Plants act as massive pumps that move water from the soil into the atmosphere, and this single function shapes rainfall patterns, river flows, and groundwater supplies across the planet. The process, called transpiration, accounts for roughly 57 to 65 percent of all the water that evaporates from land surfaces. But transpiration is only the most visible piece of a much larger story that involves roots redistributing underground moisture, canopies intercepting rainfall, and the water vapor plants exhale seeding new storms hundreds of kilometers downwind.
Transpiration and Its Scale
Every time a plant opens the tiny pores on its leaves (stomata) to absorb carbon dioxide for photosynthesis, water vapor escapes through those same openings. A single large oak can release hundreds of liters of water per day during peak summer. Multiply that by the billions of trees, grasses, shrubs, and crops on Earth, and the numbers become staggering. A 2023 modeling study estimated that vegetation transpiration accounts for about two thirds of total land evapotranspiration, exerting major effects on global water, energy, and carbon cycles.1Agricultural and Forest Meteorology. Spatial patterns and recent temporal trends in global transpiration modelled using eco-evolutionary optimality An earlier analysis using different methods placed the share somewhat lower, at about 57 percent, with a standard deviation of roughly 7 percentage points.2Geophysical Research Letters. Revisiting the contribution of transpiration to global terrestrial evapotranspiration The true figure likely falls somewhere in that range, depending on the dataset and model used, but the message is the same: vegetation is by far the biggest route through which water leaves the land surface and enters the atmosphere.
How Transpired Water Becomes Rain Again
Water vapor does not simply vanish once it leaves a leaf. It rises, cools, condenses into clouds, and eventually falls as precipitation, sometimes over the same forest that released it, sometimes hundreds or thousands of kilometers away. Researchers call this moisture recycling, and it is one of the most important and least appreciated ways plants shape climate. A study using a decade of atmospheric moisture simulations identified four major terrestrial moisture recycling hubs: the Amazon Basin, the Congo Rainforest, South Asia, and the Indonesian Archipelago.3PubMed Central. Network motifs shape distinct functioning of Earth’s moisture recycling hubs These regions do not just receive rain; they actively produce the atmospheric moisture that feeds rainfall farther inland and in neighboring regions. The Amazon, for example, relies heavily on directed moisture connections where transpired water is funneled progressively deeper into the continent through what some scientists call “flying rivers.”
This means that the forests generating that moisture are not just responding to the water cycle; they are driving it. Remove the vegetation and you lose the pump that keeps the atmospheric moisture supply going.
Deforestation and Disappearing Rainfall
When forests are cleared, the transpiration pump shuts down in that area, and the consequences extend well beyond the cleared patch. Research on the southern Amazon basin found that historical deforestation was responsible for roughly 52 to 72 percent of observed precipitation declines in the region, because removing trees suppressed the forest-sourced moisture that would otherwise return as rain.4PubMed Central. Historical deforestation drives strong rainfall decline across the southern Amazon basin Deforestation also increased atmospheric stability and moisture outflow, meaning less vapor stayed overhead long enough to form local storms.
Globally, the picture is complicated. A multi-model study found that climate changes triggered by deforestation dominate runoff changes over about 64 percent of global land areas. The deforestation-driven climate feedback caused a strong decrease in global runoff of roughly 6 mm per year, which nearly canceled out the direct land-surface effect of increased runoff from bare ground (about 5 mm per year), leaving a small net reduction.5PubMed Central. Deforestation-induced runoff changes dominated by forest-climate feedbacks In other words, the intuitive idea that cutting down trees increases river flow because less water is being transpired is often wrong once you account for the rainfall that those trees were generating. You may get more runoff per rainstorm, but fewer rainstorms overall.
What Roots Do Below Ground
The aboveground water story gets most of the attention, but roots quietly reshape how water moves through the soil in ways that matter enormously for groundwater recharge, flood control, and neighboring plant survival.
