Transpiration is the continuous process where plants release water vapor into the atmosphere, driving the global water cycle. This physiological function is a necessary consequence of photosynthesis, requiring plants to open small pores in their leaves to take in carbon dioxide. The process is intense during the growing season when plants are actively photosynthesizing and temperatures are warmer. Understanding the scale of water movement through plants is important for managing agricultural water use and predicting regional climate patterns.
The Mechanism of Transpiration
Transpiration is the movement of water through a plant followed by its evaporation from aerial parts, primarily the leaves. The driving force is the difference in water vapor concentration between the inside of the leaf and the outside air. Water is absorbed from the soil by the roots and travels upward through the plant’s vascular tissue, the xylem.
The accepted explanation for this upward movement is the cohesion-tension theory. As water evaporates from the leaf surface, it creates a negative pressure, or tension, within the xylem that extends down to the roots. Strong cohesive forces between water molecules allow them to stick together, forming a continuous column that is pulled upward to replace the lost moisture.
Water vapor exits the leaf through microscopic pores called stomata, typically found on the leaf’s underside. Each stoma is surrounded by guard cells that regulate the opening and closing of the pore. Plants must open these pores to allow carbon dioxide to enter for photosynthesis, but this action results in the loss of water vapor. The plant must balance the need to take in carbon dioxide with the risk of losing too much water.
Factors Controlling Water Loss Rates
The amount of water a plant transpires is highly variable, depending on atmospheric and biological factors. The primary atmospheric driver is the vapor pressure deficit (VPD), which measures the difference between the air’s saturation capacity and its current moisture content. A high VPD, resulting from high temperatures and low relative humidity, significantly increases water loss from the leaves. Wind also enhances transpiration by continually removing the humid air layer, known as the boundary layer, that surrounds the leaf surface.
Solar radiation and temperature play a direct role, as light stimulates the opening of the stomata for photosynthesis. Higher temperatures cause faster evaporation rates and prompt the plant to increase transpiration for evaporative cooling. However, soil water availability is a limiting factor. If roots cannot absorb water fast enough, the plant loses turgor, causing stomata to close and reducing the transpiration rate.
Plant characteristics also influence water loss rates. Plants with smaller leaves or thick, waxy cuticles, such as those in arid environments, naturally transpire less to conserve moisture. Conversely, plants with a large total leaf area, or canopy, have a greater surface for evaporation. The density and location of the stomata also determine the plant’s ability to regulate water loss.
Quantifying Water Transpired During the Growing Season
The total amount of water transpired over a growing season is substantial, varying widely based on environmental factors. A single mature oak tree can transpire approximately 100 gallons (378.5 liters) of water per day during the peak season. This daily rate can accumulate to over 40,000 gallons (151,000 liters) of water lost to the atmosphere annually.
For annual crops, water use is often discussed at the field scale. An acre of corn, for example, can transpire about 400,000 gallons (1.5 million liters) of water over its entire growing season. This figure represents the water lost through the plants, which constitutes the majority of the total water used by the field.
Scientists and water managers use the term evapotranspiration (ET) to estimate water usage, combining transpiration (water loss from the plant) and evaporation (water loss from the soil and wet surfaces). While young corn fields lose more water through soil evaporation, a mature corn canopy can account for 90% to 98% of the total ET through transpiration alone.
A high-yielding corn crop in the Midwestern United States typically requires a seasonal water depth equivalent to 20 to 30 inches (51 to 76 cm). Of this total, transpiration accounts for about 55% to 60%. The precise amount of water lost is calculated using specialized models like the Penman-Monteith equation, which integrates weather data such as solar radiation, air temperature, humidity, and wind speed to estimate water consumption for various crop types.