What is a landscape water budget?
The Alliance for Water Efficiency defines a landscape water budget as the amount of supplemental irrigation water a landscape requires, based on plant types, the size of the landscape, irrigation equipment, and weather factors including rainfall and evapotranspiration. Rather than one figure for a whole property, a budget is built hydrozone by hydrozone, a hydrozone being an area of plants with a similar water requirement, then summed to a property total.
The Alliance for Water Efficiency built its own approach on EPA WaterSense's Water Budget Tool, adapting it from an annual figure to a monthly one, since outdoor water use is highly seasonal and a monthly timescale better matches how water bills are typically read. This calculator follows that same monthly framing, and this is a personal planning budget, not a regulatory calculation.
If you specifically need California's Model Water Efficient Landscape Ordinance figures, the Maximum Applied Water Allowance (MAWA) or Estimated Total Water Use (ETWU), those follow the ordinance's own exact formula and requirements rather than this general approach; use the dedicated MAWA calculator or ETWU calculator instead.
How this landscape water budget calculator works
For each hydrozone you enter, the calculator multiplies area, plant factor and local ETo, following the same core equation behind EPA WaterSense's Water Budget Tool, then adds the zones together.
1. Each zone's budget
US units
Gallons (per zone) = Area (sq ft) × Plant factor × Monthly ETo (in) × 0.623
Metric units
Liters (per zone) = Area (m²) × Plant factor × Monthly ETo (mm)
If your ETo is reported per day or per week rather than per month, the calculator converts it using 30.4 days or 4.35 weeks per month, an average month length.
2. Plant factor
Plant factor ranges commonly used for this purpose, sourced from the Water Use Classification of Landscape Species (WUCOLS), run from 0.1-0.3 for low water use plants, 0.4-0.6 for moderate, and 0.7-1.0 for high. This calculator uses the middle of each range.
3. Property total and meter comparison
Total budget
Total = Sum of each zone's gallons (or liters)
If you enter your actual metered use for the month, the calculator shows the difference and the percentage above or below the calculated budget, as a planning comparison rather than a pass-or-fail check.
When this calculator isn't the right tool
Use a different approach for:
- California MWELO compliance figures (MAWA, ETWU). See the MAWA calculator or ETWU calculator.
- A single plant's canopy-based water need rather than a whole property. See the shrub watering calculator or the landscape coefficient calculator for a more detailed species/density/microclimate breakdown.
- Net and gross depth or a run time for a single zone. See the irrigation water requirement calculator.
Landscape water budget calculation example
A property with two zones: 2,000 sq ft of moderate-water-use planting (PF 0.5) and 500 sq ft of low-water-use planting (PF 0.2), with monthly ETo of 6 in.
- Zone 1: 2,000 × 0.5 × 6 × 0.623 ≈ 3,738 gal.
- Zone 2: 500 × 0.2 × 6 × 0.623 ≈ 374 gal.
- Total budget: 3,738 + 374 ≈ about 4,110 gallons.
The same calculation in metric
200 m² of moderate-water-use planting (PF 0.5), monthly ETo of 150 mm:
- Zone budget: 200 × 0.5 × 150 = about 15,000 liters.
Comparing with a meter reading
100 sq ft of high-water-use planting (PF 0.85), monthly ETo of 4 in, with 250 gallons of actual metered use:
- Budget: 100 × 0.85 × 4 × 0.623 ≈ 212 gal.
- Comparison: 250 actual vs. 212 budget ≈ about 18% above budget.
All examples use the same formulas the calculator runs; enter your own property's numbers above for your result.
Practical guidance
Group plants into hydrozones the way the Alliance for Water Efficiency describes: by similar water need, ideally matching how your irrigation system is actually zoned, rather than by garden bed or plant species alone. A lawn, a bed of drought-tolerant shrubs, and a vegetable garden are typically three separate hydrozones even if they're all in view of each other.
Update ETo through the season rather than reusing one figure all year; outdoor water use is highly seasonal, which is exactly why this budget works on a monthly rather than annual basis. If your water bill doesn't separate indoor and outdoor use, compare your coolest, lowest-use winter month against this budget's off-season ETo to estimate your baseline indoor use, then track how far summer months run above that baseline.
Check local watering rules. Many water utilities set watering days, hours or restrictions, especially during drought, and some tie billing tiers to a water budget of their own. Where those apply, they take priority over this planning estimate.
