What is ETWU and how is it calculated?
Estimated Total Water Use, or ETWU, is the annual water-use figure at the center of California's Model Water Efficient Landscape Ordinance (MWELO). It's an estimate of how much water a landscape design will use in a typical year, based on the local climate, how much area is planted with which kind of plants, and how efficiently the irrigation system delivers water. Under the ordinance's performance pathway, this estimate must be shown to be less than or equal to the project's Maximum Applied Water Allowance (MAWA), the water budget MWELO sets for that same landscape.
A hydrozone is a group of plants with similar water needs, irrigated together on the same schedule so none of them is over- or under-watered. MWELO groups hydrozones into water-use categories — low, moderate and high — each with a plant factor from WUCOLS (Water Use Classification of Landscape Species), plus a separate category for Special Landscape Areas. ETWU is calculated hydrozone by hydrozone and then added together for the whole project.
| Category | Plant factor range | Typical plants |
|---|---|---|
| Low water use | 0 – 0.3 | Many California natives, Mediterranean shrubs, succulents |
| Moderate water use | 0.4 – 0.6 | Many shrubs and groundcovers, some ornamental grasses |
| High water use | 0.7 – 1.0 | Cool-season turfgrass, many annuals and vegetables |
| Special Landscape Area | up to 1.0 | Edible gardens, recycled-water areas, active-play turf |
Look up the plant factor for your actual species in WUCOLS rather than guessing from the category alone; two low-water-use plants can still sit at very different points inside that 0 to 0.3 range.
MWELO applies to many landscape projects in California above a minimum size. Local jurisdictions administer it and can adopt their own equivalent ordinance with different thresholds or stricter requirements, so check with your local planning or building department before relying on this estimate for a submission.
How this ETWU calculator works
The calculator finds an ET adjustment factor (ETAF) for each hydrozone from its plant factor and irrigation efficiency, multiplies that by the local ETo and the hydrozone's area, and adds the hydrozones together. It builds your MAWA the same way, using a fixed ETAF for the whole landscape instead of one per hydrozone.
1. ET adjustment factor per hydrozone
ETAF
ETAF = Plant factor ÷ Irrigation efficiency
For a Special Landscape Area, MWELO sets ETAF at up to 1.0 directly; no plant factor or irrigation efficiency is needed.
2. ETWU per hydrozone
US units
Gallons/year = ETo (in/yr) × 0.62 × ETAF × Area (sq ft)
0.62 rounds the exact 144 ÷ 231 ≈ 0.6234 gallons per square foot-inch, the same constant this site's other volume calculators use. This calculator carries the unrounded value internally so results still convert cleanly between US and metric units.
Metric units
Liters/year = ETo (mm/yr) × ETAF × Area (m²)
MWELO itself is written in US units only. This metric version uses the exact physical relationship that one millimeter of water over one square meter equals one liter, so no separate conversion constant is needed.
3. Total ETWU
Total
Total ETWU = Low-use ETWU + Moderate-use ETWU + High-use ETWU + Special Landscape Area ETWU
4. Maximum Applied Water Allowance (MAWA)
MAWA
MAWA = ETo × 0.62 × [(ETAFtype × LA) + ((1 − ETAFtype) × SLA)]
LA is the total landscape area (every hydrozone plus any Special Landscape Area). ETAFtype is 0.55 for residential landscapes and 0.45 for non-residential landscapes. SLA is the Special Landscape Area portion of LA. The metric version drops the 0.62 constant the same way the ETWU formula does.
5. Compliance check
MWELO's performance pathway requires that a project's total water use cannot exceed its maximum allowance, so this calculator flags a design as within budget only when total ETWU is less than or equal to MAWA, and shows both numbers so you can see the margin either way.
When this calculator isn't the right tool
It's built for the MWELO water-budget calculation itself. Use a different approach for:
- A MAWA figure on its own, without breaking out ETWU by hydrozone. See the MAWA calculator.
- A general landscape water budget outside a MWELO-style ordinance. See the landscape water budget calculator.
- The Irrigation Association's landscape coefficient (KL) method, which uses a density and microclimate factor instead of MWELO's plant factor and irrigation efficiency. See the landscape coefficient calculator.
- A full official submission, which needs your jurisdiction's actual Water Efficient Landscape Worksheet, a named plant list per hydrozone, and often an effective-precipitation credit that this planning-level estimate leaves out. See the effective rainfall calculator if you need to estimate that credit separately.
