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.
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.
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
Total surface drop
= (5 days) (daily evaporation rate)
= (5 days) (1/3 inch per day)
= 1.7 inches
= .14 foot
The volume of water lost from the pool is given by the drop
in surface level times the area of the pool.
Volume
= (surface area) (depth of surface drop)
= 63 ft2
= 472 gallons
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.
