The handbook of Technical irrigaTion informaTion - Hunter Industries

Handbook of Technical Irrigation InformationFormulas – Industry MandatesAB 1881, California Calculation of Estimated Total Water Use (ETWU)This formula is used to calculate the estimated amount of water used in a landscape. The ETWU must be lessthan the Maximum Applied Water Allowance (MAWA), as shown in the previous formula, in order to receive projectapproval. The use of this formula is mandated by California State Assembly Bill 1881.ETWU =(Et o )(0.62)( PF x HA + SLA)IEWhere:ETWU = Estimated Total Water Use per year (gallons)Et o = Reference Evapotranspiration (inches)PF = Plant Factor from WUCOLS (see Section 491)HA = Hydrozone Area [high, medium, and low water use areas] (square feet)SLA = Special Landscape Area (square feet)0.62 = Conversion FactorIE = Irrigation Efficiency (minimum 0.71)Example:Using example 1 from page 11, determine the ETWU of an irrigated landscape area of 50,000 square feet withoutany Special Landscape Areas (SLA=0, no edible plants, recreational areas, or use of recycled water). The ET o valueis 51.1 inches annually, and the plant water use type, plant factor, and hydrozone area are shown in the table below.Hydrozone Plant Water Use Type(s) Plant Factor (PF)**Plant Factor from WUCOLSETWU = (51.1)(0.62)( 24,700 0) +0.71= 1,102,176 gallons per yearHydrozone Area (HA)(square feet)PF x HA (square feet)1 High 0.8 7,000 5,6002 High 0.7 10,000 7,0003 Medium 0.5 16,000 8,0004 Low 0.3 7,000 2,1005 Low 0.2 10,000 2,000Sum 24,700Compare ETWU with MAWA: For this example MAWA = (51.1) (0.62) [(0.7 x 50,000) + (0.3 x 0)] = 1,108,870gallons per year. The ETWU (1,102,176 gallons per year) is less than MAWA (1,108,870 gallons per year). In thisexample, the water budget complies with the MAWA.12