Environmental Science Calculators
Evaporation Rate Calculator Using Evap Pan Data
Accurately determine the rate of water evaporation from an evaporation pan and estimate the equivalent rate for a lake or reservoir. This tool for calculating evaporation rate using evap pan data is essential for water management, agriculture, and hydrology.
Estimated Lake Evaporation Rate (per Day)
Raw Water Level Change
Corrected Pan Evaporation (Epan)
Formula
What is Calculating Evaporation Rate Using Evap Pan Data?
Calculating the evaporation rate using evap pan data is a fundamental method in hydrology, agriculture, and meteorology to measure the amount of water lost to the atmosphere from a free water surface. The standard instrument for this is the **Class A Evaporation Pan**, a cylindrical container with specific dimensions. By measuring the change in water level over a set period (usually 24 hours), and accounting for any rainfall, we can determine the pan evaporation rate. This figure is crucial for managing water resources, scheduling irrigation for crops, and understanding local climate dynamics.
However, water evaporates at a different rate from a small, exposed metal pan than it does from a large, deep body of water like a lake or reservoir. The pan absorbs more solar radiation, and wind affects it differently. Therefore, the direct pan measurement is converted into a more realistic estimate for a larger water body using a **pan coefficient (Kp)**. This process of calculating evaporation rate using evap pan data provides a reliable and standardized way to estimate true water loss.
The Formulas for Calculating Evaporation Rate
The calculation is a two-step process. First, you determine the evaporation from the pan itself, correcting for any precipitation. Second, you apply the pan coefficient to estimate the evaporation from a nearby lake or reservoir.
1. Corrected Pan Evaporation (Epan)
This formula calculates the actual amount of water that evaporated from the pan.
Epan = (Winitial – Wfinal) + P
2. Estimated Lake Evaporation (Elake)
This formula uses the pan evaporation value to estimate the evaporation from a large water body.
Elake = Kp × Epan
For more detailed irrigation planning, you might explore our irrigation scheduling calculator.
| Variable | Meaning | Unit (auto-inferred) | Typical Range |
|---|---|---|---|
| Epan | Corrected Pan Evaporation | mm or inches | 1 – 15 per day |
| Elake | Estimated Lake Evaporation | mm or inches | 0.5 – 12 per day |
| Winitial | Initial Water Level in Pan | mm or inches | ~200 mm (for a Class A pan) |
| Wfinal | Final Water Level in Pan | mm or inches | Less than initial level (without rain) |
| P | Precipitation (Rainfall) | mm or inches | 0+ |
| Kp | Pan Coefficient | Unitless | 0.6 – 0.85 |
Practical Examples
Understanding the calculation with realistic numbers helps clarify the process.
Example 1: Hot, Dry Day
Imagine a warm, sunny day with no rainfall. The conditions are perfect for high evaporation.
- Inputs:
- Initial Water Level: 200 mm
- Final Water Level: 188 mm
- Rainfall: 0 mm
- Pan Coefficient: 0.75
- Calculation Steps:
- Pan Evaporation (Epan): (200 – 188) + 0 = 12 mm
- Lake Evaporation (Elake): 0.75 × 12 mm = 9 mm
- Result: The estimated evaporation from a nearby lake would be 9 mm for the day.
Example 2: Cool Day with Light Rain
On a cooler, more humid day, evaporation is lower, and we must account for rainfall.
- Inputs:
- Initial Water Level: 200 mm
- Final Water Level: 198 mm
- Rainfall: 3 mm
- Pan Coefficient: 0.65
- Calculation Steps:
- Pan Evaporation (Epan): (200 – 198) + 3 = 2 + 3 = 5 mm
- Lake Evaporation (Elake): 0.65 × 5 mm = 3.25 mm
- Result: Despite the water level only dropping by 2 mm, the actual pan evaporation was 5 mm. The estimated lake evaporation is 3.25 mm. This highlights why knowing the evapotranspiration vs evaporation distinction is important for total water loss.