ETWU and MAWA calculation example
A residential project has an ETo of 46 inches per year. It has 800 sq ft of low water-use planting (plant factor 0.2, drip), 500 sq ft of moderate water-use planting (plant factor 0.5, drip) and 1,200 sq ft of turf (plant factor 0.8, spray). There's no Special Landscape Area.
- Low water use: ETAF = 0.2 ÷ 0.81 = 0.2469. ETWU = 46 × 0.62 × 0.2469 × 800 ≈ about 5,660 gallons.
- Moderate water use: ETAF = 0.5 ÷ 0.81 = 0.6173. ETWU = 46 × 0.62 × 0.6173 × 500 ≈ about 8,850 gallons.
- High water use (turf): ETAF = 0.8 ÷ 0.75 = 1.0667. ETWU = 46 × 0.62 × 1.0667 × 1,200 ≈ about 36,700 gallons.
- Total ETWU: 5,660 + 8,850 + 36,700 ≈ about 51,220 gallons a year.
- MAWA: total landscape area = 2,500 sq ft, all regular (no SLA). MAWA = 46 × 0.62 × (0.55 × 2,500) ≈ about 39,430 gallons a year.
- Result: ETWU (about 51,220 gal) is over MAWA (about 39,430 gal) by roughly 11,790 gallons a year — this design is over its water budget, driven mainly by the turf area.
The same calculation in metric, and passing this time
A non-residential project has an ETo of 900 mm per year: 80 m² low water use (plant factor 0.2, drip), 30 m² moderate water use (plant factor 0.4, drip) and 20 m² high water use turf (plant factor 0.8, spray). No Special Landscape Area.
- Low water use: ETAF = 0.2469. ETWU = 900 × 0.2469 × 80 ≈ about 17,780 liters.
- Moderate water use: ETAF = 0.4 ÷ 0.81 = 0.4938. ETWU = 900 × 0.4938 × 30 ≈ about 13,330 liters.
- High water use: ETAF = 1.0667. ETWU = 900 × 1.0667 × 20 ≈ about 19,200 liters.
- Total ETWU: 17,780 + 13,330 + 19,200 ≈ about 50,310 liters a year.
- MAWA: total landscape area = 130 m², non-residential ETAF 0.45. MAWA = 900 × (0.45 × 130) = 52,650 liters a year.
- Result: ETWU (about 50,310 L) is under MAWA (52,650 L) by roughly 2,340 liters a year — this design is within its water budget.
Comparing the two examples shows why turf area matters so much: swapping some high water-use area for low or moderate water-use planting, or moving it to drip, brings ETWU down toward MAWA without changing the project's total footprint.
Using your ETWU and MAWA results
Start with a correct ETo. Use the CIMIS station or the MWELO appendix city closest to the actual project site, not a nearby major city that may sit in a different climate zone; ETo can vary noticeably across short distances near the coast or in mountainous terrain. If your source gives ETo per day or per month, convert it to an annual figure with the reference ET calculator before entering it here.
Assign plant factors from WUCOLS for the plants you're actually specifying, not just the category midpoint. Group hydrozones by both water need and irrigation method together, since MWELO expects a hydrozone to be watered on one schedule; a bed with the same plants on spray in one section and drip in another is really two hydrozones for this calculation.
Remember that MWELO's irrigation efficiency values (0.75 spray, 0.81 drip) are minimums the ordinance assumes for the compliance calculation, not a claim about how efficiently a specific installed system will actually run; verify uniformity with a catch-can test or the irrigation efficiency calculator once the system is installed. Double-check which areas actually qualify as Special Landscape Area before classifying them that way; MWELO limits SLA to edible plantings, recycled-water areas and active-play turf, not just any drought-tolerant planting.
If your ETWU comes in over MAWA, the areas to revisit are usually the ones with the largest high water-use footprint, since turf area affects ETWU the most for a given square footage. Reducing turf, as modeled by the turf removal savings calculator, moving a hydrozone from spray to drip, or substituting lower water-use plants are the three levers this formula responds to directly.
