Evaporation is the process where liquid water turns into water vapor. While the mechanics are similar for both fresh and salt water, salinity plays a crucial role in determining the speed at which this phase change occurs. This calculator uses thermodynamic principles to estimate how much water you will lose over time.
Key Concept: Raoult's Law
The presence of dissolved salts reduces the vapor pressure of the water. Simply put, salt ions hold onto water molecules, making it harder for them to escape into the air. Consequently, salt water evaporates more slowly than fresh water under the same conditions. Typical seawater (35 ppt) evaporates about 2-3% slower than fresh water.
Factors Influencing Evaporation
Several environmental variables determine the evaporation rate:
Surface Area: Evaporation only occurs at the interface between water and air. A larger surface area results in linear increases in evaporation volume.
Water Temperature: Warmer water has higher kinetic energy, increasing the vapor pressure and the rate of evaporation.
Air Temperature & Humidity: These define the "thirst" of the atmosphere. Warm, dry air can hold more moisture, encouraging faster evaporation compared to cool, humid air.
Wind Speed: Moving air strips away the saturated boundary layer just above the water surface, maintaining a gradient that promotes continuous evaporation.
Salinity: Measured in parts per thousand (ppt). Higher salinity lowers the vapor pressure, slightly reducing the evaporation rate.
How to Use This Calculator
This tool is useful for:
Aquarium Hobbyists: Estimating top-off water requirements for reef tanks.
Salt Production: Calculating drying times for brine ponds.
Pool Maintenance: Determining water loss in saltwater swimming pools.
Industrial Applications: Managing desalination plant intakes or brine disposal.
Enter your environmental parameters above. If you are unsure of the wind speed, indoor air usually moves at 0.1-0.3 m/s (due to HVAC), while outdoor wind varies significantly.
var currentUnit = 'metric';
function toggleUnits() {
var radios = document.getElementsByName('units');
for (var i = 0; i Air Vapor Pressure
if (P_water > P_air) {
evapFlux = (0.089 + 0.0782 * speedMs) * (P_water – P_air);
} else {
evapFlux = 0; // Condensation or Equilibrium
}
// evapFlux is in kg / m² / hr (roughly equivalent to mm/hr or Liters/m²/hr)
// 5. Total Volume Calculations
var totalLitersPerHour = evapFlux * areaM2;
var totalLitersPerDay = totalLitersPerHour * 24;
var totalLitersPerWeekly = totalLitersPerDay * 7;
// Water Level Drop (mm per day)
// Since 1 kg/m2 = 1 mm depth
var dropMmPerDay = evapFlux * 24;
// 6. Formatting Output based on Units
var resHourly, resDaily, resWeekly, resDrop, resSpecific;
if (currentUnit === 'metric') {
resHourly = totalLitersPerHour.toFixed(2) + " Liters";
resDaily = totalLitersPerDay.toFixed(2) + " Liters";
resWeekly = totalLitersPerWeekly.toFixed(2) + " Liters";
resDrop = dropMmPerDay.toFixed(2) + " mm";
resSpecific = evapFlux.toFixed(3) + " kg/m²/hr";
} else {
// Convert back to US Gallons and Inches
// 1 Liter = 0.264172 Gallons
var galHour = totalLitersPerHour * 0.264172;
var galDay = totalLitersPerDay * 0.264172;
var galWeek = totalLitersPerWeekly * 0.264172;
// 1 mm = 0.0393701 inches
var dropInch = dropMmPerDay * 0.0393701;
resHourly = galHour.toFixed(2) + " Gallons";
resDaily = galDay.toFixed(2) + " Gallons";
resWeekly = galWeek.toFixed(2) + " Gallons";
resDrop = dropInch.toFixed(3) + " inches";
resSpecific = (evapFlux * 0.2048).toFixed(3) + " lbs/ft²/hr"; // approx conversion
}
// 7. Update DOM
document.getElementById('res-hourly').innerText = resHourly;
document.getElementById('res-daily').innerText = resDaily;
document.getElementById('res-weekly').innerText = resWeekly;
document.getElementById('res-drop').innerText = resDrop;
document.getElementById('res-specific').innerText = resSpecific;
document.getElementById('results-area').style.display = 'block';
}