Evaporation Rate — Water Loss to Atmosphere
Evaporation transfers liquid water to vapor. The Penman equation combines energy balance and aerodynamic terms; Dalton's law relates evaporation to vapor pressure deficit. Rate depends on temperature, humidity, wind speed, and surface area.
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Penman equation combines energy and aerodynamic terms for accuracy Vapor pressure deficit (VPD) drives evaporation rate Wind increases evaporation by removing saturated air Pool evaporation typically 0.1–0.5 in. per day in summer
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Why: Evaporation affects pool maintenance, cooling tower efficiency, irrigation planning, and reservoir management. Higher temperature, lower humidity, and wind increase evaporation.
How: Enter surface area, air and water temperature, relative humidity, and wind speed. Choose Penman (most accurate), Dalton, or simplified method. Results include hourly, daily, weekly, and monthly evaporation.
Run the calculator when you are ready.
🏊 Swimming Pool Evaporation
Typical residential swimming pool - 30ft × 15ft, summer conditions
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🏭 Industrial Cooling Tower
Large industrial cooling tower - high evaporation rate
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🌊 Lake Surface Evaporation
Large lake surface - natural water body evaporation
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💧 Wet Floor Drying
Wet floor surface - indoor drying scenario
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🌾 Agricultural Irrigation Loss
Irrigation canal - agricultural water loss
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🐟 Garden Pond Evaporation
Small garden pond - typical backyard scenario
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Enter Evaporation Parameters
For educational and informational purposes only. Verify with a qualified professional.
🔬 Physics Facts
Penman equation (1948) remains the standard for evaporation estimation.
— ASHRAE
Evaporation doubles roughly every 10°C temperature increase.
— NOAA
Wind removes saturated air, increasing evaporation rate.
— USGS
Pool evaporation: typically 0.25–0.5 in. per day in hot, dry climates.
— EPA
📋 Key Takeaways
- • Evaporation rate depends on temperature, humidity, wind speed, and surface area—higher temps and lower humidity increase evaporation
- • The Penman equation is the most accurate method, combining energy balance and aerodynamic approaches
- • Vapor pressure deficit (saturation minus actual vapor pressure) drives evaporation—larger deficit means faster evaporation
- • Wind removes saturated air from surfaces, maintaining the vapor pressure gradient and sustaining evaporation
- • Applications span pools, cooling towers, lakes, agriculture, and industrial drying—each with different rate considerations
What is Evaporation Rate?
Evaporation rate is the rate at which liquid water transforms into water vapor and escapes into the atmosphere. It is a critical parameter in water management, climate studies, industrial processes, and environmental engineering. Understanding evaporation rates helps predict water loss from pools, lakes, reservoirs, cooling towers, and other water bodies.
The evaporation rate depends on several environmental factors:
- Surface Area: Larger surface areas result in higher total evaporation rates
- Temperature: Higher temperatures increase molecular motion and evaporation rate
- Humidity: Lower relative humidity creates a larger vapor pressure deficit, increasing evaporation
- Wind Speed: Wind removes saturated air from the surface, maintaining evaporation
- Atmospheric Pressure: Lower pressure can slightly increase evaporation rates
Evaporation Formulas Explained
Penman Equation
Where E is evaporation rate (kg/h), v is wind speed (m/s), A is surface area (m²), Xs is maximum humidity ratio, and X is current humidity ratio. This is the most accurate method for calculating evaporation rates.
Dalton's Law Approximation
Where Ps is saturation vapor pressure, Pa is actual vapor pressure, and f(v) is a wind factor. This method relates evaporation directly to vapor pressure deficit and wind effects.
Simplified Formula
A simplified empirical formula useful for quick estimates. Less accurate than Penman or Dalton methods but provides reasonable approximations for typical conditions.
Applications of Evaporation Rate Calculations
Swimming Pools
Calculate water loss from pools to determine refill requirements, estimate chemical usage, and plan maintenance schedules. Essential for pool operators and homeowners.
Industrial Cooling Towers
Design and operate cooling towers efficiently. Calculate make-up water requirements, optimize cooling performance, and manage water consumption in industrial processes.
Lakes and Reservoirs
Estimate water loss from natural and artificial water bodies. Critical for water resource management, irrigation planning, and environmental impact assessments.
Agriculture
Plan irrigation systems, estimate water requirements for crops, and manage water resources in agricultural operations. Essential for sustainable farming practices.
Drying Processes
Calculate drying times for wet surfaces, floors, and materials. Useful for construction, cleaning operations, and industrial drying processes.
Climate Studies
Understand water cycle dynamics, study regional climate patterns, and model hydrological processes. Important for meteorology and environmental science research.
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