Evaporation Rate Calculator

What is an Evaporation Rate Calculator?

An evaporation rate calculator is a tool used to estimate the amount of water that transforms from a liquid to a gaseous state (vapor) from a given surface over a specific period, typically a day. This process is crucial in various fields, including hydrology, agriculture, meteorology, and environmental science. The evaporation rate calculator considers factors like water surface area, air and water temperature, relative humidity, and wind speed to provide an estimate.

Anyone managing water resources, such as farmers planning irrigation, reservoir operators, swimming pool owners, or scientists studying water cycles, would find an evaporation rate calculator useful. It helps in understanding water loss and planning accordingly.

Common misconceptions include thinking evaporation only happens when water is hot (it happens at all temperatures above freezing, just slower when colder) or that it’s solely dependent on temperature (humidity and wind play huge roles).

Evaporation Rate Calculator Formula and Mathematical Explanation

The rate of evaporation from a free water surface can be estimated using various empirical and semi-empirical formulas. A common approach involves the difference in vapor pressure between the water surface and the overlying air, modified by wind speed. One simplified form, related to the Penman and Meyer equations, can be expressed as:

E = C * (e_s – e_a) * (1 + k * U)

Where:

E is the evaporation rate (e.g., in mm/day).
e_s is the saturation vapor pressure at the water surface temperature (in kPa).
e_a is the actual vapor pressure of the air (in kPa).
U is the wind speed at a certain height above the surface (e.g., in m/s).
C and k are empirical coefficients. For our evaporation rate calculator, we use a simplified form: E ≈ (e_{s_water} – e_a) * (0.5 + 0.54 * U) for mm/day with U in m/s.

The saturation vapor pressure (e_s) can be calculated using the Tetens equation (or a similar formula):

e_s(T) = 0.6108 * exp(17.27 * T / (T + 237.3))

where T is the temperature in degrees Celsius.

The actual vapor pressure (e_a) is calculated from the relative humidity (RH) and the saturation vapor pressure at air temperature:

e_a = (RH / 100) * e_s(T_air)

Our evaporation rate calculator uses these relationships to estimate the evaporation rate in mm/day and then multiplies by the surface area to get the total volume lost in Liters/day (since 1mm over 1m² is 1 Liter).

Variables Table

Variable	Meaning	Unit	Typical Range
A	Water Surface Area	m²	0.1 – 1,000,000+
T_air	Air Temperature	°C	-10 – 50
T_water	Water Surface Temperature	°C	0 – 40
RH	Relative Humidity	%	10 – 100
U	Wind Speed	m/s	0 – 20
e_s	Saturation Vapor Pressure	kPa	0.6 – 7.4 (for 0-40°C)
e_a	Actual Vapor Pressure	kPa	0.1 – 7.4
E	Evaporation Rate	mm/day or Liters/day/m²	0 – 20+

Variables used in the evaporation rate calculation.

Practical Examples (Real-World Use Cases)

Example 1: Swimming Pool Evaporation

A homeowner has a swimming pool with a surface area of 30 m². The air temperature is 30°C, water temperature is 28°C, relative humidity is 50%, and the average wind speed is 1.5 m/s.

Surface Area: 30 m²
Air Temperature: 30°C
Water Temperature: 28°C
Relative Humidity: 50%
Wind Speed: 1.5 m/s

Using the evaporation rate calculator, the estimated evaporation would be around 6-7 mm/day, leading to a total water loss of approximately 180-210 Liters per day from the pool surface.

Example 2: Small Reservoir Water Loss

A small farm reservoir has a surface area of 5000 m². On a hot day, air temperature is 35°C, water surface temperature is 30°C, relative humidity is 40%, and wind speed is 3 m/s.

Surface Area: 5000 m²
Air Temperature: 35°C
Water Temperature: 30°C
Relative Humidity: 40%
Wind Speed: 3 m/s

The evaporation rate calculator might estimate an evaporation rate of about 10-12 mm/day under these conditions, resulting in a substantial water loss of 50,000-60,000 Liters (50-60 cubic meters) per day.

