5.1 Evaporation and transpiration processes
4 min read•july 30, 2024
Evaporation and transpiration are key processes in the water cycle, moving water from Earth's surface to the atmosphere. These processes, collectively called , are influenced by weather, soil, and plant characteristics.
Understanding the differences between evaporation and transpiration is crucial for modeling water movement in ecosystems. Factors like , , and affect both processes, while plant-specific traits play a big role in transpiration rates.
Evaporation vs Transpiration
Evaporation and Transpiration Processes
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- Evaporation converts liquid water into water vapor and removes it from the surface of the Earth (water bodies, soil surfaces, and vegetation surfaces)
- Transpiration loses water from plants through stomata in their leaves, converting liquid water within the plant tissues into water vapor
- Evaporation and transpiration collectively referred to as evapotranspiration (ET) represent the total water flux from the Earth's surface to the atmosphere
Differences between Evaporation and Transpiration
- Evaporation occurs from open water surfaces, bare soil, and vegetated surfaces
- Transpiration limited to water loss through plant stomata
- Rate of evaporation influenced by meteorological factors (temperature, humidity, wind speed, and )
- Transpiration additionally influenced by plant characteristics and availability
Factors Influencing Evaporation
Meteorological Factors
- Temperature affects evaporation rates by controlling the kinetic energy of water molecules
- Higher temperatures lead to increased evaporation rates due to greater molecular motion and easier escape of water molecules from the surface
- Humidity, or the amount of water vapor in the air, influences evaporation rates through the concept of vapor pressure deficit (VPD)
- A higher VPD, resulting from low humidity, promotes faster evaporation rates
- Wind speed affects evaporation rates by removing the saturated air layer above the evaporating surface, allowing for the continuous movement of water vapor away from the surface
- Solar radiation provides the energy required for the phase change of liquid water to water vapor
- Higher levels of solar radiation, particularly in the form of direct sunlight, increase evaporation rates
Surface Characteristics
- Soil texture and structure influence evaporation rates from soil surfaces
- Sandy soils tend to have higher evaporation rates compared to clay soils due to their larger pore spaces and lower water retention capacity
- Presence of surface water (lakes, reservoirs, and wetlands) enhances evaporation rates compared to dry soil surfaces due to the readily available water supply
Vegetation's Role in Transpiration
Stomata and Transpiration Process
- Vegetation plays a crucial role in the transpiration process, as plants lose water through stomata in their leaves to the atmosphere
- Stomata are small openings on the leaf surface that allow for gas exchange ( uptake for photosynthesis and water vapor release) between the plant and the atmosphere
- Transpiration process driven by the water potential gradient between the plant leaves and the atmosphere, with water moving from areas of high water potential (leaf cells) to areas of low water potential (atmosphere)
Factors Influencing Transpiration Rates
- Solar radiation, temperature, humidity, wind speed, and soil moisture availability influence transpiration rates
- Higher levels of these factors generally increase transpiration rates
- Plant characteristics (leaf area index (LAI), leaf morphology, and stomatal density) affect transpiration rates
- Plants with higher LAI and more stomata tend to have higher transpiration rates
Impact on Water Balance
- Transpiration represents a major pathway for water loss from vegetated landscapes
- In many ecosystems, transpiration accounts for a substantial portion of the total evapotranspiration, often exceeding evaporation from soil and water surfaces
- and transpiration rates influence soil moisture dynamics, groundwater recharge, and surface runoff by determining the partitioning of precipitation into these different components of the
Potential vs Actual Evapotranspiration
Potential Evapotranspiration (PET)
- PET is the amount of water that would be evaporated and transpired from a vegetated surface if there were sufficient water available in the soil to meet the atmospheric demand
- PET represents the maximum possible evapotranspiration rate under a given set of meteorological conditions, assuming an adequate water supply and a reference crop (usually a well-watered grass or alfalfa)
- PET influenced by meteorological factors (temperature, humidity, wind speed, and solar radiation) but does not consider the actual water availability in the soil
