Or Shine

Rain or Shine Report for Nov 13
Mississippi does in 5 days what takes the Kissimmee 5 years
Lake-O-ulator lets you do the math

WET SEASON RAIN SUMMARY

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South Florida received 35 inches of wet season rain this year (SF rainfall summary). That’s only 5 inches down from its 40 inch wet season average. However, only around 25 inches fell on the Lake. Twice as much — 50 inches — fell in Miami Dade.

The Southwest Coast (Naples and Ft Myers) got 35 inches, which may seem middle of the road compared to the Lake and Miami extremes. But its actually 15 inches below the SW Coast’s wet season average over the past 5 years.

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I tabulate the wet season for the 6-month rainy half of the year from the start of May to the end of October. Our core 4 months of the wet season are June, July, August, and September, when between 7-10 inches of rain fall on average depending on which part of south Florida you are in.


Krisanna makes its pass through the Caloosahatchee’s Ortona S-78 Lock on its way to the WP Franklin and then into the Gulf. The lock lowered the boat 7 ft.

May and October are the shoulder seasons of the wet season, and typically receive about half the rainfall as the core summer season months.
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But not always. Around 10-inch of October rain fell across Miami-Dade, whereas just under 3 inches fell in Naples and Ft Myers.

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Big Cypress National Preserve received around 40 inches of wet season rain. That’s about 9 inches below its 5-year average of 49 inches over the past 10 years. Around 60 inches of rain fell in Big Cypress during 1995, and about 52 inches in 2005, in comparison.

Over 45 inches of wet season rain fell on the Upper Kissimmee in 2004 and 2005 in comparison to 23 and 32 inches over the past two wet seasons. As you’ll see below, that’s been a big contributor (or, better stated, lack of a contributor) to Lake O’s recent decline.

LAKE-O-ULATOR

The South Florida Water Management District has developed a handy online calculator for computing the depth, size, and volume of water in Lake Okeechobee. I’ve unofficially dubbed it the Lake-O-ulator until a more official name is christened.

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I used the Lake-O-ulator to discover that the lake (at 10.30 ft msl) currently holds 2.1 million acre-feet (MAF) of water, covers an area of 505 square miles, and measures a maximum depth of around 10 ft.

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Do you remember back when the lake crested above 18 ft msl? That occurred most recently in October 2004 following Jeanne, but also occurred during the “El Nino of Century” (there were two of them) wet winters of 1983 and 1998, and also in fall of 1995 at the end of consecutive “wet” dry and “wet” wet season.

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The lake’s surface area swelled to its outer reaches of its diked perimeter, covering over 700 square miles, holding around 5.3 MAF, and measuring a maximum depth of 18 ft.

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Compare that to a few months back when the hit its historic low (at 8.8 ft msl). The lake’s surface area shriveled to around 460 square miles, held only 1.7 MAF, and measured a maximum depth of around 8 ft.

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The Lake-O-ulator is also very useful for making rapid back-of-the-envelope water budget calculations.

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For example, over the past 20 years, the Kissimmee River has discharged an average of 1.2 million acre-feet into the lake per year — but as we’ve seen, Kissimmee inflows are an expanding and contracting slice of the lake’s water pie — sometimes filling the plate, other times making only the smallest of slivers.
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This year’s 180,000 acre-feet and last year’s 500,000 acre-feet of inflow from the Kissimmee, respectively, was a fraction of the 2,200,000, 1,800,000, and 2,400,000 acre-feet that flowed through the Kissimmee’s S65E structure in 2003, 2004, and 2005, respectively.

Rainfall is similarly a mercurial slice of the Lake’s water pie — when it rains it can pour, but when a drought hits it unfolds in slow motion, often taking months of speculation and maneuvering before the final length and severity of the drought plays out.

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Of all the uncertain variables in south Florida’s water cycle, evaporation is a constant performer. You can count on evaporation to serve up the same slice of water budget pie year in and year out.

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The size of that slice for the lake is a little over 2 million acre feet each year. That’s a year-in and year-out certainty — not a sometimes, maybe, or half of the time — but pretty much an all the time.

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That makes evaporation the slow — but steady — turtle that beats the Kissimmee rabbit to the finish line over the course of a 365-day year. Evaporation pulls +2 million acre feet from the lake each year in comparison to only around 1.2 million acre feet from the Kissimmee.

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If we momentarily suspend our disbelief and throw evaporation out of the water cycle, it would take the Kissimmee about 5 years to fill Lake Okeechobee from bottom to top (0-18 ft). In comparison, it would only take 5 days for the Mississippi River to do the same, and about 4 months for Florida’s Apalachicola River.

Incidently, the Mississippee was recently cited as a contributing factor to offshore origin of red tide blooms, predicated by westerly wind pattern. Freshwater discharge from the Mississippee typically tends to move east, but a periodic shift in the winds can move the massive freshet of water to the west, and that’s when it plays a role in setting the red tide stage. Below is a comparison of outflows from the Mississippee, Apalachicola, and Caloosahatchee Rivers since 1990.

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One last point on the Lake-O-ulator. Historically, prior to the 1880s, not only was Lake stage routinely at or above 20 ft mean sea level, twice its current elevation, but the bathymetry of the lake held that water with ease since the lake shore had not yet subsided (-10 ft). That means that the footprint of the present day Lake Okeechobee, when it was full and when it was flowing south into the Everglades, probably held around 10-12 million acre feet of water (educated guess) and was routinely at or over a 20 foot depth.

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When that happened, not only did the lake overflow its southern banks and feed into the downstream Everglades, but it also backed its waters up into Indian Priairie — the original unbounded littoral wetlands of the lake back in the day’s of yore — and which formed a mini Everglades fed by a mini Lake O called Lake Istokpoga.
Anyhow, the Lake-O-ulator gives us a fun and easy way to better understand the water body that lies at the heart of the south Florida’s water cycle. And it’s online for anyone that wants to use it. Many thanks to the District for making it available and easy to use.
ARRIVAL OF FALL

Its officially Fall in Naples.

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Naples average weekly night-time low temperature finally dropped below 60°F last week. That still hasn’t happened in Miami. Miami’s weekly averaged night-time lows only reached down to 64°F in comparison. Tallahassee’s weekly-averaged night-time low touched down at 40°F, and had a bone-chilling night-time low of 27° F last week. Shiver me timbers!

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Other signs that it is Fall are here: (1) Muhly grass has flowered. (2) The 5-day forecast in the Sunday paper has been showing consecutive days of unobstructed sunshine for the past 2 weeks. That’s a big change from the panels of cloud covered sunshine that prevailed in the paper through the summer, and even through most of October.

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Now all we need is for the cypress needles to fall and the Hurricane Season to officially end, although at this point it seems pretty well done.

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The first sign of winter? That’s ornithological: snowbirds.

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FOR MORE …

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Click to the data dashboard to see up-to-date information on south Florida’s water cycle in your part of the watershed.
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