What changes your ETWU and MAWA
Every input in the formula moves the result in a predictable direction. This is where to look first if a number seems off, or if you need to bring a design back within budget.
| Factor | Effect | What to adjust |
|---|---|---|
| ETo (location) | Scales every hydrozone's ETWU, and MAWA, proportionally. | Use the CIMIS station or city closest to the exact project site. |
| Plant factor | Higher plant factor raises ETAF and that hydrozone's ETWU. | Check WUCOLS for the actual species; choose lower water-use plants where practical. |
| Irrigation efficiency | Lower efficiency raises ETAF for the same plants. | Specify drip (0.81) over spray (0.75) where the planting allows it. |
| Hydrozone area | Scales that hydrozone's ETWU directly. | Reducing high water-use area, especially turf, has the largest effect per square foot. |
| Special Landscape Area | Counted at up to ETAF 1.0 in ETWU, but also raises the SLA credit inside MAWA. | Only classify area as SLA if it meets MWELO's definition. |
| Residential vs. non-residential | Sets the ETAF used in MAWA: 0.55 residential, 0.45 non-residential. | Confirm the project's classification with the local reviewing agency. |
| Local ordinance amendments | Some jurisdictions adopt a local WELO with a lower ETAF, higher efficiency minimums, or added requirements. | Check with the local planning or building department before finalizing the design. |
Frequently asked questions
What is ETWU under MWELO?
Estimated Total Water Use (ETWU) is the annual water use MWELO calculates for a landscape design, based on its reference evapotranspiration, hydrozone areas, plant factors and irrigation efficiency. It answers the question a MWELO reviewer asks first: how much water will this design actually use in a typical year?
How is ETWU calculated?
For each hydrozone, ETWU equals reference evapotranspiration (ETo) multiplied by 0.62, by the plant factor divided by irrigation efficiency (together called ETAF), and by the hydrozone's area. Add the results for every hydrozone, including any Special Landscape Areas, to get the project's total ETWU.
What is the difference between ETWU and MAWA?
ETWU is how much water your specific design is estimated to use. MAWA, the Maximum Applied Water Allowance, is the ceiling MWELO sets for the same landscape, based on its total area and whether it is residential or non-residential. A compliant design keeps ETWU at or below MAWA.
What plant factor should I use for each hydrozone?
Use the plant factor from WUCOLS for the actual plants in that hydrozone. If you are still choosing plants, WUCOLS' categories give a starting range: 0 to 0.3 for low water use, 0.4 to 0.6 for moderate, and 0.7 to 1.0 for high water use, which includes most cool-season turfgrass.
See WUCOLS' plant search database for values by species and climate zone.
What irrigation efficiency does MWELO assume?
For calculating ETWU, MWELO assumes an irrigation efficiency of 0.75 for overhead spray devices and 0.81 for drip and other micro-irrigation devices, regardless of the specific equipment installed. A different value can be used only where the local reviewing agency approves a custom efficiency for the project.
What happens if my ETWU is higher than my MAWA?
The design does not meet the ordinance's water budget requirement, and a MWELO reviewer would ask for changes before approving it. Common fixes are reducing high water-use area, especially turf, switching some hydrozones to drip, or replacing moderate or high water-use plants with lower water-use species.
The turf removal savings calculator can help size how much turf to convert.
What counts as a Special Landscape Area?
MWELO defines Special Landscape Area as space dedicated solely to edible plants, areas irrigated with recycled water, water features using recycled water, and areas for active recreation such as parks, sports fields and golf courses where turf provides the playing surface. These areas get an ETAF of up to 1.0 in both ETWU and MAWA.
Where do I find ETo for my project?
California's CIMIS network publishes reference evapotranspiration by weather station, and MWELO's own appendix lists typical annual ETo by city and county. Use the station or city closest to your project site.
The reference ET calculator can help convert a daily or monthly figure into the annual value this calculator needs.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- Formulas match MWELO's official Water Efficient Landscape Worksheet and MAWA equation, verified by hand against both worked examples.
- Plant factor ranges, ETAF values and irrigation efficiency minimums checked against the current MWELO regulation text.
- Unit conversions between US and metric checked round trip.
Sources
- California Department of Water Resources. Model Water Efficient Landscape Ordinance, Title 23, California Code of Regulations, Chapter 2.7.
- California Department of Water Resources. Landscape Irrigation Water Budget Overview, MWELO Guidebook.
- University of California, Davis, California Center for Urban Horticulture. WUCOLS: Water Use Classification of Landscape Species.
- California Department of Water Resources. California Irrigation Management Information System (CIMIS).