Actual Evapotranspiration (AET)
- AET is the quantity of water actually removed from a surface through the combined processes of evaporation and transpiration under real-world conditions
- AET limited by the availability of water in the soil and vegetation and typically lower than or equal to PET
- When soil moisture is not limiting (well-watered crops or after significant rainfall events), AET can approach or equal PET
- In water-limited environments, where soil moisture is scarce, AET often significantly lower than PET, as evaporation and transpiration are restricted by the lack of available water
Relationship between PET and AET
- Relationship between PET and AET important for understanding the water balance and the limitations on evapotranspiration in different ecosystems and under various climate conditions
- Ratio of AET to PET, known as the evapotranspiration ratio or the crop coefficient, used to assess the water stress experienced by vegetation and to estimate rates based on PET calculations
Key Terms to Review (19)
Actual evapotranspiration: Actual evapotranspiration refers to the amount of water that is evaporated from soil and surface water and transpired by plants during a specific time period, often expressed in millimeters. This process is influenced by factors such as soil moisture availability, vegetation type, and atmospheric conditions. Understanding actual evapotranspiration is crucial for water resource management and hydrological modeling, as it provides insights into the water cycle and helps in predicting water supply and demand.
Evapotranspiration: Evapotranspiration is the combined process of water evaporation from the soil and other surfaces, along with plant transpiration from leaves. This process is crucial for understanding water movement in the environment and plays a significant role in various hydrological processes, such as water balance, surface runoff, and the overall health of ecosystems.
Humidity: Humidity is the amount of water vapor present in the air, typically expressed as a percentage of the maximum amount of water vapor that the air can hold at a given temperature. High humidity levels can influence various environmental processes, affecting everything from weather patterns to biological activities. Understanding humidity is crucial for grasping how evaporation and transpiration occur, as it directly impacts the rates at which water is transferred from the Earth's surface into the atmosphere.
Hydrological Cycle: The hydrological cycle is the continuous movement of water within the Earth and atmosphere, involving processes such as evaporation, condensation, precipitation, and infiltration. This cycle is crucial for replenishing freshwater resources, regulating climate, and sustaining ecosystems. Its components interact to transfer water from one state to another, shaping weather patterns and influencing landforms over time.
Hydrometer: A hydrometer is a scientific instrument used to measure the specific gravity or density of liquids, often employed in various fields, including hydrology. This device operates based on buoyancy principles, where it floats in the liquid and indicates the density based on the level it reaches. Hydrometers are particularly useful in understanding water properties, which is essential in analyzing evaporation and transpiration processes.
Land Surface Temperature: Land surface temperature (LST) refers to the temperature of the Earth's land surfaces, which is influenced by various factors such as solar radiation, vegetation cover, and soil moisture. LST plays a crucial role in processes like evaporation and transpiration, as it directly affects how water is absorbed and released from the land. This temperature is essential for understanding energy balance, climate patterns, and hydrological processes that impact ecosystems and weather systems.
Latent Heat Flux: Latent heat flux refers to the transfer of energy associated with the phase change of water from liquid to vapor during evaporation or from vapor to liquid during condensation. This energy transfer is crucial in influencing weather patterns and climate, as it directly impacts the energy balance of the Earth's surface and the atmosphere. Understanding latent heat flux is key to grasping how evaporation and transpiration processes contribute to the movement of water and energy within the hydrological cycle.
Lysimeter: A lysimeter is a scientific instrument used to measure the amount of water that moves through soil and the amount of water absorbed by plants, providing insights into hydrological processes. By simulating natural conditions, lysimeters help in understanding how much precipitation is intercepted by vegetation, the rate of evaporation, and transpiration processes, which are vital for estimating water balance in ecosystems.
