Water Evaporation Rate By Temperature Calculator

Evaporation Rate Calculator

Calculation Results

Temperature:

Surface Area:

Relative Humidity:

Wind Speed:

Duration:

Total Water Evaporated:

Evaporation Rate:

Formula Used (Simplified Penman-Monteith approach):

Evaporation Rate (E) ≈ 0.025 * (Vapor Pressure_s – Vapor Pressure_a) * (1 + 0.078 * Wind Speed)

Total Evaporation = E * Surface Area * Duration

Where: Vapor Pressure_s is saturated vapor pressure at air temp, Vapor Pressure_a is actual vapor pressure (based on RH).

Note: This is a simplified model for illustrative purposes. Actual evaporation can be influenced by many other factors.

Intermediate Values and Assumptions

Parameter	Value	Unit
Water Temperature		°C
Surface Area		m²
Relative Humidity		%
Wind Speed		m/s
Duration		h
Saturated Vapor Pressure (at given temp)		kPa
Actual Vapor Pressure (at given RH)		kPa
Vapor Pressure Deficit		kPa
Calculated Evaporation Rate		mm/h
Total Water Evaporated		Liters

What is Water Evaporation Rate by Temperature?

The water evaporation rate by temperature calculator is a tool designed to estimate how quickly water turns into vapor, with a primary focus on the influence of water temperature. Evaporation is a fundamental natural process where liquid water transforms into water vapor and enters the atmosphere. Temperature is one of the most significant drivers of this process. Higher temperatures provide more energy to water molecules, increasing their kinetic energy and making it easier for them to break free from the liquid surface and become a gas.

This calculator is useful for a variety of individuals and professionals, including:

• Farmers and irrigators estimating water loss from fields and reservoirs.
• Environmental scientists studying hydrological cycles and climate change impacts.
• Pool owners monitoring water loss.
• Aquaculture managers.
• Anyone interested in understanding the physical principles of evaporation in everyday scenarios.

A common misunderstanding is that temperature is the *only* factor. While crucial, other elements like surface area, relative humidity, wind speed, and even solar radiation play significant roles. This calculator incorporates these factors to provide a more comprehensive estimate, highlighting temperature's dominant effect.

Water Evaporation Rate by Temperature: Formula and Explanation

Estimating the exact water evaporation rate is complex, involving multiple atmospheric and surface variables. A widely accepted and robust method is the Penman-Monteith equation, which combines principles of energy balance and aerodynamic mass transfer. For simplicity and practical use in this calculator, we use a modified and simplified approach that captures the core relationships.

The Simplified Formula

The core idea is that evaporation is driven by the difference between the vapor pressure of the water (or air at the surface) and the vapor pressure of the surrounding air, influenced by wind speed.

1. Saturation Vapor Pressure ($P_s$): This is the maximum amount of water vapor the air can hold at a given temperature. It increases significantly with temperature.

2. Actual Vapor Pressure ($P_a$): This is the amount of water vapor actually present in the air, determined by the relative humidity (RH).

3. Vapor Pressure Deficit (VPD): The difference between saturation vapor pressure and actual vapor pressure ($VPD = P_s – P_a$). A higher VPD means the air is drier and can hold more moisture, increasing evaporation.

4. Evaporation Rate (E): A common approximation relates the rate to VPD and wind speed:

E ≈ C * VPD * (1 + K * Wind Speed)

Where:

• E is the evaporation rate (e.g., in mm/hour).
• C is a coefficient related to heat of vaporization and air density. A value around 0.025 is often used for standard conditions in calculations involving kPa and m/s, leading to mm/h.
• VPD is the Vapor Pressure Deficit (in kPa).
• Wind Speed is typically in m/s.
• K is a coefficient for wind, often around 0.078.

Total Evaporation is then calculated by multiplying the rate by the surface area and the duration.

