5.2 Measurement and estimation methods for evapotranspiration
7 min read•july 30, 2024
Evapotranspiration is a key player in the water cycle, influencing how water moves between land and air. Measuring it accurately is crucial for understanding water availability and managing resources effectively.
There are various ways to measure and estimate evapotranspiration, from direct methods like lysimeters to techniques. Each method has its strengths and limitations, so choosing the right one depends on your specific needs and available resources.
Evapotranspiration Measurement Methods
Direct Measurement Techniques
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- Lysimeters monitor changes in weight or volume of a soil column over time to provide accurate ET measurements (expensive, labor-intensive, limited to point-scale observations)
- Weighing lysimeters use a balance to measure changes in the mass of a soil column, while non-weighing lysimeters measure changes in the volume of water drained from the soil column
- Lysimeters are often used as reference methods to calibrate and validate other ET estimation techniques
- Eddy covariance measures vertical turbulent fluxes of water vapor and heat above the canopy using complex instrumentation (three-dimensional sonic anemometer, infrared gas analyzer) and data processing
- Provides continuous, high-frequency ET measurements based on the principle that vertical transport of water vapor and heat is driven by turbulent eddies in the atmospheric boundary layer
- Requires careful site selection, instrument calibration, and data quality control to ensure accurate ET measurements
Remote Sensing Techniques
- Satellite-based methods estimate ET over large spatial scales using spectral reflectance data and surface energy balance models (lower temporal resolution and accuracy compared to ground-based measurements)
- Rely on the relationship between surface and vegetation indices, such as the Normalized Difference Vegetation Index (NDVI), to estimate ET
- Examples include the Surface Energy Balance Algorithm for Land (SEBAL) and the Mapping Evapotranspiration at high Resolution with Internalized Calibration (METRIC) model
- Airborne and drone-based remote sensing techniques, such as thermal infrared imagery and multispectral sensors, provide higher spatial resolution ET estimates compared to satellite-based methods
- Allow for more detailed mapping of ET variations within fields or small watersheds
- Require ground-truthing and calibration using in-situ ET measurements or reference ET values
Estimating Evapotranspiration
Penman-Monteith Method
- A physically-based model that combines energy balance and aerodynamic principles to estimate ET using meteorological data and surface characteristics
- Recommended by the United Nations Food and Agriculture Organization (FAO) as the standard method for estimating reference evapotranspiration (ET0)
- Requires inputs such as net radiation, air temperature, humidity, , and surface resistance
- The FAO-56 is a simplified version that uses a hypothetical reference crop (grass) with specific characteristics to estimate ET0
- Assumes a reference crop height of 0.12 m, a fixed surface resistance of 70 s m-1, and an albedo of 0.23
- Can be used to estimate crop-specific ET by multiplying ET0 with a crop coefficient (Kc) that accounts for the differences in crop characteristics and growth stages
Priestley-Taylor Method
- A simplified version of the Penman-Monteith equation that assumes a constant relationship between the aerodynamic and surface resistance terms
- Estimates ET using net radiation, air temperature, and a dimensionless coefficient (α) that accounts for surface and aerodynamic resistances
- The α coefficient typically ranges from 1.2 to 1.3 for well-watered surfaces but may require local calibration for specific land cover types and climatic conditions
- Often used when detailed meteorological data are not available or for large-scale ET estimation in humid regions where the aerodynamic component of ET is less significant
- Performs well in humid regions with low advection and for well-watered surfaces such as irrigated crops or wetlands
- May overestimate ET in arid or advective conditions where the aerodynamic component of ET is significant
Other Empirical Methods
- Hargreaves-Samani method estimates ET0 using only air temperature and extraterrestrial radiation data
- Useful for regions with limited meteorological data or for historical ET estimation where detailed weather data are not available
- May underestimate ET in windy or humid conditions and overestimate ET in arid or cold regions
- Turc method estimates ET0 based on air temperature and , making it suitable for regions with limited wind speed and humidity data
- Developed for Mediterranean climates and may not perform well in other climatic regions without local calibration
- Tends to underestimate ET in arid or semi-arid regions with high wind speeds and low humidity
- Thornthwaite method estimates (PET) based on air temperature and day length, assuming a 30-day month and 12-hour day
- Useful for long-term or historical PET estimation when only temperature data are available
- May overestimate PET in arid regions and underestimate PET in humid regions due to its simplifying assumptions
Evapotranspiration Method Comparison
Advantages and Limitations
