Water Evaporation Calculator

Calculate evaporation rates based on temperature, humidity, wind speed, and surface area. Essential for hydrology, agriculture, and environmental science.

Evaporation Formula (Simplified Penman):

E = (Δ·(Rn - G) + γ·(6.43·(1 + 0.536·U)·(es - ea))) / (λ·(Δ + γ))

Where: E = Evaporation rate (mm/day), Δ = Slope of saturation vapor pressure curve, Rn = Net radiation, G = Soil heat flux, γ = Psychrometric constant, U = Wind speed, es = Saturation vapor pressure, ea = Actual vapor pressure, λ = Latent heat of vaporization

Average daily air temperature
-10°C 0°C 25°C 40°C 50°C
Relative humidity as a percentage (0-100%)
0% 25% 60% 85% 100%
Average wind speed at 2 meters height
Average hours of sunshine per day
Total surface area of the water body
Number of days to calculate total evaporation

Altitude above sea level affects atmospheric pressure
Water reflectivity coefficient (0-1)
If different from air temperature (optional)
Select evaporation calculation method
Summer Lake (25°C, 60% RH)
Desert Oasis (35°C, 30% RH)
Cool Pond (15°C, 80% RH)
Windy Reservoir (20°C, 70% RH, 5 m/s wind)
Tropical Swamp (30°C, 90% RH)
Calculating evaporation rates...

Understanding Water Evaporation

Evaporation is the process by which water changes from a liquid to a gas (water vapor). It's a key component of the hydrological cycle and is influenced by temperature, humidity, wind speed, solar radiation, and atmospheric pressure.

Penman-Monteith Equation (FAO Standard):

ET₀ = (0.408·Δ·(Rₙ - G) + γ·(900/(T+273))·u₂·(eₛ - eₐ)) / (Δ + γ·(1 + 0.34·u₂))

Where: ET₀ = Reference evapotranspiration (mm/day), Δ = Slope of saturation vapor pressure curve, Rₙ = Net radiation, G = Soil heat flux density, γ = Psychrometric constant, T = Air temperature, u₂ = Wind speed at 2m height, eₛ = Saturation vapor pressure, eₐ = Actual vapor pressure

Factors Affecting Evaporation

Temperature

Higher temperatures increase the kinetic energy of water molecules, accelerating evaporation.

Humidity

Lower relative humidity creates a larger vapor pressure gradient, increasing evaporation rate.

Wind Speed

Wind removes water vapor from the surface, maintaining the vapor pressure gradient.

Solar Radiation

Sunlight provides energy for the phase change from liquid to vapor.

Atmospheric Pressure

Lower pressure reduces the energy needed for water molecules to escape.

Water Quality

Salinity and impurities affect the vapor pressure and evaporation rate.

Evaporation Rates in Different Environments

Environment Type Typical Evaporation (mm/day) Key Factors Examples
Desert/arid 8-12 High temperature, low humidity Sahara, Atacama
Tropical 4-6 High temperature, high humidity Amazon, Congo basin
Temperate summer 3-5 Moderate temperature, variable humidity Mediterranean, Eastern US
Cool climate 1-3 Low temperature, moderate humidity Northern Europe, Canada
High altitude 4-7 Low pressure, high solar radiation Andes, Tibetan Plateau
Coastal 2-4 Moderate temperature, high humidity, wind Coastal regions

Applications of Evaporation Calculations

  • Agriculture: Irrigation scheduling and water management
  • Hydrology: Water balance studies and reservoir management
  • Climate Science: Climate modeling and climate change studies
  • Engineering: Cooling tower design and industrial water use
  • Environmental Science: Wetland management and ecosystem studies
  • Water Resources: Drinking water supply planning

Calculator Features:

  • Uses the simplified Penman equation for accurate evaporation estimation
  • Considers temperature, humidity, wind speed, solar radiation, and altitude
  • Provides multiple unit conversions for practical applications
  • Visualizes evaporation rates and the effects of different factors
  • Includes pre-set scenarios for common environmental conditions

Frequently Asked Questions

Evaporation refers specifically to water changing from liquid to vapor from open water surfaces, soil, or wet vegetation. Evapotranspiration includes both evaporation from surfaces and transpiration from plants. The calculator primarily focuses on open water evaporation.

The calculator uses the Penman-Monteith equation, which is the FAO standard method for evaporation estimation. It provides good accuracy for most practical applications but should be considered an estimate. For precise engineering or scientific work, site-specific measurements are recommended.

Wind removes water vapor from the immediate vicinity of the water surface, maintaining a steep vapor pressure gradient between the surface and the atmosphere. This prevents the air from becoming saturated with water vapor, allowing evaporation to continue at a higher rate.

At higher altitudes, atmospheric pressure is lower, which reduces the energy required for water molecules to escape into the air. However, temperature typically decreases with altitude, which counteracts this effect. The calculator accounts for both factors in its calculations.

This calculator is designed for fresh water. Salt water evaporates more slowly than fresh water due to the presence of dissolved salts, which lower the vapor pressure. For salt water, evaporation rates are typically 2-5% lower than for fresh water under the same conditions.