Total Evaporation = E * Surface Area * Duration

Variables Table

Formula Variables and Units

Variable	Meaning	Unit	Typical Range
Temperature	Temperature of the water/air	°C	-10 to 50 °C
Surface Area	The exposed surface area of the water body	m²	0.1 to 1000+ m²
Relative Humidity	Ratio of actual water vapor to saturation water vapor at air temp	%	0% to 100%
Wind Speed	Speed of air movement over the water surface	m/s	0 to 15+ m/s
Duration	The time period over which evaporation is measured	h	1 to 720 h (30 days)
$P_s$ (Saturation Vapor Pressure)	Max vapor air can hold at a given temperature	kPa	0.61 to 6.10 kPa (for 0°C to 35°C)
$P_a$ (Actual Vapor Pressure)	Current vapor pressure in the air	kPa	0 to $P_s$
VPD (Vapor Pressure Deficit)	Difference between $P_s$ and $P_a$	kPa	0 to $P_s$
E (Evaporation Rate)	Rate of water turning into vapor	mm/h	0.1 to 10+ mm/h
Total Evaporation	Total volume of water evaporated	Liters	Varies greatly

Practical Examples

Let's illustrate with a couple of scenarios:

Example 1: A Warm, Humid Day

• Inputs:
• Water Temperature: 30°C
• Surface Area: 10 m²
• Relative Humidity: 70%
• Wind Speed: 1 m/s
• Duration: 12 hours

Calculation:

• $P_s$ at 30°C ≈ 4.24 kPa
• $P_a$ = $P_s$ * (RH/100) = 4.24 * (70/100) = 2.97 kPa
• VPD = 4.24 – 2.97 = 1.27 kPa
• E ≈ 0.025 * 1.27 * (1 + 0.078 * 1) ≈ 0.033 kPa * 1.078 ≈ 0.035 mm/h
• Total Evaporation = 0.035 mm/h * 10 m² * 12 h = 4.2 Liters

Result: Approximately 4.2 liters of water will evaporate over 12 hours.

Example 2: A Cooler, Windy Day

• Inputs:
• Water Temperature: 15°C
• Surface Area: 10 m²
• Relative Humidity: 50%
• Wind Speed: 5 m/s
• Duration: 12 hours

Calculation:

• $P_s$ at 15°C ≈ 1.70 kPa
• $P_a$ = $P_s$ * (RH/100) = 1.70 * (50/100) = 0.85 kPa
• VPD = 1.70 – 0.85 = 0.85 kPa
• E ≈ 0.025 * 0.85 * (1 + 0.078 * 5) ≈ 0.021 kPa * (1 + 0.39) ≈ 0.021 * 1.39 ≈ 0.029 mm/h
• Total Evaporation = 0.029 mm/h * 10 m² * 12 h = 3.5 Liters

Result: Approximately 3.5 liters of water will evaporate over 12 hours. Notice that despite the cooler temperature, the lower humidity and higher wind speed contribute significantly to evaporation.

How to Use This Water Evaporation Rate by Temperature Calculator

Using the calculator is straightforward:

1. Enter Water Temperature: Input the current temperature of the water in degrees Celsius (°C). This is a primary factor.
2. Specify Surface Area: Enter the surface area of the water body in square meters (m²). A larger area means more potential for evaporation.
3. Input Relative Humidity: Provide the current relative humidity of the air in percentage (%). Lower humidity leads to higher evaporation.
4. Enter Wind Speed: Input the wind speed in meters per second (m/s). Higher wind speeds increase evaporation by removing moist air from the surface.
5. Set Duration: Enter the time period in hours (h) for which you want to estimate evaporation.
6. Click 'Calculate': The calculator will process your inputs and display the total water evaporated and the evaporation rate.
7. Understand the Results: The output will show the estimated amount of water lost (in Liters) and the rate of evaporation (in mm/h). The intermediate values and formula used are also provided for clarity.
8. Use 'Reset': If you need to start over or change values, click 'Reset' to return to default settings.
9. Use 'Copy Results': Click this button to copy the calculated results, units, and key assumptions to your clipboard for easy sharing or documentation.

