Big rain, wrong places

October rains plentiful, but in wrong places. Winter dry season looms.
Week of Oct 15 – 21.

Recent photos of WP Franklin S-79 navigational lock and dam. Just two years after the record 2005 flow volume (3.9 million acre feet), 2007 has been a record low year in comparison (0.1 million acre feet)

RAINFALL

Over 10 inches of rain has fallen over the past 30 days along the east coast from Miami to West Palm Beach. Only 4 inches has fallen in southwest Florida over the same period in comparison. That’s even lower than the Lake’s 5 inches. (SFL rain summary)

Going back a full year — 365 days from late October of last year — Miami-Dade and West Palm Beach have accrued around 60 inches of rain. Over the same 365 day span, only 31 inches has fallen on the Lake, and only 41 inches in southwest Florida, with Naples only receiving 37 inches of rain. Big Cypress National Preserve straddles the extremes with around 48 inches of rain over the past 365 days. But that’s still down from the 60 inches of annual rainfall Big Cypress has been averaging over the past 5 years.

South Florida wide, October has already recorded 4 inches of rain. That’s just a hair above the 10-yr October average, and we still have a week to go before the month is through. As we know, October can be a hit-or-miss month in terms of rainfall. Last year’s October total was around 1 inch. The year before it was 8 inches (thanks to Wilma). Only 2 inches fell in October 2004, and under an inch in October 2003. Of course where it falls is just as important as how much.

So any rain we muster in October is a bonus, but its a thin icing on the top of a Wet Season cake that largely flopped this year. That puts us at the edge of a Wet Season that wasn’t — or was barely — in southwest Florida — but rained with frequency and abundance along the peninsula’s eastern fringe, and now puts us all on the verge of entering into the portals of a La-Nina enhanced winter Dry Season.

FLOW

The good side of the drought and the Lake being so low is that freshwater discharges down the Caloosahatchee and St Lucie have been kept to a minimum. Last year (2006) under 1 million acre-feet was discharged down the WP Franklin dam (S79). This year (2007 to date) about a tenth of that (100,000 acre-feet) has been discharged through the S79. That compares to almost 4 million acre-feet in 2005, 1.9 million in 2004, and 2.6 million in 2003. You have to travel back in time all the way to 1972 (I was 3 years old) to find a year when discharge through the S79 was similarly low to this year’s paltry 100,000 acre feet.

Over the past 25 years, about a quarter of those years (6 years) have recorded annual flow volumes of around 2 million acre-feet or higher. 2005’s 3.9 million acre-feet tops the chart, and the three-year average of over 2.5 million acre-feet per year between 2003 and 2005 is also a 3-year high over the past 25 years.

Other peak discharge years of note include El Nino impacted years of 1998, 1995, and 1983. 1995 was on the tail end of a low-amplitude ENSO swing into a multi-year El Nino phase, even though it is not typically lumped in the same category as the high-intensity El Nino years of 1983 and 1998, it was El Nino influenced.

Large hydrologic numbers are often difficult to translate into meaningful metrics. I often read in the newspapers about flow rates through the Caloosahatchee being translated into swimming pool volumes of water, and/or into billions of gallons of water per second. While swimming pools and gallons are common terms for conceptualizing volumes of water, neither are suitable measuring sticks for making comparisons to the Caloosahatchee.

Instead, and since we’re on the cusp of the World Series — imagine being a spectator in the stands at Fenway Park — preferably higher in the upper deck since we’re going to fill it with large amounts of hypothetical water, and from which you would have your best view.

At Fenway Park, the surface area of the field between foul lines is around 2 acres. If you add in the rest of the field outside the foul lines, but still in play, the total field (turf) area is around 3 acres in size. Now imagine placing a vertical column of Caloosahatchee River water in the stadium across that 3 acres of fair-and-foul playing field.

On the higher end of the S79 flow spectrum (think 1995 and