Evaporation Rate Calculator
Estimated Evaporation Rate
What Is an Evaporation Rate Calculator?
An evaporation rate calculator is a tool that estimates how quickly water evaporates from a surface under specific weather conditions. It uses temperature, relative humidity, and wind speed to calculate the rate of water loss.
This tool solves a common problem: understanding how much water disappears into the air over time. It is widely used in hydrology, irrigation planning, reservoir management, and environmental studies. By predicting evaporation, users can manage water resources better and avoid shortages.
How the Evaporation Formula Works
This calculator uses Meyer’s empirical formula combined with vapor pressure calculations. The main equation is:
Where:
- E = evaporation rate (mm/day)
- C = coefficient based on water body type
- e_s = saturation vapor pressure
- e_a = actual vapor pressure
- u = wind speed (km/h)
The saturation vapor pressure is calculated using the Tetens equation:
The actual vapor pressure is:
Step-by-step example:
- Temperature = 25°C, humidity = 40%, wind = 15 km/h
- Calculate saturation vapor pressure using the formula
- Multiply by humidity to get actual vapor pressure
- Find vapor pressure deficit (difference between the two)
- Apply Meyer’s formula to get evaporation rate
This model assumes water temperature equals air temperature. It also limits inputs to realistic ranges, typically between -40°C and 60°C. If humidity reaches 100%, evaporation becomes zero because the air is fully saturated.
How to Use the Evaporation Rate Calculator: Step-by-Step
- Select the water body type (small/shallow or large/deep).
- Enter the air temperature in degrees Celsius.
- Input the relative humidity as a percentage.
- Enter the wind speed in km/h.
- Click the calculate button to get results.
The result shows daily evaporation in mm/day, along with weekly and monthly water loss per square meter. For example, 1 mm equals 1 liter per square meter. This helps you estimate total water loss over time and plan accordingly.
Real-World Use Cases and Insights
Water Resource Management
Reservoir managers use evaporation estimates to track water levels. In hot climates, evaporation can remove large volumes quickly, especially in shallow bodies.
Agriculture and Irrigation
Farmers rely on evaporation data to adjust irrigation schedules. High evaporation means crops need more frequent watering to stay healthy.
Common Mistakes to Avoid
- Ignoring wind speed, which greatly increases evaporation
- Using incorrect humidity values outside 0–100%
- Assuming results are exact instead of estimates
Keep in mind that solar radiation, salinity, and depth also affect evaporation but are not included in this model. This calculator provides a strong baseline estimate.
Frequently Asked Questions
What is evaporation rate?
Evaporation rate is the amount of water that turns into vapor over a given time, usually measured in mm/day. It depends on temperature, humidity, and wind.
How do I calculate evaporation rate manually?
You calculate it using Meyer’s formula with vapor pressure values. First find saturation pressure, then actual pressure, and apply the formula with wind speed.
Why does humidity reduce evaporation?
Humidity reduces evaporation because moist air cannot absorb much more water vapor. At 100% humidity, evaporation stops completely.
What is a good evaporation rate?
A good rate depends on context. Below 2 mm/day is low, 2–6 mm/day is moderate, and above 6 mm/day is high and may require water management.
Is evaporation the same as evapotranspiration?
No, evaporation is water loss from surfaces, while evapotranspiration includes both evaporation and plant transpiration.
Does wind speed affect evaporation?
Yes, higher wind speeds increase evaporation by moving moist air away and replacing it with drier air.