Evapotranspiration Explained: The Hidden Variable Behind Smart Lawn Watering

If you've ever wondered why one week your lawn looks fine on three short waterings and another week it goes crispy on the same schedule, the answer is hiding inside a single number you've probably never seen on a weather app: evapotranspiration. ET, as it's usually shortened, is the daily rate at which water actually leaves your lawn — through evaporation off the soil and transpiration through the grass blades — and replacing it is the entire point of irrigation. Every credible university turf program, EPA WaterSense controller, and FAO irrigation guide is built around it. Once you understand ET, the seemingly random behavior of your lawn starts to make perfect sense.

This guide unpacks what evapotranspiration is, what affects it, what ET rates actually look like in real lawns across the U.S., and — most importantly — how you can use ET to schedule watering more precisely than any calendar timer ever could. We'll also look at why the smart irrigation controllers that the EPA gives its WaterSense label all share one feature in common: they listen to ET, not to the clock.

What Is Evapotranspiration?

Evapotranspiration is the combined loss of water from a surface through two processes happening at the same time. The first is evaporation — water moving from wet soil, leaf surfaces, or thatch directly into the air as vapor. The second is transpiration — water pulled up through the roots of grass plants and released through tiny pores on the leaf surface called stomata as the plant breathes and photosynthesizes. Lump those two together and you have ET: the total amount of water your lawn is shipping back into the atmosphere every day.

Why does it matter? Because that's the number you need to replace with irrigation, minus whatever rain falls. If your lawn is losing 0.25 inches per day to ET and you only irrigate the equivalent of 0.10 inches per day, your soil moisture bank is overdrawn and the grass goes into stress within a few days. If you're putting down 0.40 inches per day, you're wasting water, leaching nutrients past the root zone, and inviting fungal disease. Matching your weekly irrigation to weekly ET (minus rainfall) is the entire game.

According to a 2023 USDA Forest Service / Northern Research Station study published in Water Resources Research, residential lawn ET across the United States ranges from roughly 0.6–1.3 mm per day in humid cities to 2.2–3.6 mm per day in hot, dry cities. That's a four- to fivefold difference in water demand depending entirely on where you live — and within a single city, ET in July can easily be triple what it is in April.

The Four Drivers of Evapotranspiration

ET isn't a fixed property of your lawn. It changes hour to hour and day to day based on weather. Four environmental variables dominate, and once you can see them at work, you'll never look at a forecast the same way again.

1. Solar Radiation

Sunlight is the biggest driver of ET, by a wide margin. The energy that pulls water out of soil and grass blades comes overwhelmingly from incoming solar radiation. According to the USGA Turfgrass Water Requirements review by Dr. Bingru Huang, potential turfgrass ET is primarily driven by incoming solar radiation and can reach extremely high rates on unshaded turf with unlimited soil water. This is why a partly cloudy week can cut your ET nearly in half compared with a string of sunny days, even at the same temperature.

2. Temperature

Warmer air holds more water vapor, so the vapor pressure deficit between a wet leaf and the surrounding air grows as temperatures rise. That deficit is the "pull" that drives transpiration. ET climbs steeply through the 70s, 80s, and 90s Fahrenheit. Texas A&M's water management bulletin notes that ET rates increase as summer temperatures increase, and that transpiration losses can reach 0.4 inches per day in desert climates during summer compared with about 0.20 inches per day in humid climates at similar temperatures.

3. Relative Humidity

The drier the air, the steeper the vapor pressure gradient, and the faster water moves out of the grass and soil. Two days with identical temperatures but different humidity can produce very different ET. A 95°F day at 20% humidity in Phoenix is brutal on a lawn; a 95°F day at 80% humidity in Atlanta is mild by comparison, even though the thermometers agree.

4. Wind Speed

Wind strips the saturated air layer hovering just above the grass blades and replaces it with dry air, which lets transpiration run faster. Wind also accelerates evaporation from wet soil. A windy day pushes ET up well above what temperature and sun alone would suggest, which is part of why your lawn looks rougher after a hot, breezy weekend than after a hot, calm one.

Reference Evapotranspiration (ET₀) and Crop Coefficients (Kc)

If you start reading agricultural irrigation literature, you'll quickly run into two acronyms: ET₀ (sometimes written ETo or ETref) and Kc. Both are part of the standard equation that smart controllers, agricultural extension services, and weather networks use to estimate water use for any specific crop or turfgrass.

Reference evapotranspiration (ET₀) is ET measured over a standardized reference surface — a hypothetical clipped grass field, 0.12 m tall, well-watered, with a fixed surface resistance and albedo, under whatever the local weather is doing. The FAO Penman-Monteith equation is the global standard for calculating ET₀ from temperature, humidity, wind, and solar radiation data. Most public weather stations and irrigation networks (California's CIMIS, Texas ET Network, and many state mesonets) publish a daily ET₀ value for their region.

