Evaporation Rate Calculator

Estimate the rate of water evaporation from an open surface using this evaporation rate calculator. Input the conditions below.

Understanding the Evaporation Rate Calculator

An evaporation rate calculator is a tool used to estimate the amount of water that transitions from a liquid to a gaseous state (vapor) from a given surface over a specific period. This process is influenced by various environmental factors.

What is Evaporation Rate?

Evaporation rate is the quantity of water evaporated per unit area per unit time, often expressed in millimeters per day (mm/day) or liters per square meter per day. It’s a crucial component of the water cycle and has significant implications for water resource management, agriculture, and even swimming pool maintenance. Our evaporation rate calculator helps quantify this.

Who should use it?

• Pool Owners: To estimate water loss and the need for refilling.
• Farmers and Agriculturists: To understand water loss from soil and irrigation systems.
• Reservoir and Lake Managers: To assess water balance and manage resources.
• Civil Engineers and Hydrologists: For water budget calculations and environmental impact studies.
• Industrial Facilities: Managing cooling towers and open water storage.

Common Misconceptions:

• Evaporation only happens when water is hot: Evaporation occurs at any temperature above freezing, although it is faster at higher temperatures.
• Humidity stops evaporation: High humidity slows evaporation but doesn’t completely stop it unless it’s 100%.
• Wind is the only factor: While important, wind is just one of several factors; the vapor pressure difference is the primary driver.

Evaporation Rate Formula and Mathematical Explanation

The evaporation rate calculator uses a simplified empirical formula based on the Dalton equation approach, commonly used for open water surfaces:

E (mm/day) = C * (e_{s_water} – e_{a_air})

Where C is a coefficient that includes the effect of wind. A common form is:

E (mm/day) = (0.25 + 0.086 * W_km/h) * (e_{s_water} – e_{a_air})

Here:

• E is the evaporation rate in mm/day.
• W_km/h is the wind speed in km/h at 2m height (converted from m/s input: m/s * 3.6).
• e_{s_water} is the saturation vapor pressure at the water surface temperature (in mbar).
• e_{a_air} is the actual vapor pressure of the air (in mbar).

The saturation vapor pressure (e_s) at a given temperature (T in °C) can be estimated using the Tetens equation:

e_s(T) = 6.108 * exp((17.27 * T) / (T + 237.3)) (in mbar)

So, e_{s_water} is calculated using the water temperature, and e_{s_air} using the air temperature. The actual vapor pressure of the air is then:

e_{a_air} = e_{s_air} * (Relative Humidity / 100)

Finally, the total volume of water evaporated (in liters per day) is:

Total Evaporation (liters/day) = E (mm/day) * Surface Area (m²) * (1 liter / 0.001 m³)

Since 1 mm/day over 1 m² is 0.001 m³/day or 1 liter/day, it simplifies to:

Total Evaporation (liters/day) = E (mm/day) * Surface Area (m²)

Variables Used in the Evaporation Rate Calculator

Variable	Meaning	Unit	Typical Range (Input)
Surface Area	Area of water exposed	m²	0.1 – 1,000,000+
Water Temp	Water surface temperature	°C	0 – 40
Air Temp	Air temperature above water	°C	-10 – 50
Humidity	Relative Humidity	%	10 – 100
Wind Speed	Wind speed at 2m	m/s	0 – 20
es_water	Saturation vapor pressure at water temp	mbar	Calculated
ea_air	Actual vapor pressure of air	mbar	Calculated
E	Evaporation rate	mm/day	Calculated
Total Evaporation	Total water loss	liters/day	Calculated

Table 1: Variables and typical ranges for the evaporation rate calculator.

Practical Examples (Real-World Use Cases)

Example 1: Swimming Pool Evaporation

A homeowner has a pool with a surface area of 30 m². On a summer day, the water temperature is 28°C, the air temperature is 30°C, relative humidity is 40%, and the wind speed is 1.5 m/s.

• Surface Area = 30 m²
• Water Temp = 28°C
• Air Temp = 30°C
• Humidity = 40%
• Wind Speed = 1.5 m/s (5.4 km/h)

Using the evaporation rate calculator, this would result in an evaporation rate of around 5.9 mm/day, leading to a total water loss of about 177 liters per day.

Example 2: Small Reservoir Evaporation

A small reservoir has a surface area of 10,000 m² (1 hectare). The average water temperature is 20°C, air temperature is 22°C, relative humidity is 60%, and average wind speed is 3 m/s.

• Surface Area = 10000 m²
• Water Temp = 20°C
• Air Temp = 22°C
• Humidity = 60%
• Wind Speed = 3 m/s (10.8 km/h)

The evaporation rate calculator would estimate an evaporation rate of about 4.0 mm/day, meaning a loss of 40,000 liters (40 cubic meters) per day from the reservoir.

