19.5 - Methods of Estimating Evapotranspiration
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Empirical Methods of Estimating ET
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Today, we're going to learn about Empirical Methods for estimating Evapotranspiration. Can anyone tell me a type of empirical method?
Isn't the Blaney-Criddle Method one of them?
Yes! The Blaney-Criddle Method uses the mean daily temperature and daylight hours. Does anyone remember the formula?
It's ET equals p times (0.46T + 8), right?
Exactly! Remember that p stands for the monthly percentage of annual daylight hours. What about another method?
The Hargreaves Method?
Correct! It’s based on temperature too. Let's say it together: ET equals 0.0023 times (T + 17.8) times (T_max - T_min) raised to the power of 0.5 times Ra. Great job!
Energy Balance Methods
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Let's discuss Energy Balance Methods. Who can explain the fundamental idea behind them?
They focus on energy conservation, right?
Exactly! The key equation is R_n - G - H = λE. Can anyone break that down?
R_n is net radiation, G is soil heat, H is sensible heat, and λE is latent heat times ET.
Perfect! This shows how energy budget affects ET estimation. Why do you think this is important?
Because it helps us understand how much water is actually being evaporated.
Combination Methods
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Now let’s look at Combination Methods! Who can remind me of one of these?
The Penman-Monteith Method?
Exactly! This method is widely regarded as the standard. It combines both empirical and energy approaches. Can anyone explain how we write that?
It’s ET equals that big fraction with delta and gamma terms.
Correct! You’re learning well. This method is crucial for accurate global ET estimations.
Introduction & Overview
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Quick Overview
Standard
The estimation of evapotranspiration (ET) is approached through different methods depending on data availability, accuracy, and scale. These methods are categorized into empirical, energy balance, and combination methods, each with specific techniques such as the Blaney-Criddle method, Penman method, and FAO Penman-Monteith method, among others.
Detailed
Methods of Estimating Evapotranspiration
Estimating Evapotranspiration (ET) is crucial for water resource management, and various methods exist based on data availability and desired accuracy. Methods can be broadly categorized into three groups:
1. Empirical Methods
Empirical methods primarily use observed climatological data and regression equations to estimate ET. Key examples include:
- Blaney-Criddle Method: Utilizes mean daily temperature and daylight hours, expressed as:
ET = p(0.46T + 8)
where p is the monthly percentage of annual daylight hours and T is mean monthly temperature (°C).
- Hargreaves Method: Simple and temperature-based, calculated as:
ET = 0.0023(T + 17.8)(T_max - T_min)^{0.5} Ra
where Ra represents extraterrestrial radiation.
- Pan Evaporation Method: Founded on the evaporation rate from a Class A pan, represented as:
ET = K_p * E_p
where K_p is the pan coefficient and E_p is the pan evaporation.
2. Energy Balance Methods
This category is based on the principle of energy conservation at land surface, summarized in the energy budget equation:
R_n - G - H = λE
where R_n is net radiation, G is soil heat flux, H is sensible heat flux, and λE corresponds to latent heat multiplied by ET.
3. Combination Methods
Combination methods integrate both empirical and energy balance approaches. Notable methods include:
- Penman Method: Blends energy balance with aerodynamic methods:
ET = rac{Δ(R_n - G) + γ rac{900}{T + 273}u(e_s - e_a)}{Δ + γ(1 + 0.34u)}
with variables defined as slopes, psychrometric constants, wind speed, and vapor pressure deficit.
- FAO Penman-Monteith Method: The standard method for estimating reference ET, using:
ET = rac{0.408Δ(R_n - G) + γ rac{900}{T + 273}u(e_s - e_a)}{Δ + γ(1 + 0.34u)}
This method employs a standard grass reference crop and is globally applicable, providing accuracy in ET estimations.
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Overview of Estimation Methods
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Chapter Content
Various methods are used depending on data availability, accuracy required, and scale. They are broadly categorized as:
Detailed Explanation
This chunk introduces the different methods used to estimate evapotranspiration (ET). It highlights that the choice of method depends on the availability of data, the level of accuracy needed, and the scale at which the estimation is required. Understanding this framework helps identify which methods might be most applicable in different situations.
Examples & Analogies
Think of estimating ET like measuring the distance you need to travel. If you have a GPS (lots of data), you can get an accurate reading. If you only have a rough map (limited data), you might guess or use a simpler method to figure out the distance.
Empirical Methods
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Chapter Content
(a) Empirical Methods
• Based on observed climatological data and regression equations.
Detailed Explanation
Empirical methods rely on existing observations and statistical relationships to estimate ET. These approaches use climatological data, meaning data related to weather patterns and measurements, along with regression equations to derive ET estimates. They are typically simpler and can be used where direct measurements are not feasible.
Examples & Analogies
Empirical methods are like making a recipe based on one's experience from previous cooking. Instead of measuring every ingredient precisely, you rely on what worked before and adjust based on observation.
Blaney-Criddle Method
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- Blaney-Criddle Method
• Uses mean daily temperature and daylight hours.
• Suitable for monthly PET estimates.
