How to Calculate the Rate of Evaporation

What is the Rate of Evaporation?

The rate of evaporation refers to the speed at which liquid water transforms into water vapor and escapes into the atmosphere. This natural process is fundamental to the Earth's water cycle, influencing weather patterns, climate, and local water management. Understanding how to calculate the rate of evaporation is crucial for various applications, from managing reservoir levels and irrigation schedules to designing cooling systems and predicting water loss in swimming pools.

This phenomenon is driven by the energy available to break the molecular bonds of liquid water and the vapor pressure difference between the water surface and the overlying air. The higher the energy and the greater the vapor pressure deficit, the faster the evaporation.

Who should use it: Environmental engineers, hydrologists, farmers, pool owners, HVAC professionals, and anyone interested in understanding water loss from open surfaces. Our water loss calculator can provide further insights.

Common misunderstandings: Many people assume evaporation is solely dependent on temperature. While temperature is a primary factor, wind speed, humidity, and the surface area of the water body play equally significant roles. Unit confusion is also common, as different regions use metric (liters/hour, m²) or imperial (gallons/hour, ft²) measurements, making consistent calculations vital.

How to Calculate the Rate of Evaporation: Formula and Explanation

While complex models like the Penman-Monteith equation exist for highly accurate scientific applications, a simplified empirical formula based on Dalton's Law of Evaporation is often used for practical estimations. This calculator uses a variation of this empirical approach to determine the rate of evaporation.

The general form of the simplified evaporation rate formula is:

E = K × A × (e_s - e_a) × (1 + C × W)

Where:

• E = Instantaneous Evaporation Rate (e.g., Liters per hour or Gallons per hour)
• K = Empirical coefficient (depends on water body type and unit system)
• A = Water Surface Area (e.g., square meters or square feet)
• e_s = Saturated Vapor Pressure at Water Surface Temperature (e.g., kPa or mmHg)
• e_a = Actual Vapor Pressure of Air (e.g., kPa or mmHg)
• C = Empirical wind coefficient (depends on unit system)
• W = Wind Speed (e.g., meters per second or miles per hour)

The term (e_s - e_a) represents the vapor pressure deficit, which is the driving force for evaporation. A larger deficit means faster evaporation. The actual vapor pressure (e_a) is derived from the saturated vapor pressure at air temperature and relative humidity. You can learn more about this by using our humidity converter.

Variables Table for Evaporation Calculation

Practical Examples of Evaporation Rate Calculation

Example 1: Small Garden Pond (Metric)

Imagine a small garden pond with the following conditions:

• Surface Area: 10 m²
• Water Temperature: 22 °C
• Air Temperature: 25 °C
• Relative Humidity: 70%
• Wind Speed: 1.5 m/s
• Water Body Type: Open Water
• Calculation Duration: 24 hours

Using the calculator with these inputs, the estimated instantaneous evaporation rate might be around 1.5 - 2.5 Liters/hour, leading to a total evaporation of 36 - 60 Liters over 24 hours. This demonstrates a moderate rate of water loss, which might require occasional refilling during dry periods.

Example 2: Swimming Pool (Imperial)

Consider a standard backyard swimming pool:

• Surface Area: 400 ft²
• Water Temperature: 80 °F
• Air Temperature: 85 °F
• Relative Humidity: 50%
• Wind Speed: 5 mph
• Water Body Type: Swimming Pool
• Calculation Duration: 7 days (168 hours)

With these parameters, the calculator would yield a higher instantaneous evaporation rate, potentially 0.5 - 1.0 Gallons/hour. Over a week, this could accumulate to a total evaporation of 84 - 168 Gallons. This significant water loss highlights why pool owners frequently need to top up their pools, especially in hot, dry, and windy conditions. The effect of changing wind speed on evaporation can be explored with a wind speed converter.

How to Use This Evaporation Rate Calculator

1. Select Unit System: Choose "Metric" or "Imperial" based on your preference. All input and output units will adjust automatically.
2. Enter Water Surface Area: Input the total area of the water body exposed to the atmosphere. Ensure units match your selected system (m² or ft²). If you need to calculate area, consider using a surface area calculator.
3. Input Water & Air Temperatures: Provide the average temperature of the water surface and the ambient air. Temperatures can be in Celsius or Fahrenheit. For conversions, see our temperature unit converter.
4. Specify Relative Humidity: Enter the percentage of relative humidity in the air. This value ranges from 0% (completely dry) to 100% (saturated).
5. Enter Wind Speed: Provide the average wind speed over the water surface.
6. Choose Water Body Type: Select the type of water body (Open Water, Swimming Pool, Industrial Pond) as this affects the empirical coefficients used in the calculation.
7. Set Calculation Duration: Define the period (in hours or days) for which you want to calculate the total evaporated volume.
8. Click "Calculate Evaporation": The results will appear below, showing the total evaporated volume, instantaneous rate, and intermediate vapor pressure values.
9. Interpret Results: The primary result shows the total volume evaporated over your specified duration. The instantaneous rate indicates how much water is evaporating per hour. The vapor pressure values help understand the driving force.
10. Use Reset: Click "Reset" to clear all inputs and return to default values.

Key Factors That Affect the Rate of Evaporation

The rate of evaporation is a complex process influenced by several interacting environmental and physical factors:

1. Water Surface Area: A larger exposed water surface area provides more molecules with the opportunity to escape into the atmosphere, directly increasing the total evaporation volume.
2. Water Temperature: Higher water temperatures mean water molecules have more kinetic energy, making it easier for them to break free from the liquid phase and become vapor. This significantly increases the saturated vapor pressure at the water surface.
3. Air Temperature: While not as direct as water temperature, warmer air can hold more moisture (higher saturated vapor pressure of air), but also contributes to a higher vapor pressure deficit if the water is warmer than the air.
4. Relative Humidity: Lower relative humidity in the air means the air is further from saturation, creating a larger vapor pressure deficit between the water surface and the air. This drives faster evaporation. Conversely, high humidity slows evaporation.
5. Wind Speed: Wind removes saturated air from directly above the water surface, replacing it with drier air. This maintains a high vapor pressure deficit, significantly increasing evaporation rates. Calm conditions lead to a build-up of humid air, slowing the process.
6. Atmospheric Pressure: Lower atmospheric pressure (e.g., at higher altitudes) reduces the resistance to water molecules escaping, thus increasing evaporation rates. This calculator uses standard atmospheric pressure assumptions.
7. Water Purity/Salinity: Dissolved solids (like salt in seawater or chemicals in industrial ponds) reduce the vapor pressure of water, slightly decreasing the evaporation rate compared to pure water.
8. Water Body Type/Agitation: Water bodies with agitation (like a swimming pool with jets or a turbulent river) expose more surface area to the air and can disrupt the humid layer, leading to higher evaporation rates than still bodies of water.

Frequently Asked Questions (FAQ) about Evaporation Rate