How Do You Calculate the Evaporation Rate of Water

Water Evaporation Rate Calculator

Results

Evaporation Rate (kg/m²/hr): 0

Total Water Loss (Liters/hr): 0

Total Daily Water Loss: 0 Liters

Note: 1 kg of water is approximately 1 Liter.

function calculateEvaporationRate() { var Tw = parseFloat(document.getElementById('waterTemp').value); var Ta = parseFloat(document.getElementById('airTemp').value); var Rh = parseFloat(document.getElementById('humidity').value); var v = parseFloat(document.getElementById('windSpeed').value); var A = parseFloat(document.getElementById('surfaceArea').value); if (isNaN(Tw) || isNaN(Ta) || isNaN(Rh) || isNaN(v) || isNaN(A)) { alert("Please enter valid numerical values."); return; } // Vapor Pressure Function (Magnus-Tetens Approximation) function getVaporPressure(temp) { return 6.112 * Math.exp((17.67 * temp) / (temp + 243.5)); // Result in hPa } // Saturation vapor pressure at water temperature var Pw = getVaporPressure(Tw); // Saturation vapor pressure at air temperature var Psat_air = getVaporPressure(Ta); // Actual vapor pressure in air var Pa = Psat_air * (Rh / 100); // Empirical formula (Rohwer / Meyer variant for pools) // E = (0.0175 + 0.0045 * v) * (Pw – Pa) in kg/m2/h (using hPa) // We multiply (Pw – Pa) by 0.75 if using mmHg, but with hPa we use coefficients: // A commonly accepted metric coefficient for open water: var evapRatePerMeter = (0.0175 + 0.0045 * v) * (Pw – Pa); // Ensure rate isn't negative (condensation happens but usually not what user is calculating) if (evapRatePerMeter < 0) evapRatePerMeter = 0; var totalHr = evapRatePerMeter * A; var totalDay = totalHr * 24; document.getElementById('ratePerMeter').innerText = evapRatePerMeter.toFixed(4); document.getElementById('totalLitersHr').innerText = totalHr.toFixed(2); document.getElementById('totalLitersDay').innerText = totalDay.toFixed(2) + " Liters"; document.getElementById('evapResult').style.display = 'block'; }

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Understanding Water Evaporation Rate Calculation

Calculating the evaporation rate of water is essential for managing swimming pools, cooling towers, and agricultural reservoirs. Evaporation is a phase transition process where liquid water turns into vapor. This process occurs below the boiling point and is driven by the difference in vapor pressure between the water surface and the surrounding air.

The Physics Behind the Calculation

The rate of evaporation depends on several thermodynamic and environmental variables:

• Water Temperature: Higher water temperatures increase the kinetic energy of molecules, allowing more to escape the surface. This raises the Saturation Vapor Pressure at the surface.
• Air Temperature and Humidity: Relative humidity measures how much moisture the air already holds relative to its maximum capacity. High humidity reduces the evaporation rate because the air is closer to saturation.
• Wind Speed: Wind removes the "boundary layer" of saturated air just above the water surface, replacing it with drier air, which significantly accelerates evaporation.
• Surface Area: Evaporation is a surface phenomenon. A wider surface area increases the total volume of water lost per hour.

The Formula Used

The calculator uses a standard empirical formula derived from the Dalton Law of Partial Pressures:

E = (0.0175 + 0.0045 × v) × (Pw – Pa)

Where:

• E = Evaporation rate (kg/m²/hr)
• v = Wind speed (m/s) at the surface
• Pw = Saturation vapor pressure at water temperature
• Pa = Actual vapor pressure of the air

Practical Example

Imagine you have a swimming pool with a surface area of 32 m². The water is 25°C, the air is 28°C with 50% humidity, and there is a light breeze of 2 m/s.

1. Determine Vapor Pressures: At 25°C, Pw is approx 31.67 hPa. At 28°C, saturation air pressure is 37.8 hPa. At 50% humidity, Pa = 18.9 hPa.
2. Apply Wind Factor: (0.0175 + 0.0045 × 2) = 0.0265.
3. Calculate Difference: (31.67 – 18.9) = 12.77 hPa.
4. Calculate Rate: 0.0265 × 12.77 ≈ 0.338 kg/m²/hr.
5. Total Daily Loss: 0.338 × 32 m² × 24 hours ≈ 259 Liters per day.

By understanding these calculations, pool owners can better predict water bills and chemical balance adjustments, while engineers can design more efficient thermal systems.