Evaporation is the process by which a liquid turns into a gas. In the context of swimming pools, reservoirs, or industrial tanks, calculating the evaporation rate is crucial for estimating water loss, energy costs, and chemical maintenance requirements. This calculator estimates the evaporation rate from a free water surface based on empirical engineering formulas.
The Physics of Evaporation
The rate at which water evaporates is driven principally by the difference between the vapor pressure at the water surface and the vapor pressure of the surrounding air. The formula used in this calculation is derived from standard evaporation equations (often variants of the Meyer or Dalton equations) which account for wind speed, temperature, and humidity.
General Formula: E = (C + k × V) × A × (Pw – Pa)
Where:
E = Evaporation Rate
V = Wind Speed
A = Surface Area
Pw = Saturation vapor pressure at water temperature
Pa = Actual vapor pressure of the air (based on humidity)
Key Factors Affecting Evaporation
Water Temperature: Warmer water has higher molecular energy, leading to a higher vapor pressure (Pw) and faster evaporation. Heating a pool significantly increases water loss.
Air Temperature & Humidity: These determine the vapor pressure of the air (Pa). Dry air (low humidity) has a lower vapor pressure, creating a larger deficit compared to the water, which accelerates evaporation. Conversely, 100% humidity stops net evaporation.
Wind Speed: Wind removes the layer of saturated air sitting immediately above the water surface. Higher wind speeds maintain a high vapor pressure differential, drastically increasing the evaporation rate.
Surface Area: Evaporation occurs only at the interface between air and water. A larger surface area results in proportionally higher total water loss.
Why Calculate Evaporation?
Pool Maintenance: For pool owners, understanding evaporation helps distinguish between normal water loss and structural leaks. A typical uncoverd pool can lose 1/4 inch of water per day in dry, windy conditions.
Energy Efficiency: Evaporation is a cooling process. When water evaporates, it takes heat with it (latent heat of vaporization). High evaporation rates result in massive heat loss, requiring pool heaters to work overtime and increasing energy bills.
How to Reduce Evaporation
The most effective way to reduce evaporation is to cover the water surface. A solar pool cover (bubble cover) acts as a vapor barrier, potentially reducing evaporation by up to 95%. This preserves water, retains heat, and reduces chemical consumption.
function calculateEvaporation() {
// 1. Get Input Values
var waterTempF = parseFloat(document.getElementById('waterTemp').value);
var airTempF = parseFloat(document.getElementById('airTemp').value);
var humidity = parseFloat(document.getElementById('humidity').value);
var windSpeedMph = parseFloat(document.getElementById('windSpeed').value);
var areaSqFt = parseFloat(document.getElementById('surfaceArea').value);
var errorMsg = document.getElementById('errorMsg');
var resultsDiv = document.getElementById('results');
// 2. Validate Inputs
if (isNaN(waterTempF) || isNaN(airTempF) || isNaN(humidity) || isNaN(windSpeedMph) || isNaN(areaSqFt)) {
errorMsg.style.display = 'block';
resultsDiv.style.display = 'none';
return;
}
// Basic range checks to avoid physics errors
if (humidity 100) humidity = 100;
if (windSpeedMph < 0) windSpeedMph = 0;
if (areaSqFt <= 0) {
errorMsg.innerText = "Surface Area must be greater than 0.";
errorMsg.style.display = 'block';
return;
}
errorMsg.style.display = 'none';
// 3. Convert Units for Calculation
// Temperature F to C
var waterTempC = (waterTempF – 32) * (5/9);
var airTempC = (airTempF – 32) * (5/9);
// Wind speed mph to m/s
var windSpeedMs = windSpeedMph * 0.44704;
// Area sq ft to sq meters
var areaSqM = areaSqFt * 0.092903;
// 4. Calculate Vapor Pressures (in kPa)
// Formula: P = 0.6108 * exp((17.27 * T) / (T + 237.3))
// Saturation Vapor Pressure at Water Surface (Pw)
var Pw = 0.6108 * Math.exp((17.27 * waterTempC) / (waterTempC + 237.3));
// Saturation Vapor Pressure of Air (Psat_air)
var Psat_air = 0.6108 * Math.exp((17.27 * airTempC) / (airTempC + 237.3));
// Actual Vapor Pressure of Air (Pa)
var Pa = Psat_air * (humidity / 100);
// 5. Calculate Evaporation Rate
// Using a standard empirical engineering formula approximation:
// E (kg/hr) = (0.089 + 0.0782 * V_wind_m/s) * Area_m2 * (Pw_kPa – Pa_kPa) * conversion_factors
// Note: The coefficients (0.089 and 0.0782) are common for calculating evaporation from open tanks/pools.
// Pw and Pa must be in kPa. Result is roughly kg per hour.
var evaporationRateKgHr = (0.089 + (0.0782 * windSpeedMs)) * areaSqM * (Pw – Pa);
// Handle condensation (negative evaporation)
if (evaporationRateKgHr < 0) {
evaporationRateKgHr = 0;
}
// 6. Convert Results to Imperial Units
// 1 kg water = approx 0.264172 gallons
var gallonsPerHour = evaporationRateKgHr * 0.264172;
var gallonsPerDay = gallonsPerHour * 24;
// Calculate inches of water loss per day
// Volume (gallons) = Area (sq ft) * Depth (ft) * 7.48
// Depth (ft) = Volume (gallons) / (Area (sq ft) * 7.48)
// Depth (inches) = Depth (ft) * 12
var inchesPerDay = (gallonsPerDay / (areaSqFt * 7.48)) * 12;
// 7. Update UI
document.getElementById('resGallonsHour').innerText = gallonsPerHour.toFixed(2) + " gal";
document.getElementById('resGallonsDay').innerText = gallonsPerDay.toFixed(1) + " gal";
document.getElementById('resInchesDay').innerText = inchesPerDay.toFixed(3) + " inches";
resultsDiv.style.display = 'block';
}