Estimate the amount of water lost due to evaporation based on environmental conditions.
Evaporation Rate per Square Meter:0.00 kg/m²/h
Total Water Loss (Hourly):0.00 Liters/h
Total Water Loss (Daily):0.00 Liters/day
Reduction in Water Level (Daily):0.00 mm/day
How to Calculate the Evaporation Rate of Water
Calculating the evaporation rate is critical for managing swimming pools, cooling ponds, and industrial water storage. Evaporation is a phase transition 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 Evaporation Formula
This calculator uses a mass-transfer formula often applied to swimming pools and open water surfaces. The formula used is:
E = (0.0887 + 0.0781 × v) × (Pw – Pa)
Where:
E: Evaporation rate (kg/m²/h).
v: Wind velocity above the water surface (m/s).
Pw: Saturation vapor pressure at the water temperature (mbar).
Pa: Actual vapor pressure of the air (mbar), calculated using air temperature and relative humidity.
Key Factors Affecting Evaporation
Four primary environmental factors determine how quickly water will disappear from your container or pool:
Water Temperature: Higher temperatures give water molecules more kinetic energy, making it easier for them to escape into the air.
Air Humidity: If the air is already saturated with moisture (high humidity), there is less "room" for new water vapor molecules, slowing evaporation.
Wind Speed: Wind carries away the saturated air directly above the water surface, replacing it with drier air and significantly accelerating the process.
Surface Area: Evaporation is a surface phenomenon. A shallow, wide pond will lose water much faster than a deep, narrow tank containing the same volume.
Example Calculation
Suppose you have a swimming pool with a surface area of 32 m². The water temperature is 25°C, the air temperature is 30°C, the relative humidity is 50%, and there is a light breeze of 2 m/s.
The saturation pressure of water at 25°C is approximately 31.7 mbar.
The saturation pressure of air at 30°C is approximately 42.4 mbar. At 50% humidity, the actual vapor pressure (Pa) is 21.2 mbar.
The pressure difference (Pw – Pa) is 10.5 mbar.
Using the wind factor (0.0887 + 0.0781 * 2) = 0.2449.
Evaporation Rate = 0.2449 * 10.5 = 2.57 kg/m²/h.
For the whole pool: 2.57 * 32 = 82.24 Liters per hour.
function calculateEvapRate() {
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 area = parseFloat(document.getElementById('surfaceArea').value);
if (isNaN(Tw) || isNaN(Ta) || isNaN(Rh) || isNaN(v) || isNaN(area)) {
alert("Please enter valid numeric values for all fields.");
return;
}
// Function to calculate saturation vapor pressure in mbar (hPa) using Magnus-Tetens formula
function getVaporPressure(temp) {
return 6.112 * Math.exp((17.67 * temp) / (temp + 243.5));
}
var Pw = getVaporPressure(Tw);
var PsatAir = getVaporPressure(Ta);
var Pa = PsatAir * (Rh / 100);
// Evaporation rate in kg/m2/h
// Standard empirical formula for pools/open water
var deltaP = Pw – Pa;
// If Pa > Pw, condensation might occur, but for evaporation calculation we treat as 0
if (deltaP < 0) {
deltaP = 0;
}
var ratePerSqM = (0.0887 + (0.0781 * v)) * deltaP;
var totalHourly = ratePerSqM * area;
var totalDaily = totalHourly * 24;
// Water level drop in mm/day
// 1 kg/m2 is equal to 1 mm of water depth
var mmDropDaily = ratePerSqM * 24;
document.getElementById('ratePerArea').innerHTML = ratePerSqM.toFixed(3) + " kg/m²/h";
document.getElementById('totalLossHourly').innerHTML = totalHourly.toFixed(2) + " Liters/h";
document.getElementById('totalLossDaily').innerHTML = totalDaily.toFixed(2) + " Liters/day";
document.getElementById('levelDrop').innerHTML = mmDropDaily.toFixed(2) + " mm/day";
document.getElementById('evapResults').style.display = 'block';
// Scroll to results
document.getElementById('evapResults').scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}