- Direct measurement methods (lysimeters, eddy covariance) provide accurate and continuous ET data but are expensive, labor-intensive, and limited in spatial coverage
- Often used for research purposes or to calibrate and validate other ET estimation methods
- Not suitable for large-scale or long-term ET monitoring due to their high costs and maintenance requirements
- Empirical equations (Penman-Monteith, Priestley-Taylor) are widely used for estimating ET due to their relative simplicity and lower data requirements compared to direct measurements
- May have limitations in terms of accuracy and applicability under certain conditions, such as arid or advective environments
- Require careful selection of input parameters and local calibration to ensure reliable ET estimates
- Remote sensing techniques provide spatially distributed ET estimates over large areas but may have lower temporal resolution and accuracy compared to ground-based measurements
- Useful for regional-scale water resources assessment and monitoring but may require ground-truthing and validation
- Subject to uncertainties related to sensor calibration, atmospheric corrections, and surface energy balance model assumptions
Selecting an Appropriate Method
- The choice of ET measurement or estimation method depends on factors such as data availability, spatial and temporal scales, required accuracy, and resources
- Direct measurements are preferred for site-specific studies or for calibrating and validating other ET estimation methods
- Empirical equations are suitable for most agricultural and hydrological applications, provided that the required input data are available and the equations are appropriately selected and calibrated
- Remote sensing techniques are useful for large-scale ET mapping and monitoring, especially in regions with limited ground-based observations
- A combination of methods is often used to leverage their respective strengths and compensate for their limitations
- For example, using or eddy covariance measurements to calibrate and validate remote sensing-based ET estimates
- Combining empirical equations with remote sensing data to improve the spatial and temporal resolution of ET estimates
Interpreting Evapotranspiration Data
Water Balance Applications
- ET data is a crucial component of the water balance equation, which describes the partitioning of into various components of the hydrological cycle (, infiltration, storage changes)
- ET represents the water lost from the land surface to the atmosphere through evaporation and transpiration
- Often the largest outflow component in many ecosystems, especially in arid and semi-arid regions
- ET data can be used to estimate other water balance components, such as recharge and runoff, by applying the water balance equation to a specific control volume (watershed, aquifer) over a given time period
- For example, in a simplified water balance equation, recharge can be estimated as the difference between precipitation and the sum of ET and runoff, assuming negligible storage changes
- Accurate ET estimates are essential for reliable water balance calculations and for assessing the sustainability of water resources in a given region
Hydrological Modeling Applications
- ET data is an essential input for hydrological models, which simulate the movement and storage of water in a catchment or river basin
- Hydrological models use ET estimates to quantify the atmospheric losses and update the and groundwater storage in the modeled system
- Examples include the Variable Infiltration Capacity (VIC) model and the Soil and Water Assessment Tool (SWAT)
- ET data helps to improve the representation of land surface-atmosphere interactions in hydrological models and to assess the impacts of land use and climate changes on water resources
- For example, using remote sensing-based ET estimates to calibrate and validate hydrological models in data-scarce regions
- Incorporating crop-specific ET estimates in agricultural watershed models to evaluate the effects of irrigation practices on water availability and quality
Considerations for Data Interpretation
- When interpreting ET data for water balance calculations and hydrological modeling, it is important to consider the spatial and temporal scales, the uncertainties associated with the measurement or estimation methods, and the potential sources of errors
- Spatial scale: Ensure that the ET data resolution is appropriate for the scale of the analysis (point, field, watershed, or regional)
- Temporal scale: Consider the temporal variability of ET and the frequency of the ET data (hourly, daily, monthly, or annual) in relation to the modeling objectives
- Uncertainties: Assess the accuracy and reliability of the ET data based on the measurement or estimation method used and the quality of the input data
- Potential errors: Account for factors that may introduce errors in ET estimates, such as advection, irrigation, or land cover changes
- Accurate ET data and its proper interpretation are crucial for informed decision-making in water resources planning and management
- Assessing water availability and sustainability in a given region
- Optimizing irrigation scheduling and water allocation among different users
- Evaluating the impacts of land use and climate changes on hydrological processes and water resources
Key Terms to Review (20)
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.
