Whether you are managing a residential swimming pool, an industrial cooling tank, or an aquarium, understanding the rate at which water evaporates is crucial for maintenance, cost estimation, and chemical balancing. Evaporation is the process by which liquid water turns into water vapor, and the speed of this process is heavily influenced by environmental conditions.
How This Calculator Works
This calculator utilizes a variation of the standard engineering Carrier Equation (often used by ASHRAE) to estimate evaporation from a free water surface. The formula takes into account the vapor pressure differential between the water and the air, adjusted for wind velocity.
E = (0.089 + 0.0782 × V) × (P_w – P_a) × A
Where:
E = Evaporation rate (lbs/hr)
V = Wind velocity (mph)
P_w = Saturation vapor pressure at water temperature (in Hg)
P_a = Saturation vapor pressure at air dew point (in Hg)
A = Surface area (sq ft)
Key Factors Affecting Evaporation
Several variables play a critical role in how fast your water level will drop:
1. Water Temperature vs. Air Temperature
The single biggest driver of evaporation is the difference in vapor pressure. Warm water generates higher vapor pressure. If the water is significantly warmer than the air (e.g., a heated pool on a cool night), evaporation rates skyrocket.
2. Relative Humidity
Humidity represents how much moisture the air is already holding. In high humidity (near 100%), the air is saturated and cannot accept much more water vapor, slowing evaporation down. Conversely, dry air acts like a sponge, pulling moisture from the water surface rapidly.
3. Wind Speed
Wind strips away the layer of saturated air sitting immediately above the water surface, replacing it with drier air. Higher wind speeds dramatically increase the evaporation rate. This is why covering a pool is so effective—it blocks the wind and traps the saturated air.
4. Surface Area
Evaporation occurs only at the interface between water and air. A wide, shallow body of water will evaporate much faster than a deep, narrow tank of the same volume.
Why Monitor Evaporation?
Cost Savings: Reduces water bills and energy costs associated with heating replacement water.
Chemical Balance: As water evaporates, solids and chemicals (like calcium and salt) remain, increasing concentration. This can lead to scaling or equipment corrosion.
Leak Detection: Knowing the expected evaporation rate helps you distinguish between normal water loss and a potential leak in your system (e.g., the bucket test).
function calculateEvaporation() {
// 1. Get Input Values
var area = parseFloat(document.getElementById('surfaceArea').value);
var waterTempF = parseFloat(document.getElementById('waterTemp').value);
var airTempF = parseFloat(document.getElementById('airTemp').value);
var humidity = parseFloat(document.getElementById('humidity').value);
var windSpeed = parseFloat(document.getElementById('windSpeed').value);
// 2. Validation
if (isNaN(area) || isNaN(waterTempF) || isNaN(airTempF) || isNaN(humidity) || isNaN(windSpeed)) {
alert("Please fill in all fields with valid numbers.");
return;
}
if (area <= 0) {
alert("Surface area must be greater than 0.");
return;
}
// 3. Helper Functions
// Convert Fahrenheit to Celsius
function toCelsius(f) {
return (f – 32) * 5 / 9;
}
// Calculate Saturation Vapor Pressure (in kPa) given Temp in Celsius
// Using Magnus formula approximation
function getSatVaporPressureKPa(tempC) {
return 0.61094 * Math.exp((17.625 * tempC) / (tempC + 243.04));
}
// 4. Perform Calculations
var waterTempC = toCelsius(waterTempF);
var airTempC = toCelsius(airTempF);
// Calculate Vapor Pressures in kPa
var Pw_kPa = getSatVaporPressureKPa(waterTempC); // Saturation VP at water surface
var Pa_sat_kPa = getSatVaporPressureKPa(airTempC); // Saturation VP of air
// Calculate Actual Vapor Pressure of Air based on Humidity
var Pa_actual_kPa = Pa_sat_kPa * (humidity / 100);
// Convert kPa to inHg (Inches of Mercury) as required by the Carrier/ASHRAE formula constant
// 1 kPa = 0.2953 inHg
var Pw_inHg = Pw_kPa * 0.2953;
var Pa_inHg = Pa_actual_kPa * 0.2953;
// Calculate Pressure Difference
var pressureDiff = Pw_inHg – Pa_inHg;
// If air vapor pressure is higher than water vapor pressure, condensation occurs (negative evaporation)
// For this calculator, we set evaporation to 0 if condensation is happening
if (pressureDiff 0) {
depthLossInchesPerWeek = volumeLossCuInPerWeek / areaSqIn;
}
// 5. Display Results
document.getElementById('resGallonsPerHour').innerText = gallonsPerHour.toFixed(2) + " gal/hr";
document.getElementById('resGallonsPerDay').innerText = gallonsPerDay.toFixed(2) + " gal/day";
document.getElementById('resGallonsPerWeek').innerText = gallonsPerWeek.toFixed(2) + " gal/week";
document.getElementById('resInchesPerWeek').innerText = depthLossInchesPerWeek.toFixed(2) + " inches";
// Show result container
document.getElementById('results').style.display = "block";
}