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How Do You Calculate Evaporation Rate?

Published in Evaporation Science 2 mins read

The evaporation rate of water can be calculated using a formula that considers wind speed, surface area, and humidity differences. Here's a breakdown of the calculation, based on the provided short answer:

Steps to Calculate Evaporation Rate:

  1. Calculate the wind speed factor:

    • Multiply the wind speed (measured in meters per second, m/s) by 19.
    • Add 25 to the result.
    • Formula: Wind Speed Factor = (Wind Speed (m/s) * 19) + 25

  2. Account for the surface area:

    • Multiply the wind speed factor by the surface area of the water exposed to the air (measured in square meters, m²).
    • Formula: Area Factor = Wind Speed Factor * Surface Area (m²)

  3. Determine the humidity difference:

    • Calculate the maximum humidity ratio. This is the humidity ratio of saturated air at the water's temperature.
    • Determine the current humidity ratio of the air above the water.
    • Subtract the current humidity ratio from the maximum humidity ratio. (Both humidity ratios are measured in kilograms of water per kilogram of dry air, kg/kg).
    • Formula: Humidity Difference = Maximum Humidity Ratio (kg/kg) - Current Humidity Ratio (kg/kg)

  4. Calculate the evaporation rate:

    • Multiply the area factor by the humidity difference.
    • The result is the water's evaporation rate in kilograms per hour (kg/hr).
    • Formula: Evaporation Rate (kg/hr) = Area Factor * Humidity Difference

Formula Summary:

Evaporation Rate (kg/hr) = ((Wind Speed (m/s) * 19) + 25) * Surface Area (m²) * (Maximum Humidity Ratio (kg/kg) - Current Humidity Ratio (kg/kg))

Example:

Let's say:

  • Wind Speed = 2 m/s
  • Surface Area = 5 m²
  • Maximum Humidity Ratio = 0.02 kg/kg
  • Current Humidity Ratio = 0.01 kg/kg

Then:

  1. Wind Speed Factor = (2 * 19) + 25 = 38 + 25 = 63
  2. Area Factor = 63 * 5 = 315
  3. Humidity Difference = 0.02 - 0.01 = 0.01
  4. Evaporation Rate = 315 * 0.01 = 3.15 kg/hr

Therefore, the evaporation rate is 3.15 kilograms per hour.

Important Considerations:

  • This formula provides an estimate. Actual evaporation rates can be affected by factors not included in this simplified calculation, such as water temperature, air pressure, and the presence of contaminants.
  • Accurate measurements of wind speed, surface area, and humidity ratios are crucial for a reliable result.
  • More complex models exist that account for additional variables and provide more precise evaporation rate predictions.

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