Understanding Water Evaporation Rate
Water evaporation is a fundamental process where liquid water transforms into water vapor and enters the atmosphere. This phenomenon plays a critical role in the Earth's water cycle, influencing weather patterns, reservoir management, agricultural irrigation, and industrial processes. The rate at which water evaporates is influenced by several environmental factors.
Factors Affecting Evaporation:
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Surface Area: A larger surface area exposed to the air allows for more molecules to escape into the atmosphere, thus increasing the evaporation rate.
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Water Temperature: Higher water temperatures provide more kinetic energy to water molecules, making it easier for them to break free from the liquid surface and become vapor.
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Air Temperature: Warmer air can hold more moisture. When the air temperature is higher than the water temperature, it can increase the vapor pressure deficit, driving evaporation.
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Relative Humidity: This measures how much water vapor is already in the air compared to the maximum it can hold at a given temperature. High humidity means the air is close to saturation, reducing the driving force for evaporation. Conversely, dry air (low humidity) promotes faster evaporation.
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Wind Speed: Wind removes moist air from above the water surface and replaces it with drier air. This reduces the local humidity near the surface, maintaining a higher vapor pressure deficit and thus increasing the evaporation rate.
Calculation Method:
This calculator uses a simplified empirical formula often found in hydrology and meteorology to estimate evaporation rates. A common approach, though simplified here for general use, is based on the Penman-Monteith equation or simpler mass transfer models. For this calculator, we'll use a conceptual approach that reflects the primary drivers:
The rate of evaporation (E) is roughly proportional to the difference in vapor pressure between the water surface and the air, and to a wind function.
Simplified Conceptual Formula Logic (for illustration purposes):
E = k * A * (VaporPressure_Water – VaporPressure_Air) * (1 + c * WindSpeed)
Where:
- E is the evaporation rate (e.g., mm/day or kg/m²/s)
- k is a constant depending on units and conditions
- A is the surface area
- VaporPressure_Water is the saturation vapor pressure at the water temperature
- VaporPressure_Air is the actual vapor pressure of the air (calculated from relative humidity and air temperature)
- WindSpeed is the wind speed
- c is another constant
Note: This is a highly simplified representation. Accurate evaporation calculations often involve complex psychrometric constants, aerodynamic resistance, and surface resistance factors, as seen in the full Penman-Monteith equation. This calculator provides a general estimate.
Example Usage:
Imagine a small pond with a surface area of 100 m². The water temperature is 25°C, the air temperature is 20°C, the relative humidity is 60%, and there's a gentle breeze of 2 m/s. Inputting these values will give you an estimated daily evaporation loss. For instance, a result might show that 5 mm of water could evaporate from the pond's surface over a day, equating to a significant water volume loss.
function calculateEvaporation() {
var surfaceArea = parseFloat(document.getElementById("surfaceArea").value);
var waterTemperature = parseFloat(document.getElementById("waterTemperature").value);
var airTemperature = parseFloat(document.getElementById("airTemperature").value);
var humidity = parseFloat(document.getElementById("humidity").value);
var windSpeed = parseFloat(document.getElementById("windSpeed").value);
var resultDiv = document.getElementById("result");
var evaporationRateP = document.getElementById("evaporationRate");
var dailyEvaporationP = document.getElementById("dailyEvaporation");
// Clear previous results
evaporationRateP.innerHTML = "";
dailyEvaporationP.innerHTML = "";
// Input validation
if (isNaN(surfaceArea) || surfaceArea <= 0) {
evaporationRateP.innerHTML = "Please enter a valid surface area (greater than 0).";
return;
}
if (isNaN(waterTemperature)) {
evaporationRateP.innerHTML = "Please enter a valid water temperature.";
return;
}
if (isNaN(airTemperature)) {
evaporationRateP.innerHTML = "Please enter a valid air temperature.";
return;
}
if (isNaN(humidity) || humidity 100) {
evaporationRateP.innerHTML = "Please enter a valid humidity percentage (0-100).";
return;
}
if (isNaN(windSpeed) || windSpeed < 0) {
evaporationRateP.innerHTML = "Please enter a valid wind speed (0 or greater).";
return;
}
// Constants and helper functions (simplified empirical approach)
// These constants are illustrative and would be derived from complex models in real applications.
var saturationVaporPressureConstant = 0.6108; // kPa
var boilingPointConstant = 17.27;
var boilingPointDivider = 237.3;
// Function to calculate saturation vapor pressure (kPa) using the August-Roche-Magnus formula
function calculateSaturationVaporPressure(tempC) {
return saturationVaporPressureConstant * Math.exp((boilingPointConstant * tempC) / (tempC + boilingPointDivider));
}
// Function to calculate actual vapor pressure (kPa)
function calculateActualVaporPressure(airTempC, relativeHumidity) {
var saturationVP = calculateSaturationVaporPressure(airTempC);
return (relativeHumidity / 100) * saturationVP;
}
var waterSaturationVP = calculateSaturationVaporPressure(waterTemperature);
var airActualVP = calculateActualVaporPressure(airTemperature, humidity);
// Vapor Pressure Deficit (VPD) in kPa
var vpd = waterSaturationVP – airActualVP;
// If VPD is negative (air is more saturated than water surface at its temp), evaporation is minimal or condensation might occur.
// For simplicity, we'll cap VPD at 0 or a small positive value if water temp is higher than air temp and air is very humid.
if (vpd airTemperature) {
vpd = Math.max(0.1, calculateSaturationVaporPressure(airTemperature) – airActualVP);
} else {
vpd = 0; // No evaporation if air is more "moist" or equally moist and cooler/same temp.
}
}
// Simplified mass transfer coefficient (influences how quickly vapor is removed)
// This coefficient is highly dependent on atmospheric conditions and is a simplification.
// A common form for wind: a + b * windSpeed
var windFactor = 0.1; // Base factor
var windSpeedCoefficient = 0.05; // Effect of wind speed
var massTransferCoefficient = windFactor + (windSpeedCoefficient * windSpeed); // Units depend on desired output
// Evaporation rate calculation (e.g., in mm/day, this requires a conversion factor)
// This is a conceptual model. A more accurate model might use specific units for kPa and m/s
// to directly yield mm/day or kg/m²/s, which then needs area conversion.
// Let's assume a simplified conversion factor to get an approximate daily rate in mm
// This factor combines density of water, latent heat of vaporization, and time (seconds in a day)
// and is a gross simplification for demonstration.
var simplifiedConversionFactor = 0.03; // mm of water per kPa * m²/s (highly conceptual)
var evaporationRate_mm_per_sec_per_m2 = massTransferCoefficient * vpd * simplifiedConversionFactor;
// Convert to daily rate for the entire surface area
var secondsInADay = 24 * 60 * 60;
var dailyEvaporation_mm = evaporationRate_mm_per_sec_per_m2 * secondsInADay;
var totalDailyEvaporation_liters = (dailyEvaporation_mm / 1000) * surfaceArea * 1000; // mm to meters, then m³ to liters
evaporationRateP.innerHTML = "Estimated hourly evaporation rate (per m²): " + evaporationRate_mm_per_sec_per_m2.toFixed(4) + " mm/hour";
dailyEvaporationP.innerHTML = "Estimated daily evaporation loss: " + dailyEvaporation_mm.toFixed(2) + " mm (equivalent to " + totalDailyEvaporation_liters.toFixed(2) + " liters)";
resultDiv.style.display = "block";
}
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