One of the more remarkable root behaviors is hydraulic redistribution. At night, when transpiration stops, deep-rooted plants can move water from moist soil layers into drier ones through their root system, essentially acting as living irrigation pipes. This process is most common in deep-rooted tree species with large root canal diameters and roots that span multiple soil depths. Under drought conditions, hydraulic redistribution helps maintain root function and water uptake by keeping the dry root zone at least partially hydrated.6PubMed Central. Roots to the rescue: how plants harness hydraulic redistribution to survive drought across contrasting soil textures Neighboring shallow-rooted plants can benefit as well, accessing water they could never reach on their own.
Roots also physically change the soil’s ability to absorb water. As roots grow and decay, they create macropores, channels that allow rainwater to infiltrate more quickly and deeply rather than running off the surface. Research on eroded soils showed that certain plant families, particularly legumes, maintain high infiltration rates even in severely degraded conditions because their root traits promote aggregate formation and optimize pore structure.7Geoderma. Root-mediated regulation of soil infiltration dynamics in eroded Mollisols Root-induced macropores can even boost groundwater recharge in situations where you might expect high transpiration to leave less water for the underground supply.8Environmental Earth Sciences. Aquifer recharge: unpacking the impact of deep roots and land use
Plants do not do this work entirely alone. Mycorrhizal fungi, which form symbiotic networks around and inside roots, extend the effective reach of the root system by orders of magnitude. These fungal partnerships improve water and nutrient acquisition, helping plants access moisture in soil pockets their roots alone could not reach.9PubMed Central. Role of Arbuscular Mycorrhizal Fungi in Regulating Growth, Enhancing Productivity, and Potentially Influencing Ecosystems under Abiotic and Biotic Stresses The result is a more connected soil water system than would exist without vegetation.
Canopy Interception
Before rainwater even reaches the soil, plant canopies intercept a portion of it. Some of that water evaporates directly back into the atmosphere from leaf surfaces, some drips through as “throughfall,” and some flows down stems and trunks as stemflow. The split varies dramatically by plant type and canopy density. A study of corn and soybean fields in northeast China found that corn canopies intercepted about 11 percent of gross rainfall over two growing seasons, while soybean fields intercepted about 15 percent.10Water. The Throughfall, Stemflow, and Canopy Interception Loss in Corn and Soybean Fields in Northeast China Dense tropical forests can intercept 20 to 30 percent or more. This intercepted water never reaches the ground, so it never enters streams or recharges groundwater. It is a direct return to the atmosphere, separate from transpiration.
For water management, canopy interception matters because it reduces the total amount of water available for runoff and infiltration. In forested watersheds, this effect can meaningfully lower streamflow compared to grasslands or bare soil. The flip side is that by slowing and distributing how rainfall reaches the ground, canopies reduce erosion and the kind of sudden, intense surface runoff that causes flooding.
How Plants Regulate Their Own Water Loss
Plants are not passive pipes; they actively control how much water they lose, and this regulation has consequences for everything from local soil moisture to continental river flows. The key mechanism is stomatal closure. When a plant senses that it is drying out, it shuts its stomata to conserve water, cutting off both CO₂ intake and water loss simultaneously.
The trigger for closure turns out to be more mechanical than chemical. Research on tomato plants found that changes in leaf cell pressure (turgor) and acoustic signals within the plant preceded stomatal closure, while the stress hormone ABA, long thought to be the main trigger, did not increase until after stomata had already shut.11PubMed. Signal coordination before, during and after stomatal closure in response to drought stress In maize, transpiration dropped to near zero once the water tension in the plant’s plumbing reached about −1.6 MPa, a threshold that closely matched the point where air bubbles would begin forming in the leaf veins.12PubMed. Xylem embolism and drought-induced stomatal closure in maize By closing stomata before dangerous levels of air blockage could develop, the plant kept its water-transport system functional.
