1. 6. Methods for Estimation of Evapotranspiration

Methods for Estimation of Evapotranspiration

Evapotranspiration (ET) is the sum of water loss from soil (evaporation) and water used by plants (transpiration). Estimating ET accurately is crucial for efficient water management in irrigation. Various methods have been developed to estimate ET, which can be broadly categorized into direct measurement methods and indirect (theoretical) methods.

1. Lysimeter Method

2. Pan Evaporation Method

3. Penman-Monteith Equation 

4. Blaney-Criddle Method

5. Hargreaves-Samani Method

6. Thornthwaite Method

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1. Lysimeter Method

1.1 Description:

• A lysimeter is a device that measures the amount of water percolating through the soil and the amount evaporated and transpired by plants.

1.2 Procedure:

• A block of soil containing plants is isolated in a lysimeter.
• Water inputs (precipitation or irrigation) and outputs (drainage and changes in soil moisture) are measured.
• The change in water content over time gives the ET rate.

1.3 Advantages:

• Direct measurement of ET under natural conditions.
• Provides accurate and reliable data.

1.4 Disadvantages:

• Expensive and complex to set up and maintain.
• Limited to small areas and specific conditions.

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2. Pan Evaporation Method

2.1 Description:

• Uses a standardized evaporation pan (e.g., Class A evaporation pan) to measure water evaporation.

2.2 Procedure:

• The pan is filled with water and placed in an open area.
• The decrease in water level over a specific period indicates the amount of water evaporated.
• A coefficient (pan coefficient) is used to convert pan evaporation to ET.

2.3 Advantages:

• Simple and cost-effective.
• Easy to operate and maintain.

2.4 Disadvantages:

• Influenced by pan size, location, and environmental conditions.
• Requires calibration for accurate ET estimation.

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3. Penman-Monteith Equation

3.1 Description:

• A widely used theoretical method that combines meteorological and physiological parameters.

3.2 Procedure:

• Penman Method for Estimating Evapotranspiration

  The Penman method estimates reference evapotranspiration (ET₀) by combining the effects of temperature, humidity, wind speed, and solar radiation. The Penman equation is:

  $ET_0 = \frac{ \Delta \cdot (R_n - G) + \rho_a \cdot c_p \cdot (e_s - e_a) \cdot \frac{u}{\lambda} }{ \Delta + \gamma \cdot (1 + \frac{u}{\lambda}) }$

  Where:

  • $E{T}_0$ = Reference evapotranspiration (mm/day)
  • $\mathrm{\Delta }$ = Slope of the saturation vapor pressure curve (kPa/°C)
  • $R_n$= Net radiation (MJ/m²/day)
  • $G$ = Soil heat flux density (MJ/m²/day) (often approximated as zero for daily estimates)
  • ${\rho }_a$ = Air density (kg/m³) (approx. 1.225 kg/m³ at sea level)
  • $c$ = Specific heat of air (MJ/kg·K) (0.001013 MJ/kg·K)
  • $e$ = Saturation vapor pressure (kPa)
  • $e$ = Actual vapor pressure (kPa)
  • $u$ = Wind speed (m/s)
  • $\lambda$ = Latent heat of vaporization (MJ/kg) (approx. 2.45 MJ/kg)
  • $\gamma$= Psychrometric constant (kPa/°C) (0.066 kPa/°C)

3.3 Advantages:

• Comprehensive and accurate.
• Accounts for various meteorological and crop parameters.

3.4 Disadvantages:

• Requires extensive data collection.
• Complex calculations.

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4. Blaney-Criddle Method

4.1 Description:

• An empirical method that relates ET to temperature and daylight hours.

4.2 Procedure:

• The Blaney-Criddle method is a practical and widely used approach for estimating reference evapotranspiration (ET₀) based on temperature and daylight hours. The formula is given by:

  $E{T}_0=p(0.46T+8.13)$

  Where:

  • $E{T}_0$= Reference evapotranspiration (mm/day)
  • $p$ = Mean daily percentage of annual daytime hours
  • $T$ = Mean daily temperature (°C)

4.3 Advantages:

• Simple and easy to use.
• Requires minimal data.

4.4 Disadvantages:

• Less accurate compared to more complex methods.
• Assumes uniform crop and soil conditions.

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5. Hargreaves-Samani Method

5.1 Description:

• A simplified empirical method based on temperature and extraterrestrial radiation.

5.2 Procedure:

• The Hargreaves method is a simpler method for estimating reference evapotranspiration (ET₀) using temperature and extraterrestrial radiation. It is especially useful when limited climatic data is available. The Hargreaves equation is:

  $E{T}_0=0.0023\times (T_{mean}+17.8)\times (T_{max}-T_{min}{)}^{0.5}\times R_{\mathrm{a}}$

  Where:

  • $ET_0$= Reference evapotranspiration (mm/day)
  • $T_{mean}$ = Mean daily air temperature (°C)
  • $T_{max}$= Maximum daily air temperature (°C)
  • $T_{min}$ = Minimum daily air temperature (°C)
  • $R_a$ = Extraterrestrial radiation (MJ/m²/day)

5.3 Advantages:

• Requires only temperature data.
• Simple and quick.

5.4 Disadvantages:

• Less accurate in humid or arid conditions.
• Requires adjustment for different climates.

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6. Thornthwaite Method

6.1 Description:

• An empirical method based on temperature and daylight hours.

6.2 Procedure:

• The Thornthwaite method estimates potential evapotranspiration (ET₀) primarily based on temperature and day length. This method is suitable for use when only temperature data is available. The Thornthwaite equation is:

  $E{T}_0=16{\left(\frac{10\cdot T}{I}\right)}^a$

  • $E{T}_0$ = Potential evapotranspiration (mm/month)
  • $T$ = Mean monthly temperature (°C)
  • $I$ = Heat index, calculated as the sum of monthly heat indices for the year
  • a = Empirical exponent, calculated using the equation: $a = (6.75 \times 10^{-7})I^3 - (7.71 \times 10^{-5})I^2 + (1.792 \times 10^{-2})I + 0.49239$
    

                      

6.3 Advantages:

• Simple and requires minimal data.
• Widely used for monthly ET estimation.

6.4 Disadvantages:

• Less accurate for short-term estimates.
• Assumes uniform crop and soil conditions.

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Each of these methods has its own set of advantages and limitations. The choice of method depends on the available data, required accuracy, and specific conditions of the study area.