depth.
IE = Irrigation Efficiency measures the percent of applied water that is beneficially used by plants. All irrigation systems have some inefficiencies as water is lost as runoff, overspray, or percolates past the root zone.
C = Conversion factor putting the water budget in desired volumetric terms. A factor of 0.0008333 puts the water budget into hundred cubic feet (ccf). A factor of 0.0006233 puts the water budget into thousand gallons.

Example Billing Period

Hydrozone Area (ft2) KL ETo ERain IE Water Budget (ccf)
Turf 71,775 0.8 6.00 0 0.65 442
Shrub 62,845 0.4 6.00 0 0.80 157
Fountain 1,000 1.2 6.00 0 1.00 6
Total 135,620 605

Waterfluence Application of Water Budget Equation

We use the best information available to make the most accurate water budgets possible for a site. Sites have unique circumstances, but through extensive field validation, most of our budgets are relatively accurate with true water budget values within the +/- 15 percent range shown on the water use reports. When inaccuracies occur, however, we want to correct the underlying water budget assumptions. The objective is to get the most accurate water budgets possible.

Watering Station

When actual water use deviates significantly from the water budget, as a first step, review the Site Map to make sure the irrigated or hydrozone area assumptions are correct. We commonly aggregate plants into three hydrozones: turfgrass (warm or cool season), shrubs/trees/groundcovers, and pool/ponds/fountains—all which are shown on the Site Map. Contact us if you see errors as it is relatively easy to revise the electronic Site Maps.

The KL and IE factors are more technical in nature. We commonly use a KS of 0.8 for cool season turfgrass, 0.6 for warm season turfgrass, 0.4 for shrubs/trees/groundcovers, and 1.2 for pools/ponds/fountains. We use KD ranging between 0.5 and 1.3 and KMC ranging between 0.5 and 1.4., but in most cases we use 1.0 for both. We use IE ranging from 0.5 to 0.8 depending on the type of irrigation system and landscape configuration.

ETo and rainfall come from local weather stations (e.g., CIMIS) for each site. A daily soil moisture balance equation determines how much rain is effective. Effective rainfall ranges between 10 to 50 percent of total rainfall.

References

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