Different tree species play this game with vastly different safety margins. A study of three temperate species found that some close their stomata well before any hydraulic damage occurs, while others push much closer to the edge, maintaining gas exchange at the risk of catastrophic plumbing failure during severe drought.13PubMed Central. Interaction of stomatal behaviour and vulnerability to xylem cavitation determines the drought response of three temperate tree species These differences determine which species survive drought events and which die back, which in turn reshapes how entire landscapes interact with the water cycle over time.
Water-Saving Strategies in Extreme Environments
Not all plants interact with the water cycle in the same way. In arid environments, certain species have evolved dramatically different strategies that minimize water loss while still fixing carbon. Plants using Crassulacean acid metabolism (CAM) open their stomata at night instead of during the day, when cooler temperatures mean far less water escapes per unit of CO₂ absorbed. The desert agave Agave deserti, for instance, was measured to have a transpiration ratio of just 18 grams of water lost per gram of CO₂ fixed during a winter day, and about 25 over an entire year.14PubMed Central. Water Relations and Photosynthesis of a Desert CAM Plant, Agave deserti A typical non-desert plant might lose 200 to 500 grams of water per gram of carbon gained. Across CAM species more broadly, transpiration ratios ranged from about 54 in the highly efficient Aloe vera up to nearly 500 in species that switch between CAM and conventional photosynthesis.15PubMed. Carbon isotope composition and water-use efficiency in plants with crassulacean acid metabolism
Some plants go even further by acquiring water through their leaves instead of their roots. In California’s redwood forests, where summer fog is common but rain is scarce, eight out of ten species tested were able to absorb water directly through their leaf surfaces during fog exposure.16PubMed Central. Foliar water uptake: a common water acquisition strategy for plants of the redwood forest In the Atacama Desert, one of the driest places on Earth, Tillandsia air plants survive almost entirely on atmospheric humidity, using densely packed specialized leaf scales to absorb water directly from fog and dew.17PubMed Central. Foliar Anatomy of Three Native Species of Tillandsia L. from the Atacama Desert, Chile These plants essentially reverse the usual relationship between vegetation and the water cycle: instead of moving water from soil to atmosphere, they pull water from the atmosphere into the ecosystem.
Urban Trees and Stormwater Management
In cities, where impervious surfaces like roads and rooftops prevent natural infiltration, trees play a measurable role in managing stormwater. A modeling study found that adding urban trees reduced runoff volume by 20 to 25 percent and peak flow by 16 to 25 percent, depending on the scenario and time of year.18Urban Forestry & Urban Greening. An evaluation of the stormwater runoff reduction of two distinct tree species to support urban greening as nature-based solutions A field experiment took the opposite approach, removing 31 street trees from a residential neighborhood and measuring the difference. Tree removal increased surface runoff by an estimated 198 cubic meters over one growing season, and each removed tree had been intercepting roughly 6,400 liters of rainfall over that period.19Science of The Total Environment. Quantifying the stormwater runoff volume reduction benefits of urban street tree canopy
These numbers may sound modest compared to the scale of urban flooding, and they are. Street trees are not a substitute for storm sewers. But they represent a form of distributed green infrastructure that reduces the load on drainage systems during routine storms. The benefit comes from three mechanisms operating simultaneously: canopy interception holds water on leaf and branch surfaces, root uptake and transpiration dry out the soil between storms so it can absorb more next time, and root growth improves soil structure even in compacted urban soils.
Rising CO₂ and Shifting Water Budgets
As atmospheric carbon dioxide levels climb, plants respond in a way that alters the water cycle from the bottom up. Higher CO₂ concentrations allow plants to get the carbon they need while opening their stomata less, which means they transpire less water per unit of photosynthesis. A landmark study found evidence that this CO₂-induced reduction in transpiration was contributing to increased continental river runoff, independent of changes in precipitation.20PubMed. Detection of a direct carbon dioxide effect in continental river runoff records Less water being pulled out of the soil by plants means more drains into streams and rivers.
