Calculate Rate Of Evaporation

Understand and quantify how quickly water evaporates from a surface.

What is the Rate of Evaporation?

The rate of evaporation is a measure of how quickly a liquid, typically water, turns into a gas (vapor) and disperses into the surrounding atmosphere. It's a fundamental process in hydrology, agriculture, environmental science, and many industrial applications. Understanding this rate helps in managing water resources, predicting weather patterns, designing irrigation systems, and optimizing processes involving open water bodies or wet surfaces.

This rate is not constant; it's influenced by a complex interplay of environmental factors. Misinterpreting these factors, especially regarding units, can lead to significant inaccuracies in estimations. For instance, confusing Celsius with Fahrenheit or square meters with square feet can drastically alter the calculated evaporation rate.

Anyone involved in water management, from a gardener managing a small plot to an engineer designing a large reservoir, can benefit from accurately calculating or estimating the rate of evaporation.

Rate of Evaporation Formula and Explanation

A common empirical formula to estimate the rate of evaporation (E) can be approximated by:

E ≈ A * (Vp – Vap) * (1 + 0.001 * W)

Where:

Variables and Units

Variable	Meaning	Unit (Calculated)	Typical Range
E	Rate of Evaporation	(Volume/Time) e.g., mm/day, inches/day	0.1 – 20+ mm/day
A	Surface Area	(Area) e.g., m², ft²	Varies widely
Vp	Saturation Vapor Pressure at Water Surface Temperature	hPa (hectopascals) or similar pressure units	10 – 60 hPa (approx. 10°C to 35°C)
Vap	Actual Vapor Pressure of the Air	hPa (hectopascals) or similar pressure units	Varies with temp & humidity
W	Wind Speed	m/s or mph	0.1 – 10+ m/s

*Note: The formula used in the calculator is a simplified empirical model. More complex models like Penman-Monteith consider factors like net radiation and are more accurate but require more data. This calculator provides an estimate based on common, easily measurable parameters. The output rate is normalized to a standard time unit (e.g., mm per day) for comparison.*

Practical Examples of Rate of Evaporation

Example 1: Small Pond in Summer

Inputs:

• Surface Area: 50 m²
• Surface Area Unit: m²
• Air Temperature: 30°C
• Air Temperature Unit: °C
• Relative Humidity: 60%
• Wind Speed: 3 m/s
• Wind Speed Unit: m/s
• Time Period: 1 day
• Time Period Unit: days

Calculation: The calculator will process these inputs. A higher temperature, moderate humidity, and steady wind will lead to a noticeable evaporation rate.
Estimated Results: The calculator might show a total evaporation of approximately 5-8 mm over the day for this pond area, translating to a rate of 5-8 mm/day.

Example 2: Large Reservoir with Strong Winds

Inputs:

• Surface Area: 1,000,000 m²
• Surface Area Unit: m²
• Air Temperature: 20°F
• Air Temperature Unit: °F
• Relative Humidity: 70%
• Wind Speed: 15 mph
• Wind Speed Unit: mph
• Time Period: 24 hours
• Time Period Unit: hours

Calculation: The lower temperature will reduce the potential for evaporation, but the large surface area and significant wind speed are strong drivers.
Estimated Results: Even with colder air, the substantial wind action over a vast area could result in a total evaporation of perhaps 8-12 mm over the 24-hour period, with a calculated rate around 8-12 mm/day. This highlights how wind can be a dominant factor.

How to Use This Rate of Evaporation Calculator

1. Enter Surface Area: Input the size of the water body or wet surface. Select the correct unit (m² or ft²).
2. Set Air Temperature: Provide the air temperature. Choose the appropriate unit (°C or °F). Note that the calculator internally converts temperature to Kelvin for some intermediate calculations.
3. Input Relative Humidity: Enter the percentage of water vapor in the air (0-100%). Lower humidity means a higher evaporation rate.
4. Specify Wind Speed: Input the wind speed. Select the correct unit (m/s or mph). Wind generally increases evaporation by removing moist air from the surface.
5. Define Time Period: Enter the duration for which you want to calculate evaporation. Choose between hours or days.
6. Click 'Calculate': The tool will process your inputs using an empirical formula.
7. Interpret Results: You'll see the estimated total evaporation for the period and the calculated average daily rate. Intermediate values like saturation vapor pressure and actual vapor pressure provide insight into the driving force for evaporation.
8. Use Reset: Click 'Reset' to clear all fields and return to default values.
9. Copy Results: Use 'Copy Results' to save the key findings.

Choosing the correct units is crucial for accurate results. Ensure your inputs consistently reflect the selected units. The calculator aims to simplify this by offering common unit choices.

Key Factors That Affect Rate of Evaporation

• Temperature: Higher temperatures increase the energy available for water molecules to escape into the air, thus increasing evaporation. The calculator uses air temperature as a proxy for surface temperature conditions.
• Relative Humidity: This indicates how much water vapor is already in the air. High humidity means the air is already "full" of water vapor, slowing down further evaporation. Low humidity "pulls" moisture from the surface more effectively.
• 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.
• Surface Area: A larger surface area exposed to the air allows for more water molecules to evaporate simultaneously. This is why reservoirs lose more water than small puddles over the same period.
• Water Properties: While not directly used in this simplified calculator, factors like water salinity, purity, and surface tension can slightly influence evaporation rates. Saltier water tends to evaporate slightly slower.
• Solar Radiation: Direct sunlight provides significant energy that directly heats the water, further increasing the rate of evaporation. This calculator infers some of this effect through air temperature but doesn't explicitly model radiation.

FAQ about Rate of Evaporation

What is the standard unit for evaporation rate?

Commonly, evaporation rates are expressed in depth per unit of time, such as millimeters per day (mm/day) or inches per day (in/day). This refers to the equivalent depth of water that would have evaporated from a free water surface.

Does temperature have the biggest impact on evaporation?

Temperature is a primary driver, as it provides the energy for phase change. However, under certain conditions, wind speed or humidity can become equally or more dominant factors, especially over large areas. For example, high winds can drastically increase evaporation even in cool, humid conditions.

How does humidity affect evaporation?

Relative humidity quantifies the amount of water vapor the air is currently holding compared to its maximum capacity at that temperature. If humidity is high (e.g., 90%), the air is nearly saturated, and evaporation slows down significantly. If humidity is low (e.g., 20%), the air can readily accept more water vapor, and evaporation proceeds quickly.

Why does wind increase evaporation?

Wind continuously sweeps away the layer of moist air that forms directly above the evaporating surface. This maintains a steeper concentration gradient for water vapor between the surface and the air, driving more rapid evaporation.

Can this calculator estimate evaporation from soil?

This calculator is primarily designed for open water surfaces. Evaporation from soil is more complex, influenced by soil type, moisture content below the surface, and surface cover (like mulch or vegetation). While the principles are similar, specific soil evaporation models are needed for accurate soil moisture loss predictions.

What is saturation vapor pressure?

Saturation vapor pressure is the maximum partial pressure that water vapor can exert at a given temperature. It represents the pressure of water vapor in air that is fully saturated – meaning it cannot hold any more water vapor at that temperature. Warmer air can hold more water vapor, so saturation vapor pressure increases with temperature.

What is actual vapor pressure?

Actual vapor pressure is the partial pressure of water vapor actually present in the air at a given time and temperature. It is always less than or equal to the saturation vapor pressure. The difference between saturation vapor pressure (at the water surface temperature) and the actual vapor pressure (of the air) is the primary driving force for evaporation.

How accurate are these empirical formulas?

Empirical formulas provide useful estimates based on observed relationships between variables. Their accuracy depends on how well the specific formula matches the conditions and the quality of the input data. For highly critical applications, more sophisticated physically-based models (like Penman-Monteith) and site-specific measurements might be required.