How to Use This Evaporation Rate Calculator

Enter Surface Area: Input the total area of the water surface exposed to the air in square meters (m²).
Input Air Temperature: Enter the average air temperature in degrees Celsius (°C).
Input Water Temperature: Enter the temperature of the water at its surface in degrees Celsius (°C).
Enter Relative Humidity: Input the relative humidity of the air as a percentage (%).
Enter Wind Speed: Input the average wind speed in meters per second (m/s) at a standard height (usually 2m or 10m, the formula is more sensitive to wind speed measured closer to the surface if coefficients aren’t adjusted).
Read the Results: The evaporation rate calculator automatically updates to show the estimated total evaporation in Liters per day, the rate in mm per day, and intermediate vapor pressure values.
Analyze the Chart: The chart shows how the total evaporation changes with varying wind speeds, keeping other factors constant, giving you an idea of the wind’s impact.

Understanding the results can help in water management decisions, such as how often to refill a pool, the water loss from a reservoir, or irrigation needs. The evaporation rate calculator provides valuable data for such planning.

Key Factors That Affect Evaporation Rate Results

Water Surface Area: Larger areas expose more water to the air, leading to higher total evaporation, though the rate per unit area (mm/day) remains the same if other conditions are uniform.
Temperature (Air and Water): Higher temperatures increase the saturation vapor pressure, especially of the water surface. This larger difference between water surface and air vapor pressure drives evaporation.
Relative Humidity: Lower relative humidity means the air is drier and can hold more water vapor, increasing the vapor pressure gradient and thus the evaporation rate.
Wind Speed: Higher wind speeds remove the moist air layer just above the water surface, replacing it with drier air, maintaining a steeper vapor pressure gradient and increasing evaporation.
Solar Radiation: While not directly in this simplified formula, solar radiation is the primary energy source that heats the water, influencing water temperature and thus saturation vapor pressure. A more complex evaporation rate calculator like Penman-Monteith includes this.
Atmospheric Pressure: Lower atmospheric pressure (at higher altitudes) can slightly increase evaporation, but it’s often a secondary effect compared to the others.
Water Quality (Salinity): Dissolved salts reduce the vapor pressure of water slightly, so saline water evaporates a bit slower than fresh water under the same conditions. Our basic evaporation rate calculator assumes fresh water.

Frequently Asked Questions (FAQ)

1. How accurate is this evaporation rate calculator?

This evaporation rate calculator uses a simplified formula. Its accuracy is good for estimations, but real-world evaporation can be influenced by many other factors not included, like direct solar radiation input, heat advection, and the exact nature of the surface. More complex models like Penman-Monteith offer higher accuracy but require more data.

2. Why is water temperature important?

Water temperature directly determines the saturation vapor pressure at the water surface. Warmer water has a higher saturation vapor pressure, creating a larger difference with the actual vapor pressure of the air, which is the main driving force for evaporation.

3. How does wind affect evaporation?

Wind removes the layer of air near the water surface that has become saturated with water vapor. This maintains a lower vapor pressure in the air immediately above the water, sustaining the gradient and enhancing evaporation.

4. Can this calculator be used for surfaces other than water?

This specific evaporation rate calculator is designed for open water surfaces. Evaporation from soil (evapotranspiration if plants are present) is more complex and depends on soil moisture content and plant characteristics.

5. What is the difference between evaporation and evapotranspiration?

Evaporation is the process of water turning into vapor from open water or bare soil. Evapotranspiration includes evaporation plus transpiration, which is the release of water vapor from plants.

6. How can I reduce evaporation from my pool or pond?

Using a pool cover, windbreaks, or floating liquid evaporation reducers can significantly decrease water loss calculated by an evaporation rate calculator.

7. Does the depth of the water body affect the evaporation rate?

The depth itself doesn’t directly affect the rate per unit area, but it influences the water temperature due to thermal inertia. Deeper water bodies change temperature more slowly.

8. What are the units used in the calculator?

Surface area is in square meters (m²), temperatures in Celsius (°C), relative humidity in percent (%), wind speed in meters per second (m/s), and the results are in millimeters per day (mm/day) and Liters per day (Liters/day).

Related Tools and Internal Resources

Water Volume Calculator: Calculate the volume of water in various containers or bodies.
Humidity Calculator: Understand and convert between different humidity measures.
Wind Chill Calculator: See how wind speed affects perceived temperature.
Dew Point Calculator: Calculate the dew point temperature based on air temperature and humidity.
Heat Index Calculator: Understand the combined effect of heat and humidity.
Weather Data Analysis Tools: Explore tools for analyzing weather patterns, relevant to evaporation studies.