Water Evaporation Rate Calculator
Estimate the volume of water lost due to evaporation from open surfaces like pools, ponds, or tanks.
Evaporation Rate: 0 Liters per hour
Daily Water Loss: 0 Liters per day
Vertical Depth Loss: 0 mm per day
How to Calculate the Water Evaporation Rate
The rate of evaporation from a water surface is influenced by a complex interaction between the water temperature, the surrounding air temperature, humidity levels, and air movement (wind speed). To calculate this accurately, we use an empirical formula derived from the Penman equation.
The Evaporation Formula
The calculator uses the mass transfer method to determine the evaporation rate (E):
E = (0.0175 + 0.022v) × (Pw – Pa)
- E: Evaporation rate in kg/(m²·h). Since 1kg of water is approximately 1 Liter, this gives us Liters per square meter per hour.
- v: Wind speed in meters per second (m/s) above the water surface.
- Pw: Saturation vapor pressure at the water temperature (expressed in hPa).
- Pa: Actual vapor pressure of the air (expressed in hPa), calculated using the air temperature and relative humidity.
Key Factors Affecting Evaporation
- Temperature: Warmer water molecules have higher kinetic energy, making it easier for them to escape the surface as vapor.
- Humidity: High humidity means the air is already saturated with moisture, significantly slowing down the evaporation process.
- Wind Speed: Wind removes the saturated "boundary layer" of air immediately above the water surface, replacing it with drier air and accelerating evaporation.
- Surface Area: A larger surface area provides more space for molecules to escape, increasing the total volume of water lost.
Example Calculation
Suppose you have a swimming pool with a surface area of 32m²:
- Water Temp: 24°C
- Air Temp: 28°C
- Humidity: 45%
- Wind Speed: 1.5 m/s
At 24°C, the saturation vapor pressure (Pw) is approx 29.8 hPa. At 28°C and 45% humidity, the air vapor pressure (Pa) is approx 17.0 hPa. Using the formula, the evaporation loss would be roughly 0.65 Liters per square meter per hour, totaling about 20.8 Liters per hour for the whole pool.
Practical Tips to Reduce Water Loss
If you find your evaporation rate is high, consider using a pool cover or solar blanket. This creates a physical barrier that prevents vapor from escaping and blocks the wind, reducing water loss by up to 90%. Additionally, planting windbreaks like hedges or fences around your water feature can lower the wind speed across the surface.