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Maximum Allowable Water Allowance
and Estimated Total Water Use
(MAWA and ETWU)
Following up on our last newsletter: Evapotranspiration (ET), the second most common question we receive is....
...what is MAWA and ETWU and why is it important to know and understand these two concepts?
AB 1881 specifies the elements of the Landscape Documentation Package. This includes MAWA and ETWU calculations. These important calculations determine the maximum amount of water that is allowed to be applied to a landscape. The actual definitions of each are as follows:
MAWA= Maximum Applied Water Allowance. This is the upper limit of the annual allowed water for an established landscape area. The calculated number is based upon the size of the Landscape and evapotranspiration (ETo).
ETWU Estimated Total Water Use. As you will shortly learn, ETWU is estimated water use based upon the types of plant material used in your design (based upon WUCOLS classifications).
This cannot exceed MAWA!
Confusing? Not really. The following example will help you to understand each of these terms and calculations. As you will see, its fairly easy to calculate each one.
Example: A hypothetical landscape project in Fresno, CA with an irrigated landscape area of 50,000 square feet without any Special Landscape Areas (SLA). (An SLA=0 means no edible plants, recreational areas or use of recycled water).
We notice from looking in CIMIS that the annual ET value for Fresno is 51.1 inches of water.
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MAWA = Maximum Applied Water Allowance (gallons per year)
MAWA = (ETo) (.62) [(0.7 x LA) + (0.3 x SLA)]
ETo = Reference ET (inches per year)
0.62 = Conversion Factor (to gallons)
0.7 = ET Adjustment Factor (ETAF)
LA = Landscape Area including any SLA (square feet)
0.3 = Additional Water Allowance for any SLA
SLA = Special Landscape Area (Square feet)
Lets plug in the numbers and chug....
MAWA = (51.1 inches) (0.62) [(0.7 x 50,000 sq ft) + (0.3 x 0) = 1,108,870 gallons per year.
To convert from gallons per year to hundred cubic feet per year we use the conversion factor of: (100 cubic feet = 748 gallons)
1,108,870/748 = 1,482 hundredcubicfeet per year.
So this is our Maximum Applied Water Allowance. The key words here to note are "Maximum" and "Allowance". Its what you're allowed to apply to this landscape!
Next, we will calculate the Estimated Total Water Use (ETWU)
The sum of the Estimated Total Water Use for all hydrozones cannot exceed the MAWA amount we just calculated above.
ETWU = (ETo)(0.62) [(PF x HA)/IE + SLA]
Where:
ETo = Reference ET (inches)
PF = Plant Factor from WUCOLS
HA = Hydrozone Area (low, medium and high water use areas) sq ft
SLA = Special Landscape Area, sq ft
0.62 = Conversion Factor
IE = Irrigation Efficiency (minimum allowed is 0.71) This is based on the emission device you select: drip, spray, rotor etc..for this example lets use spray=.71.)
For our example lets assume we have a single Hydrozone of low water plants (Pf = .2)
ETWU = (51.1) (0.62) [(50,000 * .2) +0] = 446,225 gallons per year.
If you have multiple hydrozones, we repeart this process for each zone and sum them up. For this simple example our ETWU = 446,225 Gal/year.
Now lets compare ETWU with MAWA:
MAWA 
1,108,870 

ETWA 
446,225 

As you can clearly see, the Estimated Total Water Use does not exceed the Maximum Applied Water Allowance. In this example the water budget complies with the MAWA.
Reference: California Department of Water Resources. (www.water.ca.gov)
The calculations of MAWA and ETWU are easy and straight forward to understand and generate.
Concerned? Confused? Freaked out?
Don't be, we got it covered.
AtomicIrrigation
1158 26th St., #241
Santa Monica, CA 90403
310.592.4707
www.AtomicIrrigation.com
"Atomic Irrigation........Irrigating the planet"
(in a AB 1881 world)
