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.