ASCE Standardized Reference Evapo-Transpiration Equation: The ASCE Standardized Reference Evapo-Transpiration Equation is a formula developed by the American Society of Civil Engineers to estimate the amount of water that evaporates and transpires from a reference surface, typically a well-watered grass field. This equation integrates various climatic factors, such as temperature, humidity, wind speed, and solar radiation, allowing for a standardized approach to quantify evapotranspiration in different environments. Understanding this equation is vital for effective water management, irrigation practices, and hydrological modeling.
Climate data analysis: Climate data analysis is the process of examining and interpreting meteorological and environmental data to identify trends, patterns, and relationships that inform our understanding of climate systems. This involves collecting data from various sources such as satellite imagery, ground-based measurements, and climate models to assess changes over time. The insights gained from this analysis are crucial for understanding phenomena like evapotranspiration, which is significantly influenced by climatic conditions.
Eddy covariance method: The eddy covariance method is a technique used to measure the exchange of gases, such as water vapor and carbon dioxide, between the Earth's surface and the atmosphere. It relies on high-frequency measurements of wind speed and direction, alongside concentrations of the gases, to calculate the turbulent fluxes of these gases. This method is particularly valuable in understanding evapotranspiration processes, as it provides direct measurements of how much water vapor is being released from soil and vegetation into the atmosphere.
FAO-56 Model: The FAO-56 Model is a standardized method developed by the Food and Agriculture Organization (FAO) for estimating reference evapotranspiration (ET₀), which is essential for effective irrigation management and agricultural planning. This model utilizes climatic data such as temperature, humidity, wind speed, and solar radiation to provide accurate estimations of water requirements for crops, thereby optimizing water use and enhancing agricultural productivity.
Hargreaves Equation: The Hargreaves Equation is a widely used empirical formula that estimates potential evapotranspiration (ET) based on temperature and solar radiation. It connects temperature and solar data to predict the rate at which water is lost from soil and plant surfaces, making it a practical tool for understanding water balance and irrigation needs in various climates.
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.
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.
Precipitation: Precipitation refers to any form of water, liquid or solid, that falls from the atmosphere and reaches the ground. It includes rain, snow, sleet, and hail, and plays a vital role in the water cycle as a key input in various hydrological processes like rainfall-runoff dynamics, soil moisture replenishment, and the overall water balance in ecosystems.
Remote Sensing: Remote sensing is the process of collecting information about an object or area from a distance, typically through satellite or aerial imagery. This technology plays a crucial role in monitoring and managing natural resources, as it allows for the analysis of environmental conditions, land use changes, and hydrological phenomena without direct contact.
Runoff: Runoff is the flow of water, usually from precipitation, that moves across the land surface and eventually returns to water bodies such as rivers, lakes, and oceans. This process plays a critical role in the hydrologic cycle, influencing water availability and quality while also connecting various elements such as precipitation, watershed characteristics, and the overall water balance in a given area.
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.
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 Model: The Thornthwaite Model is a mathematical approach for estimating potential evapotranspiration (PET) based on temperature and latitude. This model considers the monthly average temperatures and uses them to assess the amount of moisture that could potentially be evaporated and transpired by plants, helping to understand the water balance in various climates.
Urbanization effects: Urbanization effects refer to the changes in environmental, hydrological, and social conditions that occur as a result of increased population density and infrastructure development in urban areas. These changes can significantly influence rainfall-runoff relationships, the measurement of evapotranspiration, and the spatial and temporal patterns of precipitation data, often leading to alterations in water availability and quality.
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.
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.
WMO Guidelines: The WMO Guidelines refer to a set of standards and recommendations established by the World Meteorological Organization (WMO) aimed at ensuring the accuracy and consistency of meteorological and hydrological measurements. These guidelines are essential for measuring and estimating processes such as evapotranspiration, providing protocols that help researchers and practitioners achieve reliable data collection and analysis across different regions and contexts.