How to Use This Evaporation Rate Calculator
This tool simplifies the process of calculating evaporation rate using evap pan data. Follow these steps for an accurate estimation:
- Select Your Unit System: Choose between “Metric (mm)” and “Imperial (inches)”. All input fields will update to reflect your choice.
- Enter Initial Water Level: Input the water level you measured at the start of your 24-hour period.
- Enter Final Water Level: Input the water level measured at the end of the period.
- Enter Rainfall: If it rained during the period, enter the total amount of rainfall measured in a separate rain gauge. If there was no rain, enter 0.
- Adjust Pan Coefficient (Optional): The calculator defaults to 0.70. You can adjust this value based on the specifics of your pan’s location (see factors below). Use our guide on the best way to set up a weather station for accurate data collection.
- Review the Results: The calculator automatically updates. The primary result is the “Estimated Lake Evaporation Rate,” which is the most practical value for large-scale water management. The intermediate values show the raw change and the corrected pan evaporation (Epan).
Key Factors That Affect Evaporation Rate
Several environmental factors influence the rate of evaporation. Understanding them helps in interpreting your results and adjusting the pan coefficient for lakes.
- Solar Radiation: The primary energy source for evaporation. More sunlight means higher water temperature and more evaporation.
- Temperature: Warmer air can hold more moisture, increasing the potential for evaporation.
- Humidity: High relative humidity in the air reduces the “moisture gradient” between the water and the air, slowing down evaporation. Dry air accelerates it.
- Wind Speed: Wind removes the layer of humid air just above the water’s surface, replacing it with drier air and thus increasing the evaporation rate.
- Pan Characteristics: The color, material, and cleanliness of the pan can affect how much heat it absorbs. A dirty or dark-colored pan will have a higher evaporation rate than a clean, reflective one.
- Surrounding Environment: A pan placed on dry, bare soil will experience more heat advection (transfer of heat through the air) than one placed over irrigated green grass, leading to higher evaporation rates. This is a key reason the **pan coefficient** is necessary.
Frequently Asked Questions (FAQ)
An evaporation pan is a small, metal object that heats up much faster and to a higher temperature than a large, deep lake. This extra energy causes water to evaporate more quickly from the pan. The pan coefficient is a scientifically determined factor used to scale down the pan measurement to a more realistic lake value.
The value typically ranges from 0.60 to 0.85. A common average for a Class A pan surrounded by a dry surface is around 0.70. For a pan surrounded by green crops (similar to a lake environment), the coefficient might be higher, around 0.80.
This calculator assumes a 24-hour period to provide a **daily evaporation rate calculation**. If your period is different, you would need to normalize the result. For example, if you measure over 48 hours, you would divide the final calculated evaporation by 2 to get the average daily rate.
Measurements are typically taken with a hook gauge or a stilling well inside the pan to minimize errors from wind-induced ripples on the water surface.
The pan coefficients are most accurately determined for a **what is a Class A evaporation pan**. If you use a different type of pan, the standard coefficients may not be accurate, and you might need to find a specific coefficient for your equipment.
You add rainfall because it refills the pan, masking the total amount of water that actually evaporated. For example, if the water level dropped by 2 mm but it rained 5 mm, the total evaporation was actually 7 mm (the 2 mm drop plus the 5 mm of rain that also evaporated).
Yes, you can use the **pan evaporation formula** as a good estimate for swimming pool water loss. The pan coefficient might be slightly different, but a value of 0.7-0.8 would provide a reasonable approximation. It’s much more accurate than just measuring the pool’s water level, which can be affected by splashing.
Evaporation is the loss of water from a free water surface. Evapotranspiration is the combined loss of water from evaporation AND transpiration (the release of water vapor from plants). This calculator only measures evaporation. To understand total water loss from a vegetated area, one must consider both. See our article for details: Understanding Evapotranspiration.