Or Shine

Rain or Shine Report for Dec 12
Dry season still young, Tropical moisture beckons
Evaporation as Black Matter of SFL’s water cycle, and
winter Tortoise to rainfall’s summer Hare

I’ve heard some chatter of a wave that may be a rain maker later in the week. But seeing is believing, and my brand of hydrology has always been about counting the raindrops after they fall. I leave the forecasting and prognosticating to the experts.

That being said, it has been a quiet start to the dry season. We haven’t had any major fronts of note or systems move through. Those “wetting rains”, as they call them, can make a big difference, and can rewind the dry season clock weeks, sometimes months — and help stave off extreme the boughts of drought that can plague our region during springs descent into our “heart of dryness.”

We’re still early in the dry season, so that is months ahead. But water restrictions have already been put in place, and water managers are keeping a close eye to the water system in their neck of the woods.

Here’s an update on recent rain trends.

The December rain cup — south Florida wide — is still empty, but we’re only a third of the way in, so still plenty of time to go. Over the past 10 years, south Florida has averaged 2.3 inches of December rain. Last December, we received the average almost right on the nose — 2.4 inches.

But last dry season as a whole — from November 1st through April 30th — was well below average — only accumulating 8.4 inches over that 6-month period. That’s not much rain when you consider we receive the same 8 inches — even more — during our typical summer rainy season month. Weather experts predicted low winter rainfalls as a result of the La Nina — El Nino’s twin opposite.

That brings me to evapotranspiration. I’ve compared it before to the black matter of the south Florida’s water cycle universe. It accounts for approximately half of our water cycle, on average, and even exceeds rainfall over Lake Okeechobee due to the lake’s cool water surface stifling upward convectional storm building during the summer — but because you can’t see it, or feel it, it rarely factors into the quitodian vernacular of the “state of the watershed” as does its twin-opposite counterpart — rainfall.

Part of it too is that evaporation is a constant performer, or as near as a constant performer you’ll find in the water cycle. In the race to the end of the annual water cycle race, evaporation is the winter-season Tortoise to the summer-seasons Hare.

The Hare is fast — it will run great lengths ahead — and at the apogee of its summer-season sprint runs the hydrograph up to its annual high-water mark — rising at or above the water ring around the cypress trunks and reaching its watery arm into the pineland and tree island high grounds.

But the Hare is a mercurial performer — good for a half a year at best — sleeping, as the analogy goes, for most of the winter.

That’s when the Tortoise catches up. Truth be told the Tortoise never sleeps. It runs all year long. Slowly but steadily, but eventually, come late spring, sunshine willing, overtakes the sleeping Hare.

That’s also about the time that the Tortoise seems to get a bit of a leap in his step, thanks to increased transpiration from the spring bloom and drop below ground into the shallow aquifer. From October through February, water stage in Big Cypress National Preserve drops around 3 inches per month. But the water recession starts accelerating 2 to 3 times that rate from late March into early May.

How far will it drop? That has as much to do with when the Hare wakes up from its 6-month sojourn, and how fast it sprints through the summer and early fall months.

But in the meanwhile, lets keep an eye on this tropical system. It could wake the Hare up from its winter slumbers and help it gain a little more ground on the Tortoise. At least for the time being.

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