Evaporation Rate Calculator

Calculate Water Evaporation

Estimate the rate of water evaporation from a surface based on environmental factors.

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 period of time. This process, evaporation, is influenced by several environmental factors, and the calculator uses mathematical models to quantify the rate based on these factors. Our evaporation rate calculator specifically helps determine water loss from open surfaces like swimming pools, ponds, lakes, reservoirs, and even moist soil or industrial tanks.

Anyone managing water resources, from homeowners with pools to engineers managing reservoirs or farmers monitoring soil moisture, can benefit from using an evaporation rate calculator. It helps in understanding water balance, planning water replenishment, and designing systems where evaporation is a significant factor. For example, knowing the evaporation rate from a pool helps estimate how much water needs to be added regularly.

Common misconceptions about evaporation include thinking it only happens when water is hot (it happens at all temperatures above freezing, just faster when warmer) or that wind is the only major factor (temperature difference and humidity are equally, if not more, important). An evaporation rate calculator considers all these key elements.

Evaporation Rate Calculator Formula and Mathematical Explanation

The rate of evaporation from a water surface is primarily driven by the difference in water vapor pressure between the water surface and the overlying air, as well as the efficiency of vapor transport away from the surface (influenced by air velocity). While many complex formulas exist, our evaporation rate calculator uses a widely recognized empirical approach based on Dalton’s law, modified to include wind effects:

E = C * A * (e_s – e_a) * (1 + k * v)

Where:

• E is the total evaporation rate (e.g., kg/h or L/h).
• C is an empirical mass transfer coefficient that depends on various factors including atmospheric pressure and is adjusted for units (our calculator uses a value that results in mm/day or L/day with other inputs).
• A is the surface area (m²).
• e_s is the saturation vapor pressure at the water surface temperature (kPa).
• e_a is the actual vapor pressure of the air (kPa).
• k is a wind speed coefficient (we use 0.1).
• v is the air velocity (m/s) over the surface.

The saturation vapor pressure (e_s at water temp, and also used to find e_a at air temp) can be estimated using the Tetens equation or similar approximations:

e_s(T) = 0.61078 * exp((17.27 * T) / (T + 237.3)) (where T is in °C, e_s in kPa)

The actual vapor pressure of the air (e_a) is calculated as:

e_a = (Relative Humidity / 100) * e_s(Air Temperature)

The evaporation rate calculator combines these to first find the rate per unit area (e.g., mm/day) and then multiplies by the surface area for total volume loss.

Variables Used in the Evaporation Rate Calculator

Variable	Meaning	Unit	Typical Range in Calculator
A	Surface Area	m²	0.1 – 1,000,000+
Tair	Air Temperature	°C	-50 – 100
Twater	Water Temperature	°C	0 – 100
RH	Relative Humidity	%	0 – 100
v	Air Velocity	m/s	0 – 50
es	Saturation Vapor Pressure	kPa	0.6 – 101.3 (depending on T)
ea	Actual Vapor Pressure	kPa	0 – 101.3 (depending on T & RH)
Erate	Evaporation Rate per Area	mm/day	0 – 50+
Etotal	Total Evaporation Volume	L/day	Varies with area

Practical Examples (Real-World Use Cases)

Example 1: Swimming Pool Evaporation

Imagine a swimming pool with a surface area of 30 m². The air temperature is 30°C, the water temperature is 28°C, the relative humidity is 40%, and there’s a light breeze of 1.5 m/s.

Using the evaporation rate calculator with these inputs, we might find an evaporation rate of around 6-8 mm/day. This translates to a total volume loss of 180-240 Liters per day (30 m² * 6-8 mm/day). The pool owner would need to add this much water daily to maintain the level, not accounting for splash-out or rain.

Example 2: Small Reservoir Water Loss

Consider a small reservoir with a surface area of 10,000 m² (1 hectare). On a day with air temperature 20°C, water temperature 18°C, relative humidity 70%, and average wind speed of 2 m/s.

The evaporation rate calculator might estimate a rate of 2-3 mm/day. This means the reservoir could lose 20,000-30,000 Liters (20-30 cubic meters) of water per day due to evaporation. For water resource managers, this is a significant volume to factor into water balance studies, especially over a dry season.

How to Use This Evaporation Rate Calculator

1. Enter Surface Area: Input the area of the water surface exposed to the air in square meters (m²).
2. Input Air Temperature: Provide the average air temperature in degrees Celsius (°C) just above the water surface.
3. Input Water Temperature: Enter the temperature of the water at its surface in degrees Celsius (°C).
4. Set Relative Humidity: Input the average relative humidity of the air as a percentage (%).
5. Enter Air Velocity: Provide the average wind speed or air velocity across the water surface in meters per second (m/s).
6. View Results: The calculator automatically updates, showing the primary evaporation rate (in mm/day), total volume loss (Liters/day), and intermediate values like vapor pressures.
7. Analyze Chart and Table: Observe the chart to see how air velocity affects evaporation at different humidity levels, and the table for pre-calculated examples.
8. Reset or Copy: Use the “Reset” button to return to default values or “Copy Results” to save the output.

The results from the evaporation rate calculator help in estimating water make-up requirements, understanding water losses in reservoirs, or even in agricultural or industrial processes involving open water bodies.

Key Factors That Affect Evaporation Rate Results

• Water Temperature: Higher water temperature means more water molecules have enough energy to escape into the air, increasing saturation vapor pressure at the surface and thus the evaporation rate.
• Air Temperature: Higher air temperature allows the air to hold more moisture, but its main effect is through its influence on the actual vapor pressure when combined with relative humidity.
• Relative Humidity: Lower relative humidity means the air is drier and can absorb more water vapor, leading to a larger vapor pressure difference and higher evaporation. High humidity reduces evaporation. See our Relative Humidity Calculator.
• Air Velocity (Wind Speed): Higher wind speeds remove the moist air layer just above the water surface and replace it with drier air, maintaining a steeper vapor pressure gradient and increasing evaporation. The Wind Chill Calculator shows wind’s cooling effect, related to evaporative cooling too.
• Surface Area: A larger surface area naturally leads to a larger total volume of water evaporated, even if the rate per unit area (mm/day) is the same. Our Surface Area Calculator can help estimate this.
• Vapor Pressure Difference: This is the driving force for evaporation, the difference between the saturation vapor pressure at the water surface (e_s) and the actual vapor pressure in the air (e_a). The larger the difference, the faster the evaporation. Our Dew Point Calculator is related to vapor pressure.
• Atmospheric Pressure: Although not a direct input in this simplified calculator, lower atmospheric pressure (at higher altitudes) can slightly increase evaporation rates.
• Water Salinity/Impurities: Dissolved substances like salt can slightly reduce the vapor pressure of the water, thus decreasing the evaporation rate compared to pure water.

Understanding these factors helps in interpreting the results from the evaporation rate calculator and predicting changes in evaporation under different conditions.

Frequently Asked Questions (FAQ)

What is the most important factor affecting evaporation?

The vapor pressure difference between the water surface and the air is the primary driving force, which is determined by water temperature, air temperature, and relative humidity. Wind speed significantly enhances the rate once this difference exists.

How accurate is this evaporation rate calculator?

This evaporation rate calculator uses a common empirical formula. Accuracy depends on how well the input conditions represent the average over the period and the applicability of the empirical coefficient to the specific site. It provides a good estimate for many practical purposes.

Can I use this for liquids other than water?

No, this calculator is specifically designed for water evaporation. Other liquids have different vapor pressure characteristics and would require different formulas and coefficients.

How does evaporation affect water temperature?

Evaporation is a cooling process. As water evaporates, it takes energy (latent heat of vaporization) from the remaining water, which can lower the water temperature, especially if the evaporation rate is high.

Does the shape of the water body matter?

The surface area is the primary geometric factor. The shape might influence wind patterns over the surface, but the area is the direct input for total volume calculation in the evaporation rate calculator.

What is the difference between evaporation and transpiration?

Evaporation is the phase change from liquid to gas from open water or bare soil. Transpiration is the release of water vapor from plants. Evapotranspiration combines both.

How can I reduce evaporation from my pool or pond?

Using a cover (like a pool cover), reducing water temperature (if possible), or creating windbreaks can reduce evaporation. The evaporation rate calculator can help quantify the potential savings.

At what temperature does water evaporate?

Water evaporates at any temperature above freezing (0°C or 32°F), but the rate increases significantly with temperature.