Selecting Correct Units: Ensure all inputs are in the specified units (Celsius, m², %, m/s, hours). The calculator is designed to work with these standard units.

Interpreting Results: The calculated evaporation is an estimate. Real-world conditions can vary, but this tool provides a good approximation based on the provided data and a simplified physical model.

Key Factors That Affect Water Evaporation Rate

While temperature is a major driver, several other factors significantly influence how quickly water evaporates:

1. Temperature (Water & Air): Higher temperatures increase the kinetic energy of water molecules, making them more likely to escape into the atmosphere. This is the most direct influence.
2. Surface Area: A larger exposed surface area of water directly correlates with a higher potential evaporation rate, as there are more molecules at the surface available to transition into vapor.
3. Relative Humidity: This measures how much water vapor is already in the air compared to the maximum it can hold at that temperature. High humidity means the air is already saturated, slowing down evaporation. Low humidity allows for much faster evaporation.
4. Wind Speed: Wind removes the layer of humid air that forms just above the water surface, replacing it with drier air. This maintains a steeper vapor pressure gradient, accelerating evaporation.
5. Solar Radiation: Direct sunlight adds energy to the water, increasing its temperature and thus promoting evaporation. This factor is implicitly considered when water temperature rises but can be a separate direct input in more complex models.
6. Atmospheric Pressure: Lower atmospheric pressure (e.g., at higher altitudes) slightly increases the evaporation rate because it's easier for molecules to escape the surface.
7. Water Salinity/Purity: Dissolved salts or other substances in water can slightly reduce the rate of evaporation compared to pure water because they interfere with the escape of water molecules.

FAQ: Water Evaporation Rate by Temperature

Q1: How does temperature directly impact evaporation?

A1: Higher temperatures provide water molecules with more energy, increasing their motion and the likelihood they will break free from the liquid surface and become vapor. It's the primary driver for increased evaporation.

Q2: Is this calculator for any type of water body?

A2: The calculator is generally applicable to open water bodies like ponds, lakes, swimming pools, or even a large pan of water. For very complex systems like oceans or glaciers, more specialized models are needed.

Q3: What units does the calculator use?

A3: The calculator uses Celsius (°C) for temperature, square meters (m²) for surface area, percentage (%) for relative humidity, meters per second (m/s) for wind speed, and hours (h) for duration. Results are in mm/h for rate and Liters for total volume.

Q4: Can I change the units for temperature (e.g., to Fahrenheit)?

A4: This version of the calculator specifically uses Celsius. To use Fahrenheit, you would need to convert your Fahrenheit temperature to Celsius before entering it into the calculator (Formula: °C = (°F – 32) * 5/9).

Q5: How accurate is the simplified formula used?

A5: The simplified formula provides a good approximation for many common scenarios. However, actual evaporation can be affected by factors not precisely modeled here (like detailed radiation, specific atmospheric conditions, or surface disturbances). For critical scientific or engineering applications, a full Penman-Monteith equation or on-site measurements might be necessary.

Q6: What does a negative vapor pressure deficit mean?

A6: A negative vapor pressure deficit is physically impossible in this context. It would imply actual vapor pressure exceeds saturation vapor pressure, which means condensation is occurring, not evaporation. The calculator ensures inputs result in a non-negative VPD.

Q7: How does wind speed affect evaporation rate?

A7: Wind increases evaporation by constantly replacing the layer of moist air above the water surface with drier air. This maintains a steeper gradient for water vapor to move from the water into the atmosphere.

Q8: Can this calculator predict evaporation for ice or snow?

A8: No, this calculator is specifically designed for liquid water. Evaporation from ice or snow (sublimation) follows different physical principles and requires different calculation methods.