Skip to content
WateringTimes

Water budgets

The Two Numbers That Decide Your Landscape Water Budget

Checked against the sources below · August 27, 2026

Two numbers decide whether a landscape plan passes the state water budget check: estimated total water use and the budget cap. The first comes from your choices, the plants, the irrigation gear and the square footage of each zone. The second is set for you, from your city's climate and the size of the landscape. Keep the first at or under the second and the water budget part of plan check is done.

The number you build: estimated water use

Estimated total water use is the yearly water your design would draw, in gallons, computed zone by zone. Each zone follows one formula: the plants' water use rating, divided by irrigation efficiency, times the zone's square footage, times your city's annual reference evapotranspiration (ETo), times 0.62. Thirstier plants push the number up. A bigger area pushes it up. More efficient irrigation pulls it down.

You only control some of those inputs. The efficiency values are fixed by the state landscape ordinance, the Model Water Efficient Landscape Ordinance, not by your equipment brand: 0.75 for spray heads and rotors, 0.81 for drip. The plant rating comes from WUCOLS, the state's plant water classification. ETo comes from the ordinance's own city table in Appendix C, and the 0.62 simply converts inches of water over square feet into gallons.

Add every zone together and you have the total. On the official worksheet this figure is called ETWU, estimated total water use. It is the number the plan reviewer puts on the left side of the comparison.

The number you are given: the budget cap

The cap is the most water the whole project can use in a year. For a home it is ETo times 0.62 times 0.55 of the regular landscape area. Notice what is missing: plants. Nothing you plant changes the cap. It is built entirely from climate and square footage.

There are two adjustments. Non-residential projects use 0.45 instead of 0.55, so a business gets a tighter budget for the same area. And areas dedicated to edible plants, recreational areas, and areas irrigated with recycled water count at full value, 1.0, instead of 0.55. The ordinance calls these Special Landscape Areas. One catch for homeowners: recreational area means park and sports-field type space for active play or public assembly, and the definition states outright that private single-family residential landscapes are not recreational areas (§490.2(a)(68)). For a home, the practical route to the 1.0 counting is a dedicated edible garden or recycled water.

On the worksheet the cap is called MAWA, the maximum applied water allowance. Think of it as the budget line the design has to fit inside. You cannot negotiate it up; the only things that move it are the size of the landscape and how much of it qualifies as edible or recreation area.

Why plant choice is the biggest lever

The plant rating sits at the front of the formula, so it scales everything behind it. High water plants such as cool season lawn carry a rating around 0.8. Low water shrubs carry around 0.2. That is a fourfold gap before area, climate or irrigation enters the math, and nothing else in the formula comes close.

Irrigation helps too, just less. Drip divides the zone by 0.81 instead of 0.75, so the same plants on drip always compute lower than on spray. It is a real cut, but it cannot rescue a thirsty zone on its own. The order that works: pick lower water plants first, then put them on drip, and the two moves multiply.

That is why the fastest fix for a plan that runs over is almost never fancier hardware. It is changing what grows in the biggest, thirstiest zone.

One Sacramento yard, start to finish

Take a Sacramento home with 1,200 square feet of landscape and an annual ETo of 55.2 inches. The design has an 800 square foot lawn on spray, high water at 0.8, and 400 square feet of low water shrubs on drip at 0.2. The lawn computes to (0.8 ÷ 0.75) × 800 × 55.2 × 0.62 = 29,204 gallons a year. The shrubs compute to (0.2 ÷ 0.81) × 400 × 55.2 × 0.62 = 3,380 gallons. Estimated total water use: 32,584 gallons.

The cap for those 1,200 square feet is 55.2 × 0.62 × 0.55 × 1,200 = 22,588 gallons. The design is over by roughly 10,000 gallons. The lawn is the problem: that one zone uses more water than the entire budget for the whole yard.

Now replace the lawn zone with low water plants on drip. The same 800 square feet drops to 6,760 gallons a year. Together with the shrubs, the design now sits far below the 22,588 gallon cap, with room to spare for a small thirsty accent zone somewhere else.

What happens at plan check

Plan check runs the exact comparison you just did: estimated total water use at or under the cap. It arrives on a worksheet with your zones, ratings, areas and irrigation types filled in. If the total fits under the cap, the water budget portion of the review is satisfied and the reviewer moves on to the rest of the package.

If the total is over, the cap does not bend. The comparison is arithmetic, so the fix is always on the design side: swap plants, convert zones to drip, shrink the irrigated area, or some mix of the three, then resubmit the worksheet.

The good news is that none of this needs to happen at the counter. The formula has no judgment calls in it, so you can test a design in minutes before anyone draws a planting plan. Run your zones through the water budget calculator, see which side of the cap you land on, and adjust while changes are still free.

Questions people ask

Where does my ETo number come from?
From Appendix C of the state landscape ordinance, which lists annual reference evapotranspiration for California cities. Sacramento's value is 55.2 inches per year. Use the figure for your own city, since the same design can pass in one climate and run over in another.
Does switching to drip really lower my estimated use?
Yes. The ordinance fixes irrigation efficiency at 0.75 for spray and 0.81 for drip, and the formula divides by that value, so every drip zone computes lower than the same zone on spray. It is a helpful cut but a modest one, and it will not bring a large lawn under the cap by itself.
My design is over the cap. What actually moves the number?
Start with the biggest, thirstiest zone, because the plant rating multiplies everything else in the formula. In the Sacramento example, converting an 800 square foot lawn on spray to low water plants on drip takes that zone from 29,204 gallons to 6,760. Hardware tweaks and small area trims help at the margins, but plant swaps do the heavy lifting.
Do vegetable gardens make the budget harder to meet?
No, the opposite. Areas permanently dedicated to edible plants, such as orchards and vegetable gardens, count at full value in the cap instead of the reduced 0.55, under the ordinance's Special Landscape Area definition. Areas irrigated with recycled water get the same treatment; recreational areas do too, but that category means parks and sports fields, and the definition excludes private single-family residential landscapes outright.
Is the cap different for a business than for a home?
Yes. Residential projects build the cap from 0.55 of the regular landscape area, while non-residential projects use 0.45. For the same square footage and climate, a commercial site has a noticeably tighter water budget.
What is the 0.62 in the formula?
It converts inches of water spread over square feet into gallons per year. The ordinance fixes it at exactly 0.62, so use that value as written rather than a more precise conversion. Both the estimated use and the cap carry the same multiplier, so it never changes which side of the comparison you land on.

Related on WateringTimes

Sources

WateringTimes is an informational gardening and landscape reference for California. Watering numbers are estimates computed from public state data. Nothing on this site is regulatory or professional advice of any kind. Check your water supplier’s current rules before relying on any schedule.