The picture gets more complicated in the long run. Higher CO₂ also stimulates plant growth (the “greening” or fertilization effect), which means more leaf area transpiring water and partially offsetting the per-leaf reduction. A projection study for the western United States through 2099 found that these two counteracting CO₂ effects were roughly compensatory, leaving warming as the dominant driver of projected runoff declines at large river basin scales.21Water Resources Research. The Compensatory CO2 Fertilization and Stomatal Closure Effects on Runoff Projection From 2016–2099 in the Western United States So while CO₂ makes individual stomata more water-efficient, more vegetation overall can cancel that gain. The net effect on any given watershed depends on the balance between these opposing forces and on how much warming accompanies the CO₂ increase.
The Reforestation Paradox
Planting trees is widely seen as an environmental good, and for carbon sequestration and biodiversity, it usually is. But its effects on water availability are more ambiguous than most people expect. A systematic review found that in about 80 percent of reported cases, forest restoration or expansion decreased annual water yield from a watershed, while only 6 percent of cases showed an increase. Baseflow, the steady groundwater-fed flow that sustains rivers during dry weather, also decreased in the majority of cases.22PubMed Central. Impacts of forest restoration on water yield: A systematic review The reason is straightforward: trees transpire far more water than grasslands or bare soil, and that water leaves the catchment via the atmosphere rather than flowing into streams.
The situation is more nuanced than “trees reduce water,” though. Some of the water transpired by a new forest returns as precipitation downwind, partially offsetting the local loss. And where reforestation improves soil structure enough to increase infiltration and groundwater recharge, dry-season streamflow might actually rise even as total annual yield falls.23WIREs Water. Reforestation effects on low flows: Review of public perceptions and scientific evidence Whether this happens depends heavily on climate, soil type, the species planted, and how degraded the soil was before planting. In water-scarce regions, large-scale tree planting without careful hydrological planning can reduce the water supply available for agriculture and human use, even as it delivers other benefits.
How Scientists Trace Plant Water Use
One reason our understanding of plants in the water cycle keeps improving is the development of isotope tracing techniques. Water molecules containing different isotopes of hydrogen and oxygen have slightly different physical properties, and these differences leave fingerprints that researchers can use to figure out exactly where a plant’s water came from: shallow soil, deep groundwater, fog, or a mixture. Process-based mixing models now allow researchers to estimate root water uptake across different soil depths continuously rather than as one-time snapshots.24Ecohydrology. A process‐based water stable isotope mixing model for plant water sourcing This kind of work is revealing that many trees are surprisingly flexible in their water sourcing, shifting between shallow and deep water depending on the season and conditions, which makes their role in the water cycle harder to predict but also more resilient than a fixed-depth root model would suggest.
When Plants Reshaped the Water Cycle Forever
The relationship between plants and water is not just a current feature of Earth’s system; it is one that fundamentally altered the planet’s surface when land plants first became widespread. During the Late Devonian period, roughly 370 million years ago, the expansion of land plants dramatically increased the weathering of rock and the flow of nutrients like phosphorus into rivers and oceans. Modeling of this period suggests that the resulting boost in riverine phosphorus promoted ocean eutrophication and oxygen depletion, while the burial of organic matter drew down atmospheric CO₂ from roughly ten times preindustrial levels to seven to nine times, cooling the climate by about 0.5 to 1.5 degrees Celsius.25Communications Earth & Environment. The expansion of land plants during the Late Devonian contributed to the marine mass extinction The development of root systems, the creation of true soils, and the initiation of large-scale transpiration transformed how water moved across landscapes, from a system dominated by rapid surface runoff on barren rock to one where water infiltrated, was stored, cycled through organisms, and returned to the atmosphere. Every feature of the modern water cycle that involves soil, groundwater, and plant-driven evaporation traces back to that transition.