Evaporation Rate Calculator

Evaporation transfers liquid water into the atmosphere when energetic surface molecules escape as vapor. Its rate depends on the difference between the vapor pressure at the water surface and the vapor already present in the surrounding air. Warm water has a higher saturation vapor pressure, dry air produces a larger vapor pressure deficit, and moving air replaces the moist boundary layer above the surface. A larger exposed area supplies proportionally more surface from which evaporation can occur. This calculator first estimates saturation vapor pressure with the widely used Magnus relationship: 0.6108 times exp(17.27T divided by T plus 237.3), where T is water temperature in degrees Celsius and pressure is in kilopascals. It multiplies that pressure by one minus relative humidity as a decimal to obtain vapor pressure deficit. The screening rate is then 0.1 times surface area times vapor pressure deficit times a wind factor of one plus 0.5 times wind speed. Under this model one kilogram of water is approximately one liter of volume lost. The calculation is intentionally an estimate rather than a site-specific engineering guarantee. Actual evaporation is influenced by solar radiation, air temperature, atmospheric pressure, water chemistry, turbulence, pool occupancy, covers, shading, heat supplied to the water, and the geometry of surrounding structures. Indoor pools also depend strongly on ventilation and whether the air near the water is already saturated. Natural lakes may store heat and experience varying wind over a long fetch, so daily weather data and calibrated hydrological models are preferable for water-balance studies. Use water temperature rather than air temperature when it is known, because saturation conditions are established at the liquid surface. Relative humidity must be between zero and one hundred percent. Wind speed should represent conditions just above the water; values from a distant weather station may not describe a sheltered pool or tank. Surface area is the horizontal water area, not the wetted wall area. The hourly result can be multiplied by twenty-four for a constant-condition daily estimate, as displayed here. This tool is useful for comparing scenarios, estimating makeup-water demand, exploring how covers reduce exposed area, and checking the order of magnitude of water loss. For equipment sizing, regulated facilities, irrigation scheduling, or environmental permits, use a method approved for the application and local climate. Measurements over several representative days can be used to calibrate the model coefficient and improve predictions for a particular site.