The crop coefficient (Kc) is a multiplier that converts ET₀ into the actual ET of a specific plant. The full equation is simple: ETc = Kc × ET₀. If today's reference ET₀ is 0.25 inches and your lawn's Kc is 0.80, your lawn's actual water loss today is 0.25 × 0.80 = 0.20 inches.

So what's the Kc for your lawn? The answer depends on whether your grass is cool-season or warm-season, plus a handful of other factors like mowing height and turf quality goals. According to a literature review published by the UC ANR Center for Landscape & Urban Horticulture, average crop coefficients fall in these ranges:

Turfgrass Type	Typical Kc Range	Common Average
Cool-season (Kentucky bluegrass, tall fescue, perennial ryegrass)	0.60 – 1.04	~0.80
Warm-season (bermudagrass, zoysia, St. Augustine)	0.54 – 0.85	~0.70

That difference is exactly why warm-season grasses use roughly 20% less water than cool-season grasses under the same weather — they're more efficient at the leaf level, with structural features (waxy cuticles, C4 photosynthesis) that throttle transpiration. If you want a deeper look at how species choice changes your watering schedule, see our lawn watering schedule by grass type.

What ET Looks Like Across the Year

Here's where ET stops being abstract and starts being practical. The USGA turfgrass water review reports typical ET rates of 3–8 mm per day for cool-season grasses and 2–5 mm per day for warm-season grasses. Converted to inches per week, that gives you a working range you can sanity-check against the irrigation recommendations you've probably heard before:

Season / Condition	Approximate ET	Weekly Water Need
Cool, cloudy spring (cool-season)	1–2 mm/day	~0.3–0.6 inches
Mild summer (cool-season)	3–5 mm/day	~1.0 inch
Peak summer, hot & dry (cool-season)	6–8 mm/day	~1.5–2.0 inches
Peak summer (warm-season)	3–5 mm/day	~0.8–1.0 inch
Dormant / cool fall	1 mm/day or less	< 0.3 inches

That's where the familiar "one inch per week" rule of thumb comes from — but you can see it's an average that masks a wide actual range. A cool-season lawn in Denver in July may genuinely need close to two inches per week, while the same lawn in early May may need barely a third of an inch. NC State Extension emphasizes the same point in its turfgrass water requirements bulletin: warm-season grasses typically require about one inch of water per week during summer, while cool-season grasses can require up to two inches per week under the same conditions.

Why Calendar Timers Get ET Wrong Every Time

A traditional sprinkler timer doesn't know about any of this. It runs the same schedule on a hot, dry, windy Tuesday as it does on a cool, cloudy Sunday after a thunderstorm. Even if you program it well for an average July week, it will overwater you for 20+ weeks of the year and underwater you for 2 weeks in peak summer. This is the core of what EPA WaterSense identifies as the central problem with U.S. outdoor water use.

According to EPA WaterSense, residential outdoor water use in the United States accounts for nearly 8 billion gallons per day, and as much as 50% of that water is wasted due to overwatering caused by inefficiencies in irrigation methods and systems. The agency labels two categories of controllers that solve this problem: weather-based controllers that automatically adjust schedules using local ET data, and soil moisture-based controllers that water only when actual soil moisture falls below a threshold. EPA estimates that replacing a clock-based controller with either kind of WaterSense-labeled controller can reduce average home irrigation water use by up to 30% and save an average home up to 15,000 gallons per year.

That's not a small number. If everyone in the U.S. with an automatic sprinkler system switched to an ET- or sensor-driven controller, EPA estimates the country would save 390 billion gallons of water and $4.5 billion in water bills annually. The reason these controllers work is simple: they're solving the right equation. They're matching irrigation to actual ET, not to a fixed timer.

Two Practical Ways to Use ET in Your Own Lawn

Method 1: The DIY ET Lookup

Most regions have a public ET data source. California's CIMIS network, Texas ET Network, and various state mesonets publish daily ET₀ for stations near you. You can also pull "actual ET" or "potential ET" from many National Weather Service forecasts and from agricultural apps. Once you have ET₀:

• Multiply it by your Kc (use ~0.80 for cool-season lawns, ~0.70 for warm-season) to get your daily lawn ET in inches.
• Add it up over 5–7 days.
• Subtract any rainfall that infiltrated. Very light rain events (a few hundredths of an inch) often evaporate before reaching the root zone and shouldn't be counted in full. The Penn State Extension "Principles of Turfgrass Irrigation" bulletin walks through how to factor rainfall into the soil-water balance.
• Irrigate to replace the remaining deficit, ideally in 1–3 sessions so the water reaches the root zone without runoff.

This is exactly how golf course superintendents and agricultural irrigators have scheduled water for decades. The math isn't hard; the part that gets you is doing it every single day, accurately, and adjusting for site-specific factors like shade, slope, and soil type that change how much of that ET your sprinklers actually need to replace.

