Evaporation Rate Calculator
Water Evaporation Rate Calculator
Estimate the rate of water evaporation from a surface based on environmental factors.
Results:
Saturation Vapor Pressure at Water Temp (es): 0.00 kPa
Actual Vapor Pressure in Air (ea): 0.00 kPa
Evaporation Rate per m²: 0.0 mm/day (L/m²/day)
Formula used (simplified): E = (0.5 + 0.54 * u) * (es – ea) * 10, where E is evaporation rate in mm/day per m², u is wind speed (m/s), es and ea are saturation and actual vapor pressures (kPa). Total Liters/day = E * Area.
Evaporation Rate Analysis
Chart: Estimated Total Evaporation vs. Wind Speed at current and lower humidity.
Evaporation Rate at Different Wind Speeds
| Wind Speed (m/s) | Evaporation Rate (mm/day/m²) | Total Evaporation (L/day) |
|---|---|---|
| 0 | … | … |
| 1 | … | … |
| 2 | … | … |
| 3 | … | … |
| 4 | … | … |
Table: How evaporation rate changes with wind speed, keeping other factors constant.
Understanding the Evaporation Rate Calculator
This article delves into the evaporation rate, its calculation, influencing factors, and how to use our evaporation rate calculator.
What is Evaporation Rate?
The evaporation rate is the quantity of water that is converted from a liquid to a vapor from a given surface area over a specific period. It’s a crucial parameter in hydrology, agriculture, environmental science, and even for managing swimming pools or reservoirs. Understanding the evaporation rate helps in water resource management, irrigation scheduling, and predicting water losses.
This evaporation rate calculator is designed for anyone needing to estimate water loss from an open water surface, such as lake managers, farmers, pool owners, and environmental engineers. It uses environmental data to provide an estimate of the evaporation rate.
Common misconceptions include thinking evaporation only happens when water boils (it happens at all temperatures) or that it’s solely dependent on temperature (wind and humidity are also very significant).
Evaporation Rate Formula and Mathematical Explanation
Several empirical and theoretical formulas exist to estimate the evaporation rate. Our calculator uses a simplified formula based on the mass transfer principle, similar in form to many engineering approximations for open water evaporation:
E = K * (es – ea)
Where:
- E is the evaporation rate (e.g., in mm/day or kg/m²/day).
- K is a mass transfer coefficient, often dependent on wind speed (u). In our calculator, we use K ≈ (0.5 + 0.54 * u) * 10 when es and ea are in kPa to get E in mm/day. The factor of 10 adjusts for units (kPa to mb, as the 0.5 and 0.54 are often cited with mb).
- es is the saturation vapor pressure at the water surface temperature (Tw).
- ea is the actual vapor pressure of the air above the water.
The saturation vapor pressure (es and esa at air temperature Ta) can be estimated using equations like the Tetens equation:
es(T) = 0.6108 * exp((17.27 * T) / (T + 237.3)) (in kPa, where T is in °C)
The actual vapor pressure (ea) is calculated from the relative humidity (RH) and the saturation vapor pressure at air temperature (esa):
ea = (RH / 100) * esa(Ta)
The total evaporation in Liters per day is then E (in mm/day) multiplied by the surface area A (in m²), as 1 mm over 1 m² is 1 liter.
Variables Table
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| A | Surface Area | m² | 0.1 – 1,000,000+ |
| Tw | Water Temperature | °C | 0 – 40 |
| Ta | Air Temperature | °C | 0 – 50 |
| RH | Relative Humidity | % | 10 – 100 |
| u | Wind Speed | m/s | 0 – 15 |
| es | Saturation Vapor Pressure at Tw | kPa | 0.6 – 7.4 |
| ea | Actual Vapor Pressure at Ta, RH | kPa | 0.1 – 7.4 |
| E | Evaporation Rate per m² | mm/day | 0 – 20+ |
Practical Examples (Real-World Use Cases)
Example 1: Swimming Pool Evaporation
A homeowner has a pool with a surface area of 30 m². On a summer day, the water temperature is 26°C, the air temperature is 30°C, relative humidity is 50%, and the average wind speed is 1.5 m/s.
- A = 30 m²
- Tw = 26 °C
- Ta = 30 °C
- RH = 50 %
- u = 1.5 m/s
Using the evaporation rate calculator with these inputs might show an evaporation rate of around 5-7 mm/day per m², resulting in a total loss of 150-210 liters per day from the pool. This helps the owner understand water top-up needs.
Example 2: Small Pond Evaporation
A farmer has a small irrigation pond with a surface area of 500 m². In a dry period, water temp is 22°C, air temp 28°C, RH 40%, and wind speed 3 m/s.