What changes the budget
| Factor | Effect | What to adjust |
|---|---|---|
| Zone grouping | Lumping very different plants into one zone blurs the real water need. | Split zones by actual water need, matching irrigation valves where possible. |
| Plant factor | Choosing the wrong water-use category over- or understates a zone's need. | Check the plant's WUCOLS category rather than guessing. |
| Season | ETo rises sharply in hot, dry, windy months and drops in cool, calm ones. | Recalculate monthly rather than using one annual average. |
| Rainfall | This budget doesn't subtract effective rainfall. | Expect actual irrigation need to run below budget after a wet month. |
| System efficiency | This budget is a net water-use figure, not a gross irrigation depth. | See the irrigation water requirement calculator to add efficiency and leaching. |
Frequently asked questions
What is a landscape water budget?
The Alliance for Water Efficiency defines a landscape water budget as the amount of supplemental irrigation water a landscape requires, based on plant types, the size of the landscape, irrigation equipment, and weather factors including rainfall and evapotranspiration. A budget is built for each hydrozone and summed to a property total, which this calculator does at a monthly scale so it lines up with how most water bills are read.
What is a hydrozone?
The Alliance for Water Efficiency defines a hydrozone as an area of plants that have a similar water requirement, ideally grouped on the same irrigation zone so that zone's schedule matches what those plants actually need. A property's lawn, a bed of drought-tolerant shrubs, and a vegetable garden would typically be different hydrozones.
How do I choose a plant factor for each hydrozone?
Plant factor ranges commonly used for this purpose, sourced from the Water Use Classification of Landscape Species (WUCOLS), run from 0.1-0.3 for low water use plants, 0.4-0.6 for moderate, and 0.7-1.0 for high. Choose the category that best matches the plants in that hydrozone, or enter your own value if you have a more specific figure.
Why is the budget monthly rather than weekly or annual?
The Alliance for Water Efficiency built its landscape water budget approach on EPA WaterSense's Water Budget Tool, which works at an annual scale, and adapted it to a monthly scale because outdoor water use is highly seasonal and a monthly timescale better aligns with how water billing typically occurs. This calculator follows that same monthly framing.
How do I compare this to my water meter or bill?
Enter your actual metered or billed water use for the month alongside the calculated budget, and the calculator shows the difference in gallons and as a percentage. This is a personal planning comparison, not a regulatory pass-or-fail check.
Is this the same as California's MWELO water budget (MAWA or ETWU)?
No. This calculator is a general property planning budget, not tied to any regulation. For California's Model Water Efficient Landscape Ordinance figures specifically, Maximum Applied Water Allowance (MAWA) and Estimated Total Water Use (ETWU), use the dedicated MAWA calculator and ETWU calculator instead, which follow that ordinance's exact formulas and requirements.
See the MAWA calculator or ETWU calculator.
Where do I find ETo for my property?
Reference evapotranspiration is published by state and regional agricultural weather networks, such as CIMIS in California or AgriMet in the Pacific Northwest, and often by local Cooperative Extension offices or water agencies. Many report it daily or weekly rather than monthly; this calculator accepts any of those periods and converts it for you.
What if my property has more than four hydrozones?
Group similar zones together where you can, since a hydrozone is meant to represent an area of plants with similar water needs rather than every individual bed. If you genuinely need more than four distinct budgets, run this calculator more than once and add the totals together.
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
- The per-zone, total-budget and meter-comparison math were verified by hand, and all three worked examples were traced against the code.
- The budget formula, hydrozone definition and monthly framing are checked against the Alliance for Water Efficiency and EPA WaterSense sources below.
- The page states explicitly that this is a personal planning budget, not a California MWELO compliance calculation, and links to the dedicated MWELO tools for that purpose.
Sources
- Alliance for Water Efficiency. Water and Energy Savings Estimates for CII Landscapes Upgrade Projects (hydrozone and landscape water budget definitions; monthly-scale rationale).
- EPA WaterSense. Water Budget Tool (base ET × plant factor × area methodology).
- California Department of Water Resources / City of Pinole. Water Efficient Landscaping ordinance text (WUCOLS plant factor ranges).
- Western Water (Western Municipal Water District). Water Budget Calculation (comparing a calculated budget with metered billing).
- Food and Agriculture Organization of the United Nations. Allen, R.G. et al. FAO Irrigation and Drainage Paper No. 56: Crop Evapotranspiration (reference ET context).