Method 2: Let a Smart Controller Do It

The whole point of a smart controller is to do this math for you, automatically, every day. A weather-based smart controller pulls ET data from local weather stations or an internal calculation and adjusts run times accordingly. A soil moisture-based controller skips ET entirely and just watches actual soil moisture, watering when readings drop below a threshold — a more direct measurement that adapts perfectly to your specific soil and microclimate.

The Soildrops 8-zone smart controller can run in either mode. Its Smart mode uses local weather data to estimate ET and skip or shorten watering accordingly. Its Autopilot mode pairs the controller with one or more wireless soil moisture sensors (±3% accuracy) and waters only when actual soil moisture has been depleted. In practice, Autopilot mode often outperforms pure ET-based scheduling because it sidesteps two big sources of error: differences between the weather station and your actual yard, and differences between "potential" ET and what's actually happening in your soil. If you want both — ET-aware scheduling plus the ground truth of soil moisture — the starter kits bundle the controller with sensors.

For a side-by-side comparison of the two sensor approaches, see our post on rain sensors vs. soil moisture sensors.

Adjusting Watering When ET Spikes

Heat waves, drought, and unusually windy stretches all push ET well above seasonal averages. If you're watching your lawn turn straw-colored during a 100°F week, the ET math is almost certainly screaming at you. A few practical adjustments:

• Don't just water longer in one session. Soils have a maximum infiltration rate. If you double your run time, you may double your runoff instead of doubling the water that reaches roots. Use cycle-and-soak — multiple short cycles spread across the day — to let water soak in. Our guide to the cycle and soak method covers this in detail.
• Water in the early morning. ET is lowest before sunrise. Watering at 4–7 a.m. puts water down when the lawn can actually use it, before solar radiation starts pulling it back out. Our post on the best time to water walks through the science.
• Raise your mowing height. Taller grass shades the soil and reduces evaporation. Even one notch up on the mower can measurably drop ET on hot days.
• Consider deficit irrigation. Many lawns survive — and even benefit — from being watered at 70–80% of ET, particularly cool-season grasses that naturally slow down in summer heat. The University of Nebraska Extension's Water Wise guidance notes that Kentucky bluegrass health can typically be maintained at around 70–80% of ET, and buffalograss can survive at 40–50%.

FAQ

What does "ET" mean in a weather forecast?

ET means evapotranspiration — the daily amount of water that evaporates from soil plus transpires through plants. It's usually reported as inches or millimeters per day. The number published by most weather services is reference ET (ET₀), measured over a standardized short grass surface, which you then multiply by a crop coefficient to get your specific lawn's water loss.

How do I find ET data for my area?

Many states publish daily ET₀ on their agricultural extension or mesonet websites — California's CIMIS, Texas ET Network, Oklahoma Mesonet, Nebraska Mesonet, and others. The Montana Climate Office and similar state climate offices also publish daily ET. If you'd rather not look it up manually, a weather-based smart controller pulls this data automatically.

Is evapotranspiration the same as evaporation?

No. Evaporation is water moving from a wet surface (soil, leaves, puddles) into the air. Transpiration is water released by plants through their leaf pores. Evapotranspiration is the sum of both processes happening simultaneously on a vegetated surface. On a lawn, transpiration typically dominates once the grass has filled in and shaded the soil.

Does ET tell me exactly how much to water?

It tells you the demand side — how much water your lawn lost. To turn that into irrigation run time, you also need to know your sprinklers' precipitation rate (inches per hour) and your irrigation efficiency (typically 50–75% for spray heads). The Penn State Extension Principles of Turfgrass Irrigation bulletin walks through the conversion. Or use a soil moisture sensor, which skips the math and just tells you whether the soil actually needs water.

Why is my smart controller still overwatering?

The most common reason is that it's running on a generic ET algorithm with no site-specific calibration — meaning it doesn't know your soil holds water differently than the weather station's reference surface, that your grass species has a lower Kc, or that one of your zones is in shade. Pairing the controller with a soil moisture sensor closes that gap by replacing assumptions with measured ground truth. EPA WaterSense lists both categories — weather-based and soil-moisture-based — as valid for water savings, and the two combined tend to outperform either alone.

The Bottom Line

Evapotranspiration is the missing variable that makes traditional sprinkler timers so wasteful. Once you know how much water your lawn is actually losing day to day — whether by reading ET₀ off a state weather network and multiplying by your crop coefficient, or by letting a smart controller and soil moisture sensor do the work — your watering stops being a guess and starts being a calculation. That's the difference between an average home wasting 50% of its outdoor water and the same home cutting irrigation by 30% with a WaterSense-labeled controller. The math is the same one EPA, FAO, and every land-grant university extension service publishes in their irrigation guides. The only question is whether you do the math, or whether you let a controller listen to ET for you. For a fuller foundation on the soil side of the equation, see our pillar guide to soil moisture.