Transpiration vs. Evaporation: What's the Difference?

Key Difference

Transpiration is the physiological process of loss of water from living surfaces while evaporation is the loss of water from any surfaces.

Comparison Chart

Transpiration Evaporation
Process Type Physiological Physical
Occurs in In plants On any free surface
Force Required Vapor pressure, osmotic pressure No force
Rate Slow Fast
Regulators Carbon dioxide, pH, hormones, and light No regulator
Factors effecting Relative humidity, wind or air movement, type of plant, temperature and water availability in soil. Temperature, surface area, concentration of the substance evaporating, pressure, inter molecular forces, flow rate of air.
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What is Transpiration?

Transpiration is the physiological process of the release of water from plants through small openings in their leaves or stomata. Plants can control the loss of water by opening and close the stomata which also help them survive in hot summer weather. Water changes into vapors and is released into the atmosphere. Transpiration is an essential process of plants. It also includes a process in which loss of water occurs in liquid form the leaves and stem of the plants; this process is called guttation. Several studies have found that 10% of the atmospheric moisture is the result of transpiration by the plants while remaining 90% results in evaporation from oceans, seas, and other water bodies. Transpiration process depends on the humidity or wetness of the air and also on how much the soil in which the plants are planted has. Water is taken in by the plants and trees through their roots and is carried to all its parts as nourishment from where it is lost as transpiration. Different atmospheric factors affect the process of transpiration for example temperature, relative humidity, air movement or wind, soil moisture availability and type of plants. Transpiration rate goes up as the temperature increases especially during the growing season because air is warmer in growing season. Increased temperature causes the plant cells which control the stomata where water is released to the atmosphere to open, whereas colder temperature causes the openings to close. When relative humidity of the air rises the transpiration rate decreases. It is easier for water to evaporate into the dry air than into more saturated air. Similarly, increased movement of the air around the plant or wind will result in a higher transpiration rate. If there is no wind, the atmospheric air around the leaf may not move very much, raising the humidity of the air around the leaf. When moisture is decreasing, plants begin to undergo premature aging, which will result in leaf loss and transpire less water. Transpiration rate also depends on the type of plants. Some plants which grow in arid regions, such as cacti and succulents, conserve precious water by transpiring less water than other plants.

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What is Evaporation?

Evaporation is the process of releasing water from different water bodies. Water changes from liquid to the gas form and goes up into the air. It happens only if energy is available to change the water into water vapor. Solar energy drives evaporation of water from lakes, oceans, moisture into the soil and other sources of water. Evaporation occurs when the surface of the liquid is exposed and allows molecules to escape and form water vapors. These vapors then make clouds. Different factors affect the evaporation process. If the air already has a high concentration of the substance evaporating, then substance will evaporate more slowly. The flow rate of air also affects evaporation rate. If fresh air is moving over the substance all the time, then the concentration of the substance in the air is less likely to go up with the time, encourages faster evaporation. This is the result of the boundary layer at the evaporation surface decreasing with flow velocity, decreasing the diffusion distance in the stagnant layer. Intermolecular forces also affect evaporation process rate. The stronger these forces keeping the molecules together in the liquid state, the more energy one must get to escape. Similarly, pressure, surface area and temperature of the substance also affect evaporation rate.

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Transpiration vs. Evaporation

  • Both transpiration and evaporation are important to the water cycle.
  • The process of releasing water into the air through both transpiration and evaporation is called evapotranspiration.
  • Transpiration is a physiological process of water loss from the plants to the air while evaporation is a physical process of water loss from the surface to the air.
  • Transpiration is present in plants naturally while evaporation starts when energy is available for water loss in the form of heat
  • The amount of water goes into air depends on the moisture content of the soil on which plant is planted and humidity of the air in the case of transpiration while depends on the heat in the case of evaporation.

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