How Fast a Naples Pool Actually Loses Water to Evaporation, Month by Month

The pool evaporation rate in Naples is not one number. Measured South Florida data puts open water loss at roughly 0.7 inches a week in December rising to about 1.3 inches a week in May, against a directly measured open water reference of 1.38 inches a week (SFWMD, 2010; USGS, 2024)¹². Converted to a daily figure, no month of the year averages a quarter inch a day.

Where does the quarter inch a day rule actually come from?

Search for the origin of the figure and the results are all one category: pool service pages, remodeling pages and leak detection marketing, each stating it as settled and none of them naming who settled it. No federal agency publishes it. No university extension publishes it. No water management district publishes it. The number circulates because it is repeated, not because it was measured.

That is a narrower statement than it sounds, and the precision is the point. Saying no Tier 1 source supports the figure is not the same as saying it has been disproved. It may well be a serviceable approximation of an annual average. What it cannot be is a test, because a threshold nobody measured gives your own measurement nothing to be checked against.

This site used it too. Until this correction, several pages here stated that losing more than about a quarter inch a day meant a leak, in the same form every competing page states it. It was carried as an internal working threshold and was never attributed to anyone, which was the right discipline as far as it went. It is being replaced site wide with the sourced band below (SFWMD, 2010)¹.

The replacement matters more than a footnote about provenance would suggest, because the unsourced figure is not conservative. It sits above the modeled loss for every month of the year, and it is furthest above it in winter. A rule that is too generous in the season with the lowest evaporation is a rule that tells a leaking pool nothing is wrong.

What a sourced number buys is the ability to check it. Everything below comes from two documents you can open, one published by the regional water manager and one by the United States Geological Survey, plus one division you can redo on a phone. If the arithmetic here is wrong, it is wrong in public and it is correctable by anyone who reads it.

What does the measured evaporation data for South Florida actually say?

Technical Paper 107 was published by the South Florida Water Management District in 2010, and it models potential evapotranspiration for the region by two methods, Penman Monteith and a simpler method the authors attribute to Abtew. Both agree on the shape of the year. The paper states that the highest evapotranspiration is in May and the lowest is in December, in both cases (SFWMD, 2010)¹.

The United States Geological Survey came at the same question from the other end and measured it. Using the Bowen ratio energy budget method at the LZ40 platform in Lake Okeechobee, annual evaporation averaged 1,825 millimeters per year, or 5.0 millimeters a day, across a four year study (USGS, 2024)². Converted, that is about 0.197 inches a day, or 1.38 inches a week.

The measurement is stable year to year, which is what makes it usable as a reference rather than an anecdote. Annual evaporation from 2013 to 2016 came in at 1,760, 1,840, 1,810 and 1,890 millimeters per year respectively (USGS, 2024)². Four consecutive years, and the spread between the highest and the lowest is narrow.

Keeping the model and the measurement separate is deliberate. A model tells you what the atmosphere over South Florida can take up given the radiation, the temperature and the wind. A measurement tells you what one specific body of open water actually lost. Neither one is your pool. Having both is worth more than having either, because they were produced independently and they land in the same range.

One category of number is deliberately excluded. Technical Paper 107 also reports Class A pan measurements from nine South Florida sites, and pan readings run materially higher than true open water evaporation (SFWMD, 2010)¹. No pan figure appears anywhere on this page. A pan is a shallow metal dish that heats fast and loses water faster than a pool does, and quoting one as pool loss would inflate the band.

How much water should a Naples pool lose per day, month by month?

The conversion is two divisions and nothing else. Inches per month is millimeters divided by 25.4. Inches per day is inches per month divided by the number of days in that month, using calendar days and a 28 day February. That is the whole method, published here so the table can be reproduced, or corrected, by anyone who disagrees with it.

The result is a band rather than a threshold. The highest monthly average is May, at about 0.184 inches a day. The lowest is December, at about 0.095 inches a day (SFWMD, 2010)¹. No month reaches a quarter inch a day. The figure the industry quotes as a normal daily loss sits above the peak month of the year, not above the average one.

One disclosure belongs with the table rather than in a footnote. The monthly columns in the paper do not sum to the annual totals the paper itself states. The Penman Monteith months add to 1,347 millimeters against a stated annual of 1,376, and the Abtew months add to 1,302 against a stated 1,315 (SFWMD, 2010)¹. Rounding in the published table is the likeliest explanation.

That gap changes how the numbers may be used. The monthly values are published here as monthly values, and the annual figure quoted anywhere on this page is the one the paper states rather than the one our column adds up to. A sum of rounded months is not a source value, and presenting it as one would be exactly the kind of quiet substitution this page exists to argue against.

The second disclosure is what the number describes. This is potential evapotranspiration for South Florida as a region, not a measurement of a swimming pool. Substituting one for the other is defensible for open water, and it is stated here rather than implied. A screened pool sits behind a barrier that cuts wind, and wind is one of the main drivers, so a screened pool should lose somewhat less than the table shows.

The cross check is the reason to trust the band at all. The directly measured Lake Okeechobee rate of 5.0 millimeters a day converts to about 0.197 inches a day (USGS, 2024)², which sits just above the modeled May peak. A model of a region and an instrument on a lake, produced by different agencies using different methods, bracket the same range.

Then the consequence, which is the part that changes what an owner does. In December the quarter inch figure is roughly two and a half times the modeled rate. A pool losing 0.20 inches a day in December is losing about double what evaporation accounts for (SFWMD, 2010)¹, and the rule of thumb tells that owner nothing is wrong. That is the winter call, arriving in the one season the rule is least able to see it.

Why is my pool losing water in January when it is not even hot?

The exception is real and it is measured. USGS reported large evaporation rates of about 10 millimeters a day during cold fronts in January of 2013, 2014, 2015 and 2016, and the model used in that study represented them well (USGS, 2024)². In inches that is about 0.39 in one day, which is more than the quarter inch figure and more than any monthly average in the table above.

The study names why, and it is not air temperature by itself. Cold fronts were associated with greater wind speeds, with drier air that accepted more water vapor, and with abrupt declines in air temperature that forced heat energy stored in the lake to become available for evaporation (USGS, 2024)². Three drivers arrive together, and the third is the counterintuitive one: the water gives up heat it had already banked.

That is the answer to the call that arrives every January in Naples, the one that opens with the observation that the pool is dropping and it is not even hot out. The monthly average for January is the second lowest of the year. A single frontal day inside that month can be the highest loss day of the year. Both statements are true and they describe the same month.

It also changes how a 24 hour result should be read in winter. A test taken during a front can read high for reasons that have nothing to do with the pool, which is an argument for repeating it on a settled day rather than for ignoring it. A pool that reads high on a calm December day is a different matter, because the band for that month is the lowest of the year.

Why do the water loss calls cluster in spring rather than in midsummer?

The paper puts the seasonal split plainly, although the sentence is garbled in the original text and is paraphrased here rather than quoted. It says that rainfall in the summer exceeds potential evapotranspiration, and that potential evapotranspiration exceeds rainfall in the winter and spring months (SFWMD, 2010)¹. The same paper puts the wet season from June through October at 66 percent of the annual rainfall for the region (SFWMD, 2010)¹.

Heat is not what makes spring the losing season. The afternoon relative humidity minimum at Fort Myers, the nearest station with a published humidity record, lands in April at 47 percent (NOAA NCEI, 2023)⁴. Naples Municipal Airport is not in that table, so this is a Fort Myers figure and it is not presented as a Naples one. Dry air takes up water faster than humid air does, whatever the thermometer reads.

Line the two curves up and the pattern is obvious. Rainfall at Naples Municipal Airport is at its lowest from November through February, but so is evaporation, so the two roughly keep pace. From March through May the evaporation curve climbs toward its annual peak while the rain has not started (NOAA NCEI, 2021)³. That gap is the spring water loss season, and it is when the calls arrive.

The station matters enough to name. The Naples normals used here are from station USW00012897, Naples Municipal Airport, in the 1991 to 2020 United States Climate Normals (NOAA NCEI, 2021)³. A station number one digit away belongs to Fort Pierce on the other coast, and the two are easy to confuse. Naming the station is how a reader checks a number instead of trusting it.

Where the ground is already wet, none of this reads on the surface. On the canal lots where the ground hides a loss, a pool losing water into saturated soil produces no wet patch and no soft ground to follow, so the seasonal band and a proper test are the only instruments available. Nothing visible in the yard is not evidence that the loss was evaporation.

What can this band tell you about your own pool, and what can it not?

Two local variables sit outside the band and both push in a known direction. A screen cage cuts wind across the surface, and wind is one of the drivers the USGS study names, so a screened pool should lose less than an open one (USGS, 2024)². A heated spa spilling into the pool pushes the other way, because warmer surface water evaporates faster. No published measurement of screened against unscreened Southwest Florida pools was found, so both are mechanisms rather than numbers.

The diagnosis itself is not this page, and it is not repeated here on purpose. Comparing your pool against a bucket of its own water over the same day removes the weather from the comparison entirely, and that procedure, including how to read a pump on result against a pump off one, is set out in how to tell a leak from evaporation on your own pool. The band only tells you whether the question is worth asking.

If the comparison says the pool is losing more than the band and more than the bucket, the next step is finding where. That is pool leak detection in Naples, which isolates and tests the plumbing lines and the shell rather than guessing at them. If the loss turns out to be equipment rather than the pool itself, it belongs with pool repair in Naples instead.

The numbers above come from a water management district table and a USGS measurement, and the conversion between them is a division you can redo in a minute. Look up the current month, write the figure down, measure your own pool over a day, and compare the two before deciding anything. If your pool sits outside the band for this month, that is worth measuring properly before it is worth worrying about. One local exception is worth knowing before you measure: equipment sitting near the water on this coast fails on a different schedule than equipment inland, and that is covered in why pool equipment fails faster near the water in Southwest Florida.