Evapotranspiration is the combined process by which water moves from the Earth’s surface into the atmosphere through two pathways: evaporation from soil, lakes, and other surfaces, and transpiration from plants. Together, these two processes return roughly 60% of all land precipitation back to the atmosphere, making evapotranspiration one of the largest movements of water in the entire hydrologic cycle.
Two Processes, One Term
Evaporation is straightforward: liquid water on any surface (soil, lakes, rivers, puddles on pavement) absorbs enough energy to change into water vapor and rise into the air. This happens constantly wherever water is exposed to the atmosphere, and the rate depends on how much energy is available and how dry the surrounding air is.
Transpiration is the biological half. Plants pull water from the soil through their roots, move it upward through specialized transport tissue called xylem, and release it as vapor through tiny pores on their leaves called stomata. These pores exist primarily so the plant can absorb carbon dioxide for photosynthesis, but every time they open, water escapes. On average, a plant loses about 400 water molecules for every single molecule of CO2 it takes in. That’s an enormous amount of water moving through vegetation and into the sky.
Scientists lump these two processes together because, from a water-budget perspective, it’s often impossible to separate them in a landscape. A farm field, a forest, or a wetland is losing water through both pathways simultaneously, and what matters for hydrology, agriculture, and climate is the total amount leaving the surface.
How Water Travels Through a Plant
Nearly all the water a land plant uses enters through its roots, drawn from the surrounding soil. From there, it crosses several cell layers before reaching the xylem, the plant’s internal plumbing. Xylem consists of open, tube-like channels that allow water to flow efficiently over long distances, from root tips all the way up to the highest leaves.
The engine driving this upward flow isn’t a pump. It’s evaporation at the leaf surface. As water molecules evaporate through the stomata, they create tension that pulls the entire column of water upward through the xylem. This is called the cohesion-tension mechanism: water molecules cling to each other (cohesion), so when one is pulled out at the top, the whole chain moves up. Water essentially flows from high water potential in the soil to low water potential in the dry air outside the leaves.
Plants face a constant tradeoff. They need their stomata open to capture CO2 and produce sugars, but open stomata mean rapid water loss. In darkness or drought, stomata close to conserve water, which slows transpiration but also halts photosynthesis. This balancing act is why transpiration rates vary so dramatically depending on conditions.
What Controls the Rate
Several environmental factors speed up or slow down evapotranspiration:
- Temperature. Higher temperatures cause stomata to open wider and give water molecules more energy to escape into the air. During the growing season, when sunlight is strongest and air masses are warmest, transpiration rates climb significantly.
- Humidity. Water evaporates more easily into dry air than into air already saturated with moisture. When relative humidity around a plant rises, the transpiration rate drops because there’s less of a gradient pulling water out of the leaves.
- Sunlight. Solar radiation provides the energy that drives evaporation. More intense sunlight means faster evapotranspiration.
- Wind. Moving air sweeps away the thin layer of humid air that builds up around leaf surfaces, replacing it with drier air. This increases the rate at which water vapor escapes.
- Soil type and moisture. Sandy soils release water readily, while clay soils hold it tightly in their smaller particles. When the soil dries out, plants can’t pull enough water to keep up with demand, and they may begin to wilt or drop leaves, reducing transpiration.
Why It Matters for Agriculture
For farmers and irrigation managers, evapotranspiration is the single most important number for deciding how much water crops need. The standard approach uses a two-step calculation. First, you determine a “reference” evapotranspiration rate (called ETo), which represents how much water a well-watered grass surface would lose under current weather conditions. Then you multiply that by a crop coefficient (Kc) specific to whatever you’re growing and its stage of development. A young corn seedling has a very different water demand than a corn plant at full height with a thick canopy.
The resulting number, crop evapotranspiration (ETc), tells you how much water your field is losing each day. By comparing that to recent rainfall and soil moisture, you can schedule irrigation precisely, applying just enough water to replace what was lost without wasting resources. This method has become the backbone of efficient irrigation scheduling worldwide.
How Evapotranspiration Is Measured
Directly measuring how much invisible water vapor leaves a landscape is tricky. The gold standard for precision is a weighing lysimeter: essentially a large, isolated container of soil and vegetation set into the ground on a sensitive scale. As water leaves through evapotranspiration, the container gets lighter, and the weight change tells you exactly how much water was lost. Lysimeters are highly accurate but expensive, and they only measure a small patch of ground, so they’re mainly used to validate other methods.
For larger areas, researchers use eddy covariance towers. These instruments measure high-frequency fluctuations in wind speed, temperature, and water vapor concentration above a landscape to calculate how much moisture is moving upward. Eddy covariance can capture real-time evapotranspiration data across a broader area, though it typically loses 10% to 30% of the energy signal, requiring corrections.
At regional and global scales, satellites have become essential. NASA’s MODIS sensor, for example, produces global evapotranspiration maps every eight days at roughly half-kilometer resolution, covering over 109 million square kilometers of vegetated land. The satellite doesn’t measure water vapor directly. Instead, it tracks vegetation characteristics like leaf area and surface reflectivity, then feeds those into a version of the same physics-based equation used on the ground. This makes it possible to monitor water loss across entire continents in near real-time.
The Standard Calculation
The internationally accepted method for estimating reference evapotranspiration is the FAO Penman-Monteith equation, adopted by the United Nations Food and Agriculture Organization. You don’t need to memorize the math, but it helps to know what goes into it: air temperature (daily highs and lows), relative humidity, solar radiation, wind speed, atmospheric pressure, and site elevation. These inputs capture the key physical forces pulling water into the atmosphere. Weather stations at airports, farms, and research sites collect these measurements, and many regions publish daily ETo values that farmers and water managers can look up.
Evapotranspiration and Climate Change
As the planet warms, evapotranspiration is increasing. Global air temperatures have risen at about 0.016°C per year, and warmer air can hold more moisture, which increases the atmosphere’s ability to pull water from surfaces and vegetation. Recent research finds that global evaporative demand has risen significantly, with reference evapotranspiration climbing by an average of 0.80 mm per year, each year. That may sound small, but compounded over decades and across millions of square kilometers, it reshapes how much water is available for rivers, reservoirs, and aquifers.
Rising evaporative demand means some agricultural regions will need more irrigation water to grow the same crops, even if rainfall stays the same. In areas where precipitation is also shifting, the combination creates a double challenge for water planners. Understanding evapotranspiration, and tracking how it changes, is becoming increasingly critical for managing water supplies in a warming world.