How to Use This Evaporation Rate Calculator

1. Enter Surface Area: Input the area of the water surface in square meters (m²).
2. Enter Water Temperature: Input the temperature of the water at the surface in degrees Celsius (°C).
3. Enter Air Temperature: Input the air temperature above the water surface in degrees Celsius (°C).
4. Enter Relative Humidity: Input the relative humidity of the air as a percentage (%).
5. Enter Wind Speed: Input the wind speed measured at a height of 2 meters above the water surface in meters per second (m/s).
6. View Results: The calculator will automatically update and display the estimated total evaporation in liters per day, the evaporation rate in mm/day, and intermediate vapor pressures.
7. Reset: Click “Reset” to return to default values.
8. Copy: Click “Copy Results” to copy the inputs and results to your clipboard.

The results from the evaporation rate calculator can help you understand the magnitude of water loss and plan accordingly, whether it’s refilling a pool or managing water resources.

Key Factors That Affect Evaporation Rate Results

Several factors influence the rate of evaporation, which our evaporation rate calculator considers:

• Surface Area: Larger surface areas expose more water to the air, increasing total evaporation volume, though the rate per unit area (mm/day) remains the same if other conditions are uniform.
• Water Temperature: Warmer water has a higher saturation vapor pressure (e_{s_water}), increasing the vapor pressure difference between the water and the air, thus increasing the evaporation rate.
• Air Temperature: Air temperature influences the saturation vapor pressure of the air (e_{s_air}) and, consequently, the actual vapor pressure (e_{a_air}) when combined with humidity. Warmer air can hold more moisture.
• Relative Humidity: Higher relative humidity means the air is closer to saturation, reducing the vapor pressure difference (e_{s_water} – e_{a_air}) and slowing down evaporation. Lower humidity accelerates it.
• Wind Speed: Wind removes the layer of moist air just above the water surface, maintaining a steeper vapor pressure gradient and increasing the evaporation rate. Our evaporation rate calculator includes wind effects.
• Solar Radiation: Although not a direct input in this simplified formula, solar radiation is the primary source of energy that heats the water, thus indirectly affecting the water temperature and evaporation.
• Atmospheric Pressure: Lower atmospheric pressure (at higher altitudes) can slightly increase evaporation, but this effect is minor compared to temperature, humidity, and wind.
• Water Salinity/Impurities: Dissolved salts or other impurities can slightly reduce the vapor pressure of the water, thus decreasing the evaporation rate compared to pure water. This calculator assumes fresh water.

Frequently Asked Questions (FAQ)

Q1: What is the main unit for evaporation rate?

A1: The evaporation rate is typically measured in millimeters per day (mm/day), representing the depth of water that would evaporate from the surface over 24 hours.

Q2: How accurate is this evaporation rate calculator?

A2: This evaporation rate calculator uses a simplified empirical formula. It provides a good estimate for open water under average conditions but can vary from actual measured rates due to local factors, complex heat transfer, and the specific formula’s limitations.

Q3: Can I use this calculator for liquids other than water?

A3: No, the formula and constants used are specific to water. Other liquids have different vapor pressures and properties, requiring different formulas.

Q4: What happens if the wind speed is zero?

A4: Even with zero wind, some evaporation occurs due to natural convection and the vapor pressure difference, as reflected by the constant term (0.25) in the formula used by the evaporation rate calculator.

Q5: Why does my pool seem to lose more water than the calculator estimates?

A5: Pools can also lose water through splashing, leaks, or filter backwashing, in addition to evaporation. The evaporation rate calculator only estimates evaporative loss.

Q6: How does altitude affect evaporation?

A6: Higher altitudes have lower atmospheric pressure, which can slightly increase the evaporation rate, but this effect is often secondary to changes in temperature, humidity, and wind associated with altitude.

Q7: Can evaporation occur when water is covered?

A7: Yes, but at a much-reduced rate. A cover limits air exchange and reduces the impact of wind and solar radiation, significantly lowering evaporation. Our evaporation rate calculator is for open surfaces.

Q8: Is the evaporation rate constant throughout the day?

A8: No, it varies as temperature, humidity, and wind speed change throughout the day and night. The calculator gives an average rate based on the input conditions. For more accurate daily totals, one might need to average conditions or use more complex models.

Related Tools and Internal Resources

Explore other tools and resources that might be helpful:

• Humidity Calculator: Calculate relative humidity, dew point, or absolute humidity given other parameters.
• Wind Speed Converter: Convert between different units of wind speed (m/s, km/h, knots, mph).
• Temperature Converter: Convert temperatures between Celsius, Fahrenheit, and Kelvin.
• Water Conservation Tips: Learn how to save water at home and in the garden.
• Pool Maintenance Guide: Tips for maintaining your swimming pool, including managing water levels.
• Reservoir Management Strategies: Information on managing water resources in reservoirs.