- A = 500 m²
- Tw = 22 °C
- Ta = 28 °C
- RH = 40 %
- u = 3 m/s
The evaporation rate calculator might estimate a higher rate per m² due to lower humidity and higher wind, perhaps 8-10 mm/day per m², leading to a total loss of 4000-5000 liters (4-5 cubic meters) per day. This is critical for water budgeting for irrigation.
How to Use This Evaporation Rate Calculator
- Enter Surface Area (A): Input the area of the water surface in square meters (m²).
- Enter Water Temperature (Tw): Input the temperature of the water at the surface in Celsius (°C).
- Enter Air Temperature (Ta): Input the air temperature above the water in Celsius (°C).
- Enter Relative Humidity (RH): Input the relative humidity of the air as a percentage (%).
- Enter Wind Speed (u): Input the average wind speed over the water surface in meters per second (m/s).
- View Results: The calculator automatically updates the “Total Evaporation” (primary result in Liters/day), and intermediate values like vapor pressures and rate per m².
- Analyze Chart and Table: The chart and table show how evaporation changes with wind speed, providing more insight.
The results help in understanding the magnitude of water loss and making informed decisions regarding water conservation or replenishment.
Key Factors That Affect Evaporation Rate Results
- Water Temperature (Tw): Higher water temperature increases the saturation vapor pressure at the surface (es), increasing the driving force (es – ea) for evaporation.
- Air Temperature (Ta) & Relative Humidity (RH): These together determine the actual vapor pressure of the air (ea). Higher air temperature can hold more moisture, but higher RH means the air is closer to saturation, reducing the (es – ea) difference and thus the evaporation rate. Lower RH increases evaporation.
- Wind Speed (u): Higher wind speed enhances the transport of water vapor away from the surface, maintaining a larger vapor pressure difference and increasing the evaporation rate.
- Surface Area (A): A larger surface area directly leads to a greater total volume of water evaporated, even if the rate per unit area (mm/day) remains the same.
- Solar Radiation: While not a direct input in this simplified calculator, solar radiation significantly heats the water, increasing Tw, and thus indirectly increasing the evaporation rate.
- Water Salinity/Purity: Dissolved substances like salts can slightly reduce the saturation vapor pressure of the water, thus reducing the evaporation rate compared to pure water at the same temperature. This calculator assumes fresh water.
Understanding these factors helps in predicting changes in the evaporation rate under different conditions. For more precise calculations, one might refer to our Penman-Monteith calculator if available, or consult specialized resources.
Frequently Asked Questions (FAQ)
- What is the most significant factor affecting the evaporation rate?
- The vapor pressure difference (es – ea) is the primary driving force, which is influenced by water temperature, air temperature, and humidity. Wind speed is also very significant in accelerating the process.
- How accurate is this evaporation rate calculator?
- This calculator uses a simplified empirical formula. Accuracy depends on how well the formula and inputs represent the specific conditions. For highly accurate measurements, direct methods (like pan evaporation adjusted by coefficients) or more complex models (e.g., Penman-Monteith) are used, which require more data like net radiation.
- Can I use this for liquids other than water?
- No, this evaporation rate calculator is specifically calibrated and designed for water. Other liquids have different vapor pressure characteristics.
- Does the depth of the water affect the evaporation rate?
- The depth itself doesn’t directly affect the rate per unit area, but it influences how quickly the water temperature changes, which in turn affects the evaporation rate.
- Why does evaporation happen even when it’s cold?
- Evaporation occurs as long as the vapor pressure at the water surface is greater than that in the air, and the water isn’t frozen. It’s slower at lower temperatures but still happens.
- How can I reduce evaporation from my pool?
- Using a pool cover is the most effective way. Windbreaks and reducing water temperature can also help reduce the evaporation rate. See our guide on pool maintenance.
- Is evaporation faster indoors or outdoors?
- It depends. Outdoors often has more wind and direct sun (increasing water temp), but indoors might have lower humidity from air conditioning, increasing the evaporation rate. Check our indoor humidity guide.
- What units does the calculator use?
- The inputs are in m², °C, %, and m/s. The results are in kPa, mm/day, and Liters/day.
Related Tools and Internal Resources
- Relative Humidity Calculator: Understand and calculate relative humidity based on temperature and dew point.
- Water Volume Calculator: Calculate the volume of water in various shapes, useful for pools and ponds.
- Heat Index Calculator: See how temperature and humidity combine to affect perceived heat, related to air’s moisture content.
- Advanced Evaporation Models: Learn about more complex models like Penman-Monteith for precise evaporation estimates.