The movement of water through a tree is known as transpiration, driven by the evaporation of water vapor from tiny pores (stomata) located primarily on the leaves. This continuous flow transports dissolved nutrients, provides raw material for photosynthesis, and acts as an internal cooling system. The amount of water a tree moves depends on its size, environment, and species-specific adaptations. While all trees require water, some species have evolved traits that make them far more prodigious consumers than others, leading to a few specific trees dominating the list of the highest daily water users.
The Champion Water Drinkers
The trees consuming the largest volumes of water are typically fast-growing species found in riparian areas where water is perpetually abundant. Mature Cottonwood trees (Populus spp.) draw between 50 and 200 gallons of water daily during the peak growing season. Some reports indicate that exceptionally large Cottonwoods can transpire over 500 gallons of water daily.
Willows (Salix spp.) are similarly prodigious drinkers, often found along streams where their roots have constant access to moisture. A mature Willow can absorb up to 100 gallons of water per day, a habit that makes them highly effective at stabilizing riverbanks. Eucalyptus trees, particularly fast-growing introduced species, also have a reputation for high water use. A large Eucalyptus can still consume between 10 and 24 gallons of water daily, a rate that can significantly impact local water tables when planted in large numbers.
Total water consumption must also consider the sheer biomass of the tree. Giant Sequoias, though not known for a high consumption rate per unit of leaf area, move immense amounts of water simply because of their colossal size. A giant Sequoia can transpire an estimated 500 gallons of water per day, placing it among the highest consumers due to its massive canopy and trunk volume.
Measuring Tree Water Usage
Quantifying the exact volume of water a tree uses involves specialized tools and concepts. Total water use is typically measured as evapotranspiration, which accounts for water lost from the soil surface through evaporation combined with the water lost from the plant through transpiration. When researchers want to isolate the tree’s contribution, they often use a technique called sap flow measurement.
Sap flow sensors are small instruments inserted directly into the water-conducting tissue of the tree, known as the xylem. These sensors use heat as a tracer, pulsing a small amount of heat into the wood and then measuring the speed at which the flowing sap carries that heat away. The faster the heat is moved, the faster the sap is flowing, allowing scientists to calculate the tree’s daily water uptake with high precision.
Another method involves using a lysimeter, which is essentially a large, isolated container of soil and the tree placed on a high-precision scale. By continuously weighing the entire system, researchers can track minute changes in mass corresponding to water loss or gain. The difference between the water supplied (rain or irrigation) and the water drained (percolation) is accurately measured, allowing for a direct calculation of the water lost to evapotranspiration.
Physiological Traits That Increase Consumption
The ability of a tree to consume large volumes of water is linked to three primary physiological traits. First, many champion species are classified as phreatophytes, meaning they have deep root systems that consistently tap into the permanent water table (phreatic zone). This constant access to groundwater allows them to maintain high transpiration rates even when surface soil is dry.
Second, high-use trees often exhibit a rapid growth rate. Sustaining this fast biomass accumulation requires a continuous and high volume of water flow to transport nutrients and maintain cell turgor.
Third, the regulation of stomata—the pores on the leaves that control water loss—plays a role. Unlike drought-tolerant species that quickly close their stomata to conserve water, high-water-use trees maintain high stomatal conductance, keeping these pores open for longer periods. This preference for maximizing carbon dioxide intake for photosynthesis is possible because their deep roots ensure a constant supply of water is available to replace what is lost.
Environmental Applications of High-Use Trees
The remarkable water consumption of these species has been deliberately harnessed for practical environmental management, a process known as phytoremediation. This technique utilizes the tree’s natural ability to absorb and process large quantities of water and dissolved substances from the soil and groundwater. Fast-growing species like Poplars and Willows are frequently planted at contaminated sites, such as old landfills or industrial areas.
The trees’ extensive root systems draw up contaminated water containing organic chemicals or heavy metals. Once inside the plant, some contaminants are degraded by enzymes in the tree, while others are stored harmlessly in the plant tissue or released as harmless vapors through the leaves. The high water use of these trees is also applied in dewatering operations, such as stabilizing slopes or actively lowering a shallow water table in marshy areas.