Penman-Monteith Equation: The Penman-Monteith Equation is a widely used formula for estimating evapotranspiration, which combines the effects of evaporation from the soil and transpiration from plants into a single equation. This equation takes into account various environmental factors such as temperature, humidity, wind speed, and solar radiation, making it a comprehensive method for understanding how water is transferred from the land to the atmosphere. It serves as a critical tool in hydrology for assessing water balance and managing water resources effectively.
Plant Water Stress: Plant water stress refers to the condition where a plant experiences a deficiency of water, which can hinder its physiological processes, growth, and overall health. This stress affects crucial functions such as photosynthesis, nutrient uptake, and transpiration, leading to reduced biomass and impaired reproductive success. Understanding plant water stress is essential for assessing how plants respond to varying moisture conditions and their ability to adapt to environmental changes.
Potential Evapotranspiration: Potential evapotranspiration (PET) refers to the maximum amount of water that can be evaporated and transpired from a land surface under optimal moisture conditions, assuming that there is sufficient water available for this process. It is a crucial concept in understanding how much water could potentially leave the soil and plant surfaces due to evaporation and plant transpiration when the environment provides adequate conditions such as temperature, sunlight, and wind. PET is used to estimate water balance in hydrological modeling and is influenced by various factors like climate, vegetation type, and land use.
Soil moisture: Soil moisture refers to the water held in the spaces between soil particles, which is crucial for plant growth and plays a vital role in the hydrological cycle. This moisture impacts various processes including runoff generation, evapotranspiration, and is influenced by precipitation and other hydrological components. Understanding soil moisture is essential for effective land management and assessing water availability in ecosystems.
Solar Radiation: Solar radiation is the energy emitted by the sun in the form of electromagnetic waves, primarily visible light, ultraviolet light, and infrared radiation. This energy drives various natural processes on Earth, including evapotranspiration, where it plays a crucial role in both evaporation from soil and water bodies and transpiration from plants. Understanding solar radiation is key to estimating energy inputs in hydrological models and assessing water balance in ecosystems.
Stomatal Conductance: Stomatal conductance is the measure of the rate at which carbon dioxide enters, and water vapor exits, a leaf through tiny openings called stomata. This process is crucial for photosynthesis and transpiration, affecting water loss and gas exchange in plants. Stomatal conductance directly impacts how efficiently plants can use water and light, influencing both their growth and the surrounding environment.
Temperature: Temperature is a measure of the average kinetic energy of the particles in a substance, influencing physical processes like evaporation and transpiration. It plays a crucial role in determining the rate at which water evaporates from surfaces and how plants lose water through transpiration, affecting overall evapotranspiration rates in an ecosystem. Variations in temperature can significantly impact these processes, as higher temperatures typically lead to increased evaporation and transpiration rates.
Thornthwaite Equation: The Thornthwaite Equation is a formula used to estimate potential evapotranspiration (PET) based on temperature and day length. It helps to quantify the amount of moisture that can potentially be returned to the atmosphere through evaporation from soil and transpiration from plants, making it crucial for understanding water balance in different climates and ecosystems.
Vegetation Cover: Vegetation cover refers to the layer of plant material, including trees, shrubs, grass, and other flora, that covers the ground in a specific area. It plays a critical role in the water cycle by influencing both evaporation and transpiration processes, which are essential for understanding how water moves through ecosystems and contributes to evapotranspiration measurements.
Water Balance: Water balance refers to the relationship between the input and output of water in a specific system, ensuring that the amount of water entering equals the amount leaving over a defined period. This concept is crucial for understanding hydrological processes, as it incorporates components such as precipitation, evaporation, transpiration, and runoff to assess the overall water availability in an area. Achieving a sustainable water balance is essential for effective water resource management and ecosystem health.
Wind Speed: Wind speed is a measure of how fast the air is moving in a specific direction, usually expressed in units like meters per second (m/s) or miles per hour (mph). It plays a critical role in the processes of evaporation and transpiration by influencing the rate at which water vapor moves away from surfaces, impacting overall evapotranspiration rates. Wind speed also affects temperature and moisture transfer in the atmosphere, directly influencing the effectiveness of these processes.