A forest canopy does more than receive rainfall. It divides incoming water among several routes, returning some directly to the atmosphere, guiding some down branches and trunks, and allowing the remainder to reach the ground. Once water enters the soil, tree roots can lift it back toward the leaves, where it becomes vapor. Together, these upward transfers form forest evapotranspiration.
The term combines evaporation and transpiration, but a canopy adds important detail to both. Rain caught on leaves may evaporate without entering a tree. Water absorbed by roots travels through living tissue before escaping through leaf pores. Moisture can also evaporate from wet bark, litter, soil, and understory vegetation beneath the crown. Separating these components is essential for understanding how forests influence runoff, soil water, and atmospheric moisture.
Rain Is Partitioned Before It Reaches the Soil
The first exchange begins during a storm. Leaves, twigs, epiphytes, and bark intercept falling droplets. A small shower may wet these surfaces without producing much water beneath the trees. Larger or longer storms eventually exceed the canopy’s storage capacity, after which water drips as throughfall or follows stems downward as stemflow.
Intercepted water is exposed to moving air across a large surface area. Some evaporates during the storm, especially when rainfall is light or intermittent, and more can disappear as the canopy dries afterward. Because this loss occurs before water enters the soil, it can reduce both streamflow and groundwater recharge. Its importance varies enormously with leaf area, canopy structure, storm frequency, wind, and the amount of energy available for evaporation.
Evergreen forests can intercept precipitation throughout the year, while deciduous stands change sharply after leaf fall. Needleleaf canopies often retain droplets differently from broad leaves, and rough bark stores more water than smooth bark. Snow creates another pattern: branches may hold it temporarily, expose it to sublimation, or release it later as clumps and meltwater.
Trees Return Soil Water Through Their Leaves
Transpiration begins after roots absorb water from pores in the soil. Water moves through the xylem, a network of conducting tissue extending from roots to leaves. Evaporation inside a leaf creates tension that helps pull a continuous water column upward, even through tall trees.
Vapor exits mainly through stomata, microscopic pores whose opening also permits carbon dioxide to enter for photosynthesis. A tree must therefore balance carbon gain against water loss. Stomata tend to close when leaves become too dry, when atmospheric demand is extreme, or when the soil cannot supply water quickly enough. They open more readily when light supports photosynthesis and the plant’s hydraulic system remains secure.
Atmospheric dryness is commonly described by vapor pressure deficit: the difference between the moisture the air contains and the amount it could hold at the same temperature. A higher deficit can initially increase evaporation from leaves, but severe conditions often trigger stomatal closure. Consequently, hotter air does not guarantee a proportional rise in transpiration.
The Canopy Changes Its Own Microclimate
Dense foliage absorbs sunlight, slows wind, shades the forest floor, and maintains humid air beneath the crown. These effects suppress some soil evaporation even while the trees create a large transpiring surface. An open canopy may expose soil and understory plants to more radiation, whereas a closed canopy shifts more of the water transfer through leaves and wet crown surfaces.
Rooting depth also changes the timing. Shallow-rooted vegetation depends strongly on recent rain, while deep-rooted trees may draw from moisture stored well below the surface during dry periods. Species, tree age, stand density, leaf area, and access to groundwater can make neighboring forests respond differently to the same weather.
Forest evapotranspiration is therefore constrained by both energy and water. In a wet environment, sunlight and atmospheric demand may set the upper limit. During drought, available soil moisture and the ability of roots and stems to transport it become more important. Prolonged stress can cause leaves to shed, branches to die, or water columns inside xylem to break, reducing later transpiration.
Measuring Vapor That Cannot Be Seen
Sap-flow sensors estimate how rapidly water moves through tree stems. Small gauges measure throughfall and stemflow, while lysimeters or soil-moisture profiles help track water below the canopy. Above the trees, eddy-covariance towers calculate exchanges of heat, carbon dioxide, and water vapor from rapid measurements of turbulent air.
Each method observes a different scale. A sensor attached to one trunk may not represent an entire stand, and a tower integrates a shifting upwind area. Satellite instruments estimate evapotranspiration from surface temperature, vegetation condition, radiation, and weather data, providing regional coverage but relying on models to divide evaporation from transpiration.
Partitioning remains one of the hardest tasks. Published estimates of the transpiration share of total evapotranspiration span a wide range because ecosystems and methods differ. Water evaporated from a wet canopy can be mistaken for transpiration unless measurements capture storms, drying periods, and plant activity separately.
A Vertical Link in the Water Cycle
Water returned by a forest may condense and fall nearby, or winds may carry it far beyond the watershed. At the same time, evapotranspiration reduces the portion of precipitation available for immediate runoff. Removing or adding tree cover therefore changes several flows at once: interception, soil storage, atmospheric moisture, stream discharge, and sometimes recharge.
The direction and size of those changes depend on climate and forest structure. There is no universal percentage of rainfall that every canopy sends back to the air. What forests consistently provide is a biologically regulated pathway from soil and intercepted precipitation into the atmosphere—a route that links roots, leaves, clouds, and rivers within the wider water cycle.