ET = p(0.46T + 8) where p = monthly % of annual daylight hours, T = mean monthly temperature (°C)
Detailed Explanation
The Blaney-Criddle Method is a specific empirical method used to estimate potential evapotranspiration (PET). It incorporates mean daily temperature and the ratio of daylight hours in a month. This makes it particularly useful for monthly calculations of ET based on climatic data.
Examples & Analogies
Imagine you are planning a picnic and decide the best time to go based on past weather patterns. If it’s usually sunny for most of the day in the peak summer months, you use that knowledge to decide on the best day for the picnic.
Hargreaves Method
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Chapter Content
- Hargreaves Method
• Simple temperature-based method.
ET = 0.0023(T + 17.8)(T − T )^0.5 * Ra
• where Ra is extraterrestrial radiation.
Detailed Explanation
The Hargreaves Method is another empirical approach that uses temperature data to estimate ET. This method also factors in extraterrestrial radiation, which represents the total solar energy received outside the Earth’s atmosphere. It's a straightforward calculation that requires minimal data.
Examples & Analogies
Using the Hargreaves method is similar to checking the weather forecast to decide whether to carry an umbrella. If the temperature is higher, you might think it's a drier day, hence less need for an umbrella (similar to less ET).
Pan Evaporation Method
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Chapter Content
- Pan Evaporation Method
• Based on evaporation from a Class A evaporation pan.
ET = K · Ep where K = pan coefficient, Ep = pan evaporation.
Detailed Explanation
The Pan Evaporation Method estimates ET by measuring the evaporation from a Class A pan, which is a standardized device. The actual ET is calculated by applying a pan coefficient to the measured evaporation. This method is easy to implement and provides real-time data.
Examples & Analogies
Think of a pan of water left outside in the sun. You can observe how much water evaporates each day. If you use that measurement to predict how much water might be lost from a farm, it’s similar to how the Pan Evaporation Method works.
Energy Balance Methods
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Chapter Content
(b) Energy Balance Methods
• Based on the conservation of energy at the land surface.
Detailed Explanation
Energy balance methods estimate ET by considering the laws of energy conservation. They calculate the balance between energy inputs (like solar radiation) and outputs (like heat loss and evaporation). This holistic approach accounts for all energy fluxes at the land surface.
Examples & Analogies
Consider a bank account where money is deposited and withdrawn. To know your balance for the day, you compute how much was added and how much was spent. Similarly, energy balance methods compute energy gains and losses to estimate ET.
Penman Method
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Chapter Content
- Penman Method
• Combines energy balance and aerodynamic approach.
Detailed Explanation
The Penman Method is a sophisticated approach that merges energy balance principles with considerations of aerodynamic data, such as wind speed. It accounts for how the atmosphere affects evaporation and provides a more accurate ET estimate by incorporating multiple environmental variables.
Examples & Analogies
This method is like cooking where you not only consider the ingredients (energy balance) but also the cooking temperature and method (aerodynamics), resulting in a well-prepared dish.
FAO Penman-Monteith Method
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Chapter Content
- FAO Penman-Monteith Method
• Recommended standard method by FAO for reference ET estimation:
0.408∆(R − G) + γ 900 u (e − e )/(∆ + γ(1 + 0.34u ))
Detailed Explanation
The FAO Penman-Monteith Method is considered a gold standard for estimating reference ET. It is widely used because it is both accurate and applicable in many global contexts. The formula integrates multiple factors, including net radiation, temperature, wind speed, and humidity to provide a reliable ET estimate.
Examples & Analogies
Using this method is like having a detailed recipe that has been perfected over time. It includes all necessary ingredients and precise steps, ensuring the best possible outcome for estimating ET.
Key Concepts
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Empirical Methods: Techniques for ET estimation using observed data.
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Energy Balance: A principle focused on conserving energy at the surface for ET calculations.
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Penman-Monteith Method: A comprehensive standard method that combines various approaches for accurate ET estimation.
Examples & Applications
Using the Blaney-Criddle method, a farmer estimates their crop ET based on temperature data from the past month.
The Penman-Monteith method is often employed in meteorological studies for precise ET calculations in various climates.
Memory Aids
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Rhymes
Blaney-Criddle's simple indeed, temperature and daylight lead.
Stories
Imagine a farmer checking temperatures daily, using that data and hours of sunshine to grow crops wisely. This is how the Blaney-Criddle method enables successful farming!
Memory Tools
Remember: PET, AET. Potential, Actual, energy balance can't be skipped!
Acronyms
E-B-C for Estimation Methods
Empirical
Balance
Combination.
Flash Cards
Glossary
- Empirical Method
Estimation techniques based on observed data and statistical correlations.
- Energy Balance
Method focusing on the conservation of energy to estimate ET.
- PenmanMonteith Method
A standard method for estimating reference ET, combining energy balance and aerodynamic approaches.
- BlaneyCriddle Method
An empirical method using temperature and daylight hours for estimating ET.
- Hargreaves Method
An empirical method of estimating ET using maximum and minimum temperature values.
Reference links
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