Evaporation Rate - kg per Hour Water Loss

An evaporation rate calculator converts the open water surface area, the wind speed just above the water, the water and air temperatures, the relative humidity, and the local atmospheric pressure into the kilograms of water lost per hour from a pool, pond, tank, or other open water surface, using the empirical (25+19v) formula published by Engineering ToolBox. Use it whenever you need a realistic estimate of how fast water disappears from a swimming pool, irrigation reservoir, cooling pond, or garden pond.

• • Pool water loss planning: estimate how many litres per hour to top up a backyard pool during a heat wave to budget water and chemicals.
• • Pond and lake water budget: size an automatic top-up valve for a koi pond or stormwater retention pond.
• • Cooling-tower make-up: calculate how much water a cooling tower or evaporative cooler pulls out so the make-up line can be sized.
• • Industrial tank loss: predict water loss from an open process tank or quench tank so emissions drift are tracked.

The result depends on the humidity ratio the air can hold at the water surface, and the absolute humidity calculator computes the matching water vapor density in g/m^3 from the same temperature, relative humidity, and pressure inputs.

The calculator reads six inputs, computes the saturation humidity ratio at the water temperature and the actual humidity ratio at the air temperature, multiplies their difference by theta = 25 + 19v and the surface area, and returns kilograms of water lost per hour together with the daily total, the litre and pound equivalents, the theta coefficient, and the latent heat removed.

• g_h: water loss in kilograms per hour from the open surface.
• v: air speed just above the water in metres per second.
• A: open water surface area in square metres.
• X_s: saturation humidity ratio at the water surface temperature, in kg water per kg dry air.
• X: current humidity ratio at the air temperature and relative humidity, in kg water per kg dry air.

Saturation vapor pressure uses the Tetens equation 610.78 * exp(17.27 * T / (T + 237.3)) in pascals, accurate to about 0.4 percent between 0 and 50 degrees C. The humidity ratio formula x = 0.62198 * pw / (pa - pw) converts that partial pressure into kg of water vapor per kg of dry air.

According to Engineering ToolBox - Evaporation from a Water Surface, the empirical evaporation coefficient is theta = (25 + 19 v) kg/m^2/h with v in m/s, and the hourly water loss is g_h = theta * A * (Xs - X).

The (Xs - X) humidity ratio gap in this formula also drives plant transpiration, and the vapor pressure deficit calculator reports the matching saturation minus actual vapor pressure in kPa from the same temperature and humidity inputs.

Four ideas drive the calculation. None dominates on its own, so changing one without re-checking the others can leave you surprised at how much (or how little) water actually disappears.

The 0.62198 in the humidity ratio formula is just the ratio of the molar mass of water to that of dry air, and the air density calculator computes the matching density of the same moist air parcel from temperature, pressure, and relative humidity.

Run the calculator in six steps, then read both the kg/h number and the latent heat output so the same condition does not catch you by surprise on the energy bill.

1. 1 Enter the surface area: Type the open water surface area in square metres. For a rectangular pool multiply length by width; for a circular pond use pi times radius squared.
2. 2 Enter the wind speed: Use the average wind speed just above the water, not the open-field gust. A backyard anemometer or the local forecast's sustained wind work; convert mph or km/h to m/s if needed.
3. 3 Enter both temperatures: Type the water surface temperature and the ambient air temperature in degrees Celsius. If you only have an air thermometer, set the water equal to the air and accept the small daytime error.
4. 4 Enter the relative humidity: Type the current relative humidity as a percent from 0 to 100. Most weather apps report this directly.
5. 5 Adjust the pressure if needed: Leave atmospheric pressure at 101.325 kPa for sea level. Reduce to about 90 kPa at 1000 m altitude and 80 kPa at 2000 m to keep the humidity ratio calculation accurate.
6. 6 Read the kg/h and the latent heat: Use the kg/h number to size top-up flow, and the latent heat output to size the heater that has to replace what evaporation steals from the water.

If the pool or tank you are sizing is hot enough that evaporation is approaching boiling, the boiling point calculator returns the matching saturation temperature from pressure and substance so you can see how close you are to that limit.

A clear kg/h number plus a kilowatt heat loss saves water, money, and chemistry. These are the most useful decisions the calculator supports.

• • Size the auto-fill line: Pick a float valve, solenoid, or supply pump rated to at least the peak kg/h rate so the pool or tank does not run dry during a hot dry afternoon.
• • Budget chemicals and water: Multiply the daily kg loss by the salt, chlorine, or pH buffer dose to know how much chemistry disappears with the water.
• • Quantify the heater load: Use the latent heat output as the heat the heater or chiller has to replace, usually 50 to 80 percent of the total HVAC load for an indoor pool.
• • Compare windbreaks and covers: Run the same pool with v = 2 m/s and v = 0.5 m/s, or with and without a cover, to see the dollar value of each intervention.
• • Plan seasonal top-up: Multiply the daily loss by the number of dry windy days in your climate window to estimate the litres to budget over a summer.

Evaporation is only one path for heat to leave a body of water; conduction through the walls and floor adds another, and the heat transfer conduction calculator returns the matching conduction loss in watts from material thickness and area.

The rate printed by the calculator is the answer at the entered conditions. The actual rate can swing widely with weather and setup, and the most common drivers are below.

• • The (25 + 19 v) coefficient is an empirical fit from the Engineering ToolBox psychrometric chart and is most accurate between about 0 and 30 degC with light to moderate winds; very high winds, sub-zero air, or strong direct sunlight can push the actual rate above or below the prediction.
• • The formula assumes a flat open water surface. Falling water in a cooling tower, splashing from a fountain, or a thin film on a plate behaves more like a wet-surface model and will usually evaporate faster than this calculator predicts for the same area.

According to Wikipedia - Evaporation, evaporation rate is controlled by concentration of the evaporating substance in the surrounding gas, flow rate of fresh air, surface area, water temperature, and atmospheric pressure.

If the body of water holds dissolved salts or glycol, the saturation vapor pressure drops and so does the rate, and the boiling point elevation calculator shows the matching temperature shift you have to add on top of the water temperature.