Evaporation Rate Calculator

Calculate Evaporation Rate

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

What is an Evaporation Rate Calculator?

An evaporation rate calculator is a tool used to estimate the amount of water that evaporates from a surface (like a lake, reservoir, swimming pool, or even moist soil) over a certain period. The rate of evaporation is influenced by several environmental factors, including the surface area of the water, the temperature of the water and the air, the relative humidity of the air, and the wind speed across the surface. Our evaporation rate calculator uses these inputs to provide an estimate.

This kind of calculator is useful for water resource management, agriculture (to estimate water loss from irrigation), swimming pool maintenance (to predict water top-up needs), and environmental studies. By understanding the evaporation rate, one can better manage water resources and predict water losses. The evaporation rate calculator helps quantify this complex process.

Who Should Use It?

• Water resource managers overseeing reservoirs and lakes.
• Farmers and agricultural consultants managing irrigation.
• Swimming pool owners and maintenance professionals.
• Environmental scientists studying water cycles.
• Civil engineers designing water storage facilities.

Common Misconceptions

A common misconception is that evaporation is solely dependent on temperature. While temperature is a major factor, humidity and wind speed play equally crucial roles. A hot but very humid day might see less evaporation than a cooler, dry, and windy day. Another is that the evaporation rate calculator provides an exact figure; it provides an estimate based on empirical formulas and average conditions.

Evaporation Rate Calculator Formula and Explanation

The evaporation rate calculator often employs empirical formulas derived from observations and physical principles. A common approach, like the one simplified in our calculator, relates evaporation to the difference in vapor pressure between the water surface and the overlying air, modified by wind speed.

The simplified formula used is:

E = C * (es_water - ea) * (1 + b * U)

Where:

• E = Evaporation rate (e.g., in mm/day)
• C = An empirical coefficient (we use around 0.42 for mm/day with wind in m/s and pressure in kPa)
• es_water = Saturation vapor pressure at the water surface temperature (kPa)
• ea = Actual vapor pressure of the air (kPa)
• b = Wind function coefficient (we use 0.5)
• U = Wind speed at a specified height (e.g., 2m) (m/s)

The saturation vapor pressure (es) can be estimated using the Tetens equation (or similar):

es(T) = 0.6108 * exp((17.27 * T) / (T + 237.3)) (T in °C, e_s in kPa)

The actual vapor pressure (ea) is calculated from the relative humidity (RH):

ea = (RH / 100) * es(Tair)

The evaporation rate calculator combines these to find E.

Variables Table

Variable	Meaning	Unit	Typical Range
A	Water Surface Area	m²	0.1 – 1,000,000+
Tair	Air Temperature	°C	0 – 50
Twater	Water Surface Temperature	°C	0 – 50
RH	Relative Humidity	%	0 – 100
U	Wind Speed	m/s	0 – 15
es	Saturation Vapor Pressure	kPa	0.6 – 12.3
ea	Actual Vapor Pressure	kPa	0 – 12.3
E	Evaporation Rate	mm/day or kg/m²/day	0 – 20+

Variables used in the evaporation rate calculation.

Practical Examples

Example 1: Swimming Pool in Summer

A swimming pool has a surface area of 50 m². On a summer day, the air temperature is 30°C, water temperature is 26°C, relative humidity is 50%, and wind speed is 1.5 m/s.

• Surface Area: 50 m²
• Air Temp: 30°C
• Water Temp: 26°C
• Humidity: 50%
• Wind Speed: 1.5 m/s

Using the evaporation rate calculator with these inputs, we find e_{s_water} ≈ 3.36 kPa, e_{s_air} ≈ 4.24 kPa, e_a ≈ 2.12 kPa. The estimated evaporation rate would be around 4-6 mm/day, meaning about 200-300 liters lost per day from the pool.

Example 2: Small Reservoir in Cooler, Drier Conditions

A small reservoir with a surface area of 10,000 m². The air temperature is 15°C, water temperature is 14°C, relative humidity is 40%, and wind speed is 3 m/s.

• Surface Area: 10,000 m²
• Air Temp: 15°C
• Water Temp: 14°C
• Humidity: 40%
• Wind Speed: 3 m/s

Here, e_{s_water} ≈ 1.60 kPa, e_{s_air} ≈ 1.70 kPa, e_a ≈ 0.68 kPa. The evaporation rate calculator would predict a rate around 2-3 mm/day, leading to a substantial water loss of 20,000-30,000 liters (20-30 cubic meters) per day over the reservoir surface.

How to Use This Evaporation Rate Calculator

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

The results help in understanding the magnitude of water loss and planning for replenishment or water conservation measures. Consider these as estimates, as real-world conditions can vary.

Key Factors That Affect Evaporation Rate Results

1. Water Surface Temperature: Warmer water has a higher saturation vapor pressure, increasing the driving force (e_{s_water} – e_a) for evaporation.
2. Air Temperature and Relative Humidity: These determine the actual vapor pressure of the air (e_a). Lower humidity or higher air temperature (if water is warmer) increases the vapor pressure deficit and thus evaporation. Our humidity calculator can help understand this better.
3. Wind Speed: Higher wind speeds remove the moist air layer just above the water surface, maintaining a steeper vapor pressure gradient and increasing evaporation. See our wind speed converter.
4. Surface Area: The total volume of water evaporated is directly proportional to the surface area exposed. Our surface area calculator might be useful.
5. Solar Radiation: Although not directly in this simplified formula, solar radiation is the primary energy source that heats the water, thus influencing water temperature and evaporation.
6. Atmospheric Pressure: Lower atmospheric pressure (at higher altitudes) can slightly increase evaporation rates, though this is often a secondary effect compared to the others.
7. Water Quality: The presence of solutes (like salt) can slightly reduce the saturation vapor pressure of the water, thus reducing evaporation compared to pure water.

Using an evaporation rate calculator requires understanding these influencing factors.

Frequently Asked Questions (FAQ)

How accurate is this evaporation rate calculator?

This evaporation rate calculator uses a simplified empirical formula and provides an estimate. Actual evaporation can be influenced by many factors not explicitly included, like solar radiation variations, precise wind profile, and water body shape. For very accurate measurements, pan evaporation data or more complex models like Penman-Monteith are used.

Can I use this for any liquid?

This calculator is specifically designed for water evaporation. The constants and formulas are based on the properties of water. Other liquids have different vapor pressures and properties.

What is the difference between mm/day and kg/m²/day?

For water, 1 mm of evaporation depth over 1 m² of area corresponds to 1 liter of water, which has a mass of approximately 1 kg. So, an evaporation rate of 1 mm/day is equivalent to a mass loss of 1 kg/m²/day.

Why does wind increase evaporation?

Wind removes the layer of air near the water surface that has become saturated with water vapor, replacing it with drier air. This maintains a larger difference in vapor pressure between the water and the air, driving more evaporation.

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 and its daily/seasonal fluctuations, which in turn affect the evaporation rate calculated by the evaporation rate calculator.

How can I reduce evaporation from my pool or pond?

Using a pool cover, windbreaks, or floating covers can significantly reduce evaporation by limiting surface exposure and wind effects. We have more water conservation tips.

Is the formula used universally applicable?

The type of formula used here is common for open water surfaces but coefficients can vary based on location and specific conditions. More complex formulas like Penman-Monteith are used for vegetated surfaces or when energy balance is explicitly considered.

What if my air and water temperatures are very different?

The difference between water surface temperature and air temperature (along with humidity) dictates the vapor pressure gradient, a key driver. Our evaporation rate calculator accounts for this.

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