dry season

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Intro - Dry season paradox?

Winter rains are the exception, not the rule

By Robert V. Sobczak

Believe it or not,

It rains during the winter in south Florida.

South Florida skies are reliably sunny in winter and spring

Even more quizzical, some of those individual rainfall events can be quite large, gulley washers even. The big difference with dry season rains is that they simply don’t add up over a monthly scale. One two inch rain event (however impressive) plus 29 zeros add up to a whopping two inches of rain. Compare that to the typically 7-9 inch totals of the core summer wet season months of June, July, August and September.

The water table reliably and steadily drops during the dry season, starting in November and lasting into May, as a result of the lack of steady daily rains. The caveat is when a big frontal storms pass through. Unlike summer storms that tend to be more local in nature, a winter cold front can dump water across the entire southern peninsula.

Another factor is the cooler temperatures. Winter storms don’t lose as much of their water to evapotranspiration back into the sky. The result is that every drop of winter rain counts as two, and also has a longer “staying power” on the landscape.

That changes in the latter half of the dry season somewhere around the vernal equinox when daylight hours start to grow, the cypress trees green out and air temperatures start to rise. Without rains in March, April and May, the swamp nosedives into a deep drought.

Cycles of flood and drought

How deep and how long will the spring drought last?

Usually not too far into May and rarely into June. It only takes a few weeks (sometimes less) for the summer rains to lift south Florida out of drought.

Isummary: Yes, dry seasons are wet, and that’s not paradoxical. Just don’t count on them too much.

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Green out marks “shift” in swamp

The green out of the cypress is a big swamp’s milestone:

The second half of the winter dry season has now begun.

The above bar chart reports the annual duration (in months) that the floor of the pond apple forest has gone dry in Big Cypress National Preserve, from 1992 to present.  The years 2005, 2007, 2009 and 2011 also coincided with major wildfires.

The cooler temperatures of the first half (Nov-Feb) put evaporation on hold.

Look for it to heat up now that the second half (Mar-May) has begun.

That means water levels will start dropping faster and — more and more — surface water will be harder and harder to find.  The place you find it last is in the center of the cypress domes and strands where the pond apple trees call home.

The surest sign that deep drought has hit the swamp?

Late March 2011

Answer: That’s when the pond apple roots become exposed. Last year it happened just for just two weeks.  Two years before that (see photo above) over four months.