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The climate number

What Is ETo? California Reference Evapotranspiration

Checked against the sources below · August 27, 2026

ETo, short for reference evapotranspiration, is the amount of water your local climate pulls out of a well-watered field of cool-season grass, measured in inches. It is California's standard yardstick for how thirsty a place is. San Francisco sits at 38.7 inches a year and Palm Springs at 68.0, and every watering schedule and water budget starts from that number.

A field of grass as the yardstick

Evapotranspiration is two losses added together: water that evaporates from the soil surface and water that plants pull up through their roots and release through their leaves. Reference evapotranspiration pins that idea to one standard scene. Picture a large field of cool-season turfgrass, four to seven inches tall, never short of water. ETo is the estimate of how much water that field loses to the weather. The state landscape ordinance, the Model Water Efficient Landscape Ordinance, defines it as a standard measure of the environmental conditions that drive plant water use.

Why grass? Because it is uniform, it grows everywhere, and a well-watered lawn is one of the thirstiest common plantings. That makes it a stable ceiling. The number says nothing about your actual yard. It describes the demand of the sky: sun, heat, wind, and dry air doing the pulling.

ETo is expressed in inches per day, month, or year. An inch of ETo means an inch of water depth lost across the whole area, the same way a rain gauge counts an inch gained. So a month with 8 inches of ETo is a month when the climate tried to take 8 inches of water off every square foot of that reference lawn.

Why the number swings so widely across California

The spread across California is huge. San Francisco, wrapped in fog and cool marine air, loses 38.7 inches a year. Los Angeles comes in at 50.0. Sacramento, out in the Central Valley heat, reaches 55.2. Riverside, further inland, hits 59.4. Among the big cities the desert tops the list: Palm Springs prints 68.0 and Victorville 68.6, and the full 676-city table stretches even wider, from 33.8 in foggy Montara on the San Mateo coast to 92.5 in Baker, deep in the Mojave.

That is why the state landscape ordinance builds its water budgets on annual ETo. The definition itself says the annual value is the basis for the maximum water allowance so that regional differences in climate are accommodated. The same landscape gets a different budget in a different city, automatically.

The practical takeaway: a yard in Palm Springs faces nearly twice the climate demand of the same yard in San Francisco. If a plan or a schedule ignores that gap, it is wrong in at least one of those two cities.

Monthly ETo is your watering calendar

The annual number sets the budget, but the twelve monthly numbers set the rhythm. Sacramento is the clearest example: 1.3 inches of ETo in January against 8.6 inches in July. The climate demands more than six times as much water in midsummer as in midwinter. That is the whole reason summer schedules run several days a week while winter irrigation often drops to zero.

Geography shapes the curve, not just the total. San Francisco's July peak is only 5.2 inches, so coastal watering seasons are flatter and gentler. Palm Springs peaks at 9.2 inches in July, a long and steep demand curve that starts climbing in early spring.

In practice you read the monthly values like a calendar. Low winter ETo plus winter rain usually means the irrigation system rests. Demand ramps up through spring, peaks in July, and tapers through fall. Matching your controller to that shape, instead of running one setting all year, is the single biggest scheduling win.

Budgets and plant needs both scale from it

ETo is the input for both sides of the water math. On the budget side, the ordinance uses your city's annual ETo as the basis for the maximum water allowance of a new landscape. Bigger climate demand, bigger allowance; the number does the regional adjusting for you.

On the plant side, every planting is described as a share of ETo. A cool-season lawn needs close to the full reference amount, because it is the reference. Low-water shrubs like lavender or rosemary get by on a fraction of it. Multiply the share by your city's ETo and you have a real monthly water need in inches.

The values themselves come from Appendix C of the ordinance: 676 cities across all 58 counties, each with twelve monthly ETo values plus an annual, in inches. Our calculator and city schedules use those exact printed numbers, so what you see here matches the adopted table line for line.

Questions people ask

Is ETo the amount I should water my yard?
No. ETo is the climate's demand on the thirstiest standard planting, a well-watered cool-season lawn. Most plants need only a fraction of it, and rain covers part of the rest. Think of ETo as the ceiling that schedules and budgets are calculated from, not a number to put on your controller.
What does one inch of ETo actually mean?
It is a depth of water, exactly like rain in a rain gauge but flowing the other way. One inch of ETo means the weather pulled a one-inch-deep layer of water off every square foot of the reference lawn. That is why irrigation amounts and ETo can be compared directly: both are inches of water over an area.
Why is the reference a lawn of cool-season grass?
Because it is a scene scientists can reproduce anywhere: a large, uniform, four- to seven-inch stand of grass that is never short of water. Its water use responds cleanly to sun, heat, wind, and humidity. Once that ceiling is fixed, every other plant can be described as a share of it.
Does ETo include rain?
No. ETo only measures what the atmosphere pulls out, not what rain puts back. In most of coastal and valley California, winter rain arrives exactly when monthly ETo is at its lowest, which is why irrigation can often stay off through the cool months. In summer the rain stops, ETo peaks, and irrigation has to carry the load.
Where do California's ETo numbers come from?
From Appendix C of the state landscape ordinance, the table adopted with the 2025 text. It lists 676 cities across all 58 counties, each with twelve monthly values and an annual total in inches. We print the values as adopted and never recompute them.
My city is not one of the examples here. How do I find its ETo?
The Appendix C table covers 676 cities, so your city or a close neighbor is very likely listed. Check your city's page in our watering schedules, which prints its monthly and annual ETo. If your exact town is missing, the nearest listed city with similar elevation and distance from the coast is the honest stand-in.

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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.