Evapotranspiration (ET): What It Is and Why It Drives Watering
Written and checked with AI using the references below. This is not human expert review.
Quick answer
Evapotranspiration (ET) is the total water your yard loses through soil evaporation and plant transpiration, measured in inches per day. ET combines two losses: water that dries from the soil and water that plants breathe out. ET rises when it is hot, windy, or sunny, and drops when it is cool or cloudy. A sprinkler timer set once for summer will overwater in cool weather. It will also underwater your yard during a heat wave.
What ET actually measures
Evapotranspiration — abbreviated ET — is the total water that moves from the land surface into the atmosphere. It combines two processes: evaporation from the soil and transpiration from plants. The U.S. Geological Survey tracks this measurement. Think of it as your yard’s daily water bill: every inch of ET is an inch of moisture that left the soil and must be replaced by rain or irrigation if you want healthy plants.
ET has two parts. Evaporation is water leaving directly from the soil surface, puddles, or shallow groundwater. Transpiration is water that plant roots pull up from the soil, move through stems and leaves, and then release as invisible vapor through tiny pores called stomata. Scientists combine both into one number because, from a watering standpoint, both represent moisture your plants can no longer use.
How transpiration works inside a plant
Transpiration follows three steps. First, roots absorb liquid water from the soil. Second, that water travels up through the plant’s tissues, carrying nutrients and supporting cell structure. Third, leaves release the water as vapor through their stomata. During a growing season, a single leaf transpires many times its own weight in water. A large oak can release roughly 40,000 gallons per year.
You can’t see transpiration happening, but a simple check makes it visible: wrap a clear plastic bag around a leafy branch for an hour. The moisture that fogs the inside of the bag is transpired water vapor.
What drives ET up or down
ET is not a fixed number. It changes every day based on several conditions:
- Temperature: Higher temperatures cause stomata to open wider, so plants release more vapor. ET climbs sharply during summer heat waves.
- Humidity: Dry air pulls moisture out of leaves faster. When humidity is high, the air is already close to saturated, so transpiration slows.
- Wind: Moving air sweeps the moist layer away from leaf surfaces and replaces it with drier air, which speeds water loss.
- Sunlight: Stronger, longer sunlight drives both evaporation from the soil and stomatal opening in leaves.
- Plant type: Drought-adapted plants like cacti and succulents transpire far less than thirsty turf grasses or vegetable crops.
- Soil moisture: When soil dries out, plants begin to shut their stomata to conserve water, which reduces transpiration — but also stresses the plant.
Because all these factors shift daily, ET on a hot, windy, sunny July afternoon in Phoenix is many times higher than ET on a cool, cloudy October morning in Seattle. That gap is exactly why how much water a lawn needs per week cannot be answered with a single number that works everywhere.
ET and irrigation scheduling
The practical value of ET is that it gives you a target: replace what was lost, no more and no less. Most homeowners set their sprinkler timers once for peak summer and never adjust them. This means the system overwaters during mild weather and may still underwater during extreme heat. Weather-based irrigation controllers fix this by reading local ET data and adjusting run times on their own.
For a do-it-yourself approach, irrigation researchers at UF/IFAS use a two-step formula:
- Start with reference ET (ETo), a standardized daily water-loss rate calculated from weather data. Then multiply it by a crop coefficient (Kc), a number that reflects how thirsty your specific plant is. The result is crop ET (ETc), the actual daily water demand:
ETc = ETo × Kc. - Subtract effective rainfall (Pe) from ETc to get the net irrigation requirement (I):
I = ETc − Pe. This is the depth of water your irrigation system must supply.
Illustrative example (based on UF/IFAS avocado orchard data, shown here to demonstrate the math only): In October, ETo = 0.12 in/day and Kc = 0.80, so ETc = 0.096 in/day ≈ 0.10 in/day. Effective rainfall that month supplies 0.06 in/day, leaving a net irrigation need of about 0.04 in/day. Over a 7-day week, that adds up to well under the “water one inch a week” rule of thumb, which ignores rainfall entirely. The actual number for your lawn will differ based on your grass type, local weather, and soil.
Once you know the depth you need to deliver, you convert it to runtime using your sprinkler’s precipitation rate (inches per hour), not a generic time recommendation. For example, a rotor head applying 0.5 in/hr will need more time to deliver a given depth than a fixed spray nozzle running at a higher rate.
Why a fixed timer fails — and what to do instead
A clock-based controller has no way to know that yesterday’s rain already delivered 0.5 inches, or that today’s cool overcast sky cut ET in half. The EPA estimates that replacing a standard clock timer with a WaterSense-labeled weather-based controller can save an average home nearly 7,600 gallons annually by avoiding chronic overwatering. Individual results depend on your system, climate, and current schedule.
If a smart controller is not in your budget, you can still schedule watering based on ET by hand. Check a local weather station or your state’s agricultural weather network for the daily ETo value. Subtract any rainfall your rain gauge recorded, then run your system only when the deficit reaches the target depth for your soil and root zone. For timing within the day, morning watering reduces evaporation loss from the soil surface compared with midday runs. This slightly lowers the ET you need to replace.
Clay soils absorb water slowly. Even when ET creates a large deficit, you may need to split the runtime into two or three shorter cycles to avoid runoff before the water soaks in. Sandy soils drain fast and hold little water, so they need more frequent, shorter applications even when daily ET is moderate. Improving clay soil structure can raise its absorption rate and make ET-based scheduling easier to apply.
Frequently asked questions
Where can I find the daily ETo value for my area?
Most US states publish daily reference ET through agricultural weather networks. For example, Florida uses FAWN (Florida Automated Weather Network) and California uses a statewide irrigation management system. Your state's land-grant university extension service is a good starting point. Look for ETo in inches per day for your nearest weather station.
Does ET matter for container plants and potted gardens?
Yes. Containers dry out faster than in-ground beds because they have limited soil volume and often sit in full sun. High temperatures, low humidity, and wind all raise ET for potted plants just as they do for turf. Check soil moisture daily in summer rather than relying on a fixed watering schedule.
How does rainfall offset my irrigation need?
Only effective rainfall — the portion that actually soaks into the root zone — counts as an offset. Light rain that evaporates quickly from the soil surface contributes little. UF/IFAS irrigation guides subtract effective precipitation (Pe) from crop ET to find the true irrigation deficit: I = ETc − Pe.
Do drought-tolerant plants have lower ET?
Generally yes. Plants adapted to arid conditions, such as cacti and succulents, transpire less water than thirsty turf or vegetable crops. Their lower crop coefficient (Kc) means their ETc is a smaller fraction of the reference ETo, so they need less irrigation to replace what is lost.
Can I use ET data to know when to stop watering in fall?
ET drops as temperatures fall and days get shorter, so the irrigation deficit shrinks. When daily ETo falls low enough that rainfall alone covers ETc, you can stop irrigating. Many weather-based controllers handle this on their own. Manual schedulers should check monthly ETo averages from their local extension service to plan a fall ramp-down.
Sources
- Evapotranspiration and the Water Cycle · USGS (usgs.gov)
- Weather-Based Irrigation Controllers · US EPA (epa.gov)
- AE458/AE458: Evapotranspiration-Based Irrigation for Agriculture: Implementing Evapotranspiration-Based Irrigation Scheduling for Agriculture · Ask IFAS - Powered by EDIS (edis.ifas.ufl.edu)
Written by AI; checked through automated evidence and editorial review. References retrieved October 11, 2026.
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