6 Methods of measurement of evaporation

 Measuring evaporation is crucial in hydrology and water resource management. Here's a detailed breakdown of the common methods:

1. Pan Measurement Method

• Principle:
  • This method directly measures the amount of water that evaporates from a standardized pan.
  • The water level in the pan is monitored, and the decrease in water level over a specific period represents the evaporation.
• Types of Evaporation Pans:
  • U.S. Class A Pan:
    • The most widely used type.
    • A cylindrical pan, typically made of unpainted galvanized iron.
    • Placed on a wooden platform to allow air circulation.
    • Measurements are taken daily.
  • Colorado Sunken Pan:
    • Buried in the ground, which provides a more realistic representation of natural evaporation conditions.
    • Reduces the influence of solar radiation and air temperature fluctuations.
  • I.S. Standard Pan:
    • A modified version of the U.S. Class A pan, with specific dimensions and materials.
• Pan Coefficient:
  • Evaporation from a pan differs from that of a large water body due to factors like size and heat capacity.
  • A pan coefficient is used to correct the pan evaporation to estimate the actual evaporation from a lake or reservoir.
  • Lake Evaporation = Pan Coefficient × Pan Evaporation.
• Advantages:
  • Relatively simple and inexpensive.
  • Provides direct measurements.
• Disadvantages:
  • Pan evaporation can be significantly different from actual lake evaporation.
  • Requires careful maintenance and monitoring.

2. Empirical Formulae

• Principle:
  • These methods use mathematical equations that relate evaporation to meteorological data, such as temperature, humidity, and wind speed.
  • Based on Dalton's law of evaporation, which states that evaporation is proportional to the difference between saturation vapor pressure and actual vapor pressure.
• Examples of Formulae:
  • Mayer Formula:
    • Considers wind velocity and the difference between saturation and actual vapor pressure.
  • Rohwer's Formula:
    • Includes atmospheric pressure in addition to vapor pressure and wind velocity.
  • Lake Hefner Formula:
    • Specifically developed for large water bodies.
• Advantages:
  • Can be used when direct measurements are unavailable.
  • Requires readily available meteorological data.
• Disadvantages:
  • Accuracy depends on the specific formula and the accuracy of the meteorological data.
  • Empirical formulae are often location-specific.

3. Water Budget Method

• Principle:
  • This method balances all water inflows and outflows to a water body to determine evaporation.
  • Based on the principle of conservation of mass.
• Equation:
  • Inflow - Outflow = Change in Storage
  • Evaporation is calculated as the difference between inflows, outflows, and changes in storage.
• Advantages:
  • Provides an integrated estimate of evaporation.
  • Can be used for large water bodies.
• Disadvantages:
  • Requires accurate measurements of all water budget components, which can be challenging.
  • Accuracy is limited by the accuracy of the other measurements.

4. Energy Budget Method

• Principle:
  • This method analyzes the energy balance at the water surface to determine the energy used for evaporation.
  • Based on the principle of conservation of energy.
• Factors Considered:
  • Solar radiation, back radiation, sensible heat transfer, heat stored in the water body, and heat used for evaporation.
• Bowen Ratio:
  • Used to estimate the sensible heat transfer, which is often difficult to measure directly.
• Advantages:
  • Provides a physically based estimate of evaporation.
  • Can be used to study the effects of different energy sources on evaporation.
• Disadvantages:
  • Requires complex measurements and calculations.
  • Accurate measurements of all energy budget components can be difficult.

In summary, the choice of method depends on the available resources, the desired accuracy, and the specific application.