Evaporation

Evaporation is the changing of water from a liquid state to a gas. It is usually used to indicate a state change below the boiling point of water. The evaporation rate can be measured by noting the change in the depth of water in a glass, a pail, a puddle or a swimming pool over a given time period (usually a day). Placing a ruler in any of these gives a scale one can use to read the drop in the surface elevation in a day or more.

The National Weather Service uses a large water pan with a diameter of 4 feet. They measure the drop in water level in one day, fill it back to the level it was before and measure the level drop the next day. They and others do this all over the United States. Others do it throughout the world. The Weather Service reports the evaporation at each location, each day, and sum all these daily values up to obtain an annual evaporation rate in inches. All the data through the US is collected and compiled into an annual evaporation map.

Annual Class A Pan Evaporation (inches)

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Looking at the annual evaporation map one can find the average annual evaporation rate for any location. It is roughly 100 inches for Tucson, Arizona.

The evaporation rate varies with temperature, wind speed, sunshine, and relative humidity. The evaporation rate also varies throughout the year. A rough daily rate is given by dividing the annual rate by 365 days.

Rough daily evaporation rate = Annual Evaporation rate / 365

For Tucson the rough evaporation rate is 100 inches/365 which is about 1/3 inch per day.

Swimming pool

If we use this rough evaporation rate, we can find how much a swimming pool water level might drop in 5 days

The volume of water lost from the pool is given by the drop in surface level times the area of the pool.

This is how much water must be added to keep the pool level unchanged.

Swimming Pool Water Loss Calculator

The following calculator allows one to use the annual evaporation rate map for the US to determine the amount of water that will be lost for a specific pool.

Lake or Reservoir Evaporation

The evaporation rate from larger water bodies such as ponds, lakes, and reservoirs is roughly 0.7 times the Class A pan evaporation rate. This is also roughly the same water use rate of heavy vegetation, tall grass, or alfalfa.

Lake water Loss Calculator

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