Rate of Evaporation of Water Calculator

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Water Evaporation Rate Calculator

Estimate the mass of water evaporating from a surface over time.

Calculation Results

Rate per Hour: 0 kg/h
Total Water Lost: 0 kg
Volume Equivalent: 0 Liters
Evaporation Depth: 0 mm

How the Evaporation Rate is Calculated

The rate of evaporation of water is a complex physical process influenced by temperature, humidity, wind, and surface area. This calculator utilizes a simplified version of the mass transfer formula (similar to the Carrier equation) used in engineering to estimate water loss from pools, tanks, and open containers.

The Core Formula:

The evaporation rate (E) is generally expressed as:
E = (a + b × v) × A × (Pw - Pa)

  • E: Evaporation rate (kg/h).
  • v: Wind velocity above the water surface (m/s).
  • A: Surface area of the water (m²).
  • Pw: Saturation vapor pressure at the water temperature (mmHg).
  • Pa: Actual vapor pressure of the ambient air (mmHg).
  • a & b: Constants (0.0175 and 0.02 respectively for standard applications).

Key Factors Affecting Evaporation

1. Water Temperature: Higher temperatures increase the kinetic energy of water molecules, allowing them to escape the surface more easily, increasing vapor pressure (Pw).

2. Air Humidity: The "Actual Vapor Pressure" (Pa) is determined by air temperature and relative humidity. High humidity reduces the "vapor pressure deficit," slowing down evaporation.

3. Wind Speed: Wind removes the saturated layer of air immediately above the water surface and replaces it with drier air, significantly accelerating the process.

4. Surface Area: Evaporation is a surface phenomenon. Doubling the surface area directly doubles the amount of water lost.

Example Calculation

Imagine a swimming pool with a surface area of 32m². If the water is 26°C, the air is 22°C with 40% humidity, and a light breeze of 2 m/s is blowing:

  • Saturation Pressure at 26°C (Pw) ≈ 25.2 mmHg
  • Saturation Pressure at 22°C ≈ 19.8 mmHg
  • Actual Air Pressure (Pa) = 19.8 × 0.40 = 7.92 mmHg
  • Evaporation Coefficient = 0.0175 + (0.02 × 2) = 0.0575
  • Rate = 0.0575 × 32 × (25.2 – 7.92) ≈ 31.8 kg per hour.

Over 10 hours, this pool would lose approximately 318 kg (or 318 liters) of water.

function calculateEvaporation() { // Get Input Values var Tw = parseFloat(document.getElementById('waterTemp').value); var Ta = parseFloat(document.getElementById('airTemp').value); var rh = parseFloat(document.getElementById('relHumidity').value) / 100; var v = parseFloat(document.getElementById('windSpeed').value); var A = parseFloat(document.getElementById('surfaceArea').value); var hours = parseFloat(document.getElementById('duration').value); // Validate if (isNaN(Tw) || isNaN(Ta) || isNaN(rh) || isNaN(v) || isNaN(A) || isNaN(hours)) { alert("Please enter valid numeric values for all fields."); return; } // Calculate Vapor Pressures using Magnus-Tetens Approximation (in hPa) // Formula: P = 6.112 * exp((17.67 * T) / (T + 243.5)) function getVaporPressure(temp) { var hPa = 6.112 * Math.exp((17.67 * temp) / (temp + 243.5)); return hPa * 0.750062; // Convert hPa to mmHg } var Pw = getVaporPressure(Tw); var Psat_air = getVaporPressure(Ta); var Pa = Psat_air * rh; // Mass Transfer Coefficient (k) // Using empirical constants for indoor/sheltered water (kg/h per m2 per mmHg) var a = 0.0175; var b = 0.02; var k = a + (b * v); // Evaporation Rate E (kg/h) var rate = k * A * (Pw – Pa); // If rate is negative (condensation), set to 0 if (rate < 0) rate = 0; var totalMass = rate * hours; var totalVolume = totalMass; // 1kg water approx 1 Liter var depthLoss = (totalVolume / A); // Liters per m2 is mm // Display Results document.getElementById('evapResult').style.display = 'block'; document.getElementById('ratePerHour').innerText = rate.toFixed(3) + " kg/h"; document.getElementById('totalMass').innerText = totalMass.toFixed(2) + " kg"; document.getElementById('totalVolume').innerText = totalVolume.toFixed(2) + " Liters"; document.getElementById('evapDepth').innerText = depthLoss.toFixed(2) + " mm"; // Smooth scroll to result document.getElementById('evapResult').scrollIntoView({ behavior: 'smooth', block: 'nearest' }); }

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