Calculate Evaporation
The exposed area of the water body or surface.
Temperature of the water surface.
Ambient air temperature above the water surface.
The amount of moisture in the air, as a percentage.
Speed of wind over the water surface. Enter 0 for still air.
The period over which evaporation occurs.
What is Evaporation?
Evaporation is a fundamental physical process where a substance, most commonly water, changes from a liquid to a gaseous state (vapor) and disperses into the atmosphere. This transformation occurs without the need for the liquid to reach its boiling point, happening continuously at the surface of any exposed liquid body.
The evaporation rate is influenced by a complex interplay of factors, including temperature, humidity, wind speed, and the surface area of the liquid. Understanding and calculating evaporation is crucial in various fields:
- Agriculture: Farmers need to know water loss from irrigation ponds and soil to manage water resources efficiently.
- Hydrology: Essential for managing reservoirs, lakes, and rivers, predicting water levels, and assessing drought risks.
- Industrial Processes: Used in cooling towers, chemical processing, and wastewater treatment to manage water balance.
- Meteorology: A key component of the water cycle, affecting weather patterns and climate.
This evaporation calculator is designed for anyone needing to estimate water loss, from environmental scientists and engineers to homeowners managing ponds or pools. It helps demystify the process by allowing you to see the impact of different environmental conditions.
Evaporation Calculator Formula and Explanation
The evaporation calculation in this tool is based on a simplified model derived from Dalton's Law of Evaporation, which states that the rate of evaporation is proportional to the difference between the saturation vapor pressure at the water surface and the actual vapor pressure of the overlying air. This model is enhanced by incorporating the effects of wind speed and surface area.
The primary formula used is:
Emm/day = C × (es(water) - ea(air)) × (1 + 0.1 × U)
Where:
- Emm/day: Evaporation rate in millimeters per day.
- C: An empirical coefficient (approximately 0.18 for this calculator, assuming water and typical conditions, when vapor pressures are in kPa and wind speed in m/s). This coefficient accounts for various unmodeled factors.
- es(water): Saturation vapor pressure at the water surface temperature. This represents the maximum amount of water vapor the air can hold at that specific temperature. It's calculated using the Magnus-Tetens approximation.
- ea(air): Actual vapor pressure of the air. This is the partial pressure exerted by water vapor present in the air, calculated from the saturation vapor pressure at air temperature and the relative humidity.
- U: Wind speed over the water surface. Wind removes saturated air from above the surface, allowing more evaporation.
Once the daily evaporation rate (Emm/day) is determined, the total volume of water evaporated is calculated by multiplying this rate by the surface area and the duration:
Total Volume = Emm/day × Surface Area × Duration
Units are carefully converted internally to ensure consistency (e.g., mm to meters, days to hours) before final volume calculation and conversion back to user-selected display units like Liters or Gallons.
Variables Table
| Variable | Meaning | Unit (Metric/Imperial) | Typical Range |
|---|---|---|---|
| Surface Area | The exposed area of the liquid from which evaporation occurs. | m² / ft² | 1 to 10,000 m² (10 to 100,000 ft²) |
| Water Temperature | The temperature of the liquid surface. | °C / °F | 0 to 40 °C (32 to 104 °F) |
| Air Temperature | The ambient air temperature above the liquid. | °C / °F | -10 to 40 °C (14 to 104 °F) |
| Relative Humidity | The amount of water vapor in the air relative to the maximum it can hold at that temperature. | % | 0 to 100% |
| Wind Speed | The speed of air movement across the liquid surface. | m/s / mph | 0 to 15 m/s (0 to 33 mph) |
| Duration | The total time period over which evaporation is calculated. | days / hours | 1 to 30 days (24 to 720 hours) |
Practical Examples of Evaporation Calculation
Example 1: Small Pond in a Warm, Dry Climate (Metric)
Imagine a small decorative pond in a park during a summer week.
- Inputs:
- Surface Area: 50 m²
- Water Temperature: 28 °C
- Air Temperature: 32 °C
- Relative Humidity: 40 %
- Wind Speed: 3 m/s
- Duration: 7 days
- Calculation (Internal):
- Saturation Vapor Pressure (Water @ 28°C): ~3.78 kPa
- Saturation Vapor Pressure (Air @ 32°C): ~4.75 kPa
- Actual Vapor Pressure (Air): 4.75 kPa * 0.40 = ~1.90 kPa
- Evaporation Rate: 0.18 * (3.78 - 1.90) * (1 + 0.1 * 3) = 0.18 * 1.88 * 1.3 = ~0.44 mm/day
- Volumetric Rate: (0.44 / 1000) m/day * 50 m² = 0.022 m³/day = 22 Liters/day
- Total Evaporation: 22 Liters/day * 7 days = 154 Liters
- Results:
- Total Evaporation: 154.00 Liters
- Evaporation Rate: 0.44 mm/day
- Volumetric Rate: 22.00 Liters/day
This shows a significant amount of water loss for even a small pond over a week in warm, dry, windy conditions.
Example 2: Swimming Pool in a Mild, Humid Climate (Imperial)
Consider a backyard swimming pool over a weekend in a relatively humid region.
- Inputs:
- Surface Area: 300 ft²
- Water Temperature: 80 °F
- Air Temperature: 75 °F
- Relative Humidity: 75 %
- Wind Speed: 5 mph
- Duration: 2 days
- Calculation (Internal Metric Conversion then Calculation):
- Surface Area: 300 ft² ≈ 27.87 m²
- Water Temp: 80 °F ≈ 26.67 °C
- Air Temp: 75 °F ≈ 23.89 °C
- Wind Speed: 5 mph ≈ 2.24 m/s
- Saturation Vapor Pressure (Water @ 26.67°C): ~3.50 kPa
- Saturation Vapor Pressure (Air @ 23.89°C): ~2.97 kPa
- Actual Vapor Pressure (Air): 2.97 kPa * 0.75 = ~2.23 kPa
- Evaporation Rate: 0.18 * (3.50 - 2.23) * (1 + 0.1 * 2.24) = 0.18 * 1.27 * 1.224 = ~0.28 mm/day
- Volumetric Rate: (0.28 / 1000) m/day * 27.87 m² = 0.0078 m³/day ≈ 2.06 Gallons/day
- Total Evaporation: 2.06 Gallons/day * 2 days = 4.12 Gallons
- Results:
- Total Evaporation: 4.12 Gallons
- Evaporation Rate: 0.01 inches/day (approx.)
- Volumetric Rate: 2.06 Gallons/day
Even with higher humidity, a noticeable amount of water can evaporate from a pool, especially with some wind.
How to Use This Evaporation Calculator
Our evaporation calculator is designed for ease of use and accuracy. Follow these steps to get your evaporation estimates:
- Select Your Unit System: At the top of the calculator, choose between "Metric" (m², °C, m/s, Liters) or "Imperial" (ft², °F, mph, Gallons) based on your preference or available data. All input and output units will adjust accordingly.
- Enter Surface Area: Input the total exposed surface area of the water body. For a rectangular pool, this would be length × width.
- Enter Water Temperature: Provide the average temperature of the water surface. This is a critical factor as warmer water evaporates faster.
- Enter Air Temperature: Input the average ambient air temperature above the water.
- Enter Relative Humidity: Input the percentage of relative humidity. Lower humidity means drier air, which can absorb more moisture, thus increasing evaporation.
- Enter Wind Speed: Input the average wind speed over the water surface. Even light breezes significantly increase evaporation. Enter 0 for completely still air.
- Enter Duration: Specify the time period (in hours or days) for which you want to calculate the total evaporation.
- Calculate: Click the "Calculate Evaporation" button. The results will instantly appear below the input fields.
- Interpret Results:
- The Total Evaporation is the primary result, indicating the total volume of water lost over your specified duration.
- Evaporation Rate (mm/day or inches/day) shows the depth of water lost per day.
- Volumetric Rate (Liters/day or Gallons/day) shows the volume lost per day.
- Intermediate values like vapor pressures provide deeper insight into the atmospheric conditions driving evaporation.
- Copy Results: Use the "Copy Results" button to quickly save the calculated values and assumptions to your clipboard for documentation or sharing.
- Reset: Click "Reset" to clear all inputs and return to default values.
Key Factors That Affect Evaporation
Several environmental and physical factors significantly influence the rate of evaporation. Understanding these helps in predicting and managing water loss effectively:
- Water Temperature: This is arguably the most critical factor. As water temperature increases, the kinetic energy of water molecules rises, making it easier for them to escape as vapor. Higher water temperature leads to higher saturation vapor pressure at the surface, increasing the driving force for evaporation.
- Air Temperature: While water temperature is more direct, warmer air can hold more moisture (has a higher saturation vapor pressure capacity) and can also contribute to heating the water body, indirectly increasing evaporation.
- Relative Humidity: The amount of moisture already present in the air. If the air is very humid (high relative humidity), it is closer to saturation and has less capacity to absorb more water vapor, thus reducing the evaporation rate. Conversely, dry air (low humidity) readily absorbs water vapor.
- Wind Speed: Wind plays a crucial role by continuously removing the layer of moist, saturated air directly above the water surface. This allows drier air to come into contact with the liquid, maintaining a steep vapor pressure gradient and accelerating evaporation. Even a slight breeze can significantly increase water loss.
- Surface Area: A larger exposed surface area means more water molecules are directly at the liquid-air interface, available to transition into vapor. Consequently, a larger surface area will result in a higher total volume of evaporation over a given time, even if the evaporation rate per unit area remains constant.
- Atmospheric Pressure: Lower atmospheric pressure (e.g., at higher altitudes) reduces the resistance to water molecules escaping into the air, generally increasing evaporation rates. Our calculator assumes standard atmospheric pressure for simplicity.
- Water Purity/Salinity: Dissolved solids, like salt in seawater, reduce the vapor pressure of water, which slightly lowers the evaporation rate compared to pure water at the same temperature. This calculator assumes fresh water. For specific applications like saltwater pools or brines, adjustments may be needed.
Frequently Asked Questions (FAQ) about Evaporation
A: This calculator provides a good estimate based on a widely accepted empirical model (modified Dalton's Law). Its accuracy depends on the quality of your input data and the consistency of conditions over the duration. It's suitable for most general applications but may not match highly precise, site-specific measurements which might account for more complex variables or microclimates.
A: Water temperature determines the saturation vapor pressure at the water's surface, representing the maximum potential for water molecules to escape. Air temperature, combined with relative humidity, determines the actual vapor pressure of the air, indicating how much moisture the air already holds and its capacity to absorb more. The difference between these two vapor pressures is the primary driving force for evaporation.
A: If wind speed is zero, evaporation will still occur, but at a significantly reduced rate. Without wind to remove the saturated air layer above the water, evaporation becomes limited by diffusion, which is a much slower process. Our formula accounts for this, showing a lower evaporation rate when wind speed is zero.
A: This calculator is primarily calibrated for fresh water. Saltwater has a lower vapor pressure due to dissolved salts, which slightly reduces its evaporation rate compared to fresh water at the same temperature. For precise saltwater calculations, a specific salinity evaporation calculator would be more accurate.
A: High relative humidity means the air is already nearly saturated with moisture. This reduces the vapor pressure difference between the water surface and the air, thereby slowing down the evaporation process. Conversely, low relative humidity (dry air) increases the vapor pressure difference, leading to faster evaporation.
A: Evaporation rates vary widely based on climate and conditions. In arid, hot, and windy regions, rates can exceed 10 mm/day (0.4 inches/day). In cooler, humid, still conditions, rates might be less than 1 mm/day (0.04 inches/day). This calculator helps quantify these variations.
A: While the underlying principles are similar, this calculator is best suited for open-air, outdoor scenarios where wind, solar radiation (indirectly via temperatures), and ambient humidity are the primary drivers. Indoor environments with controlled ventilation, air conditioning, or dehumidifiers introduce additional complexities that this simplified model doesn't explicitly cover. For indoor pools, dedicated indoor pool evaporation calculators that consider air exchange rates are often used.
A: You can use either Metric or Imperial units. Simply select your preferred system from the dropdown menu at the top of the calculator. The input fields and result displays will automatically adjust to reflect your chosen units, and all internal calculations handle the conversions seamlessly.
A: No, this calculator focuses solely on water loss due to evaporation. It does not account for water gains from rainfall, manual refilling, or other sources. To get a net water balance, you would need to subtract the calculated evaporation from any water additions.
Related Tools and Internal Resources
Explore our other useful calculators and articles to further your understanding of environmental and engineering processes:
- Water Loss Calculator: Estimate general water loss from various systems.
- Reservoir Sizing Tool: Plan the optimal dimensions for water storage based on demand and environmental factors.
- Humidity Calculator: Understand and convert between different humidity measurements.
- Cooling Tower Efficiency Calculator: Evaluate the performance of industrial cooling systems.
- Dew Point Calculator: Determine the temperature at which air becomes saturated with water vapor.
- Vapor Pressure Calculator: Calculate the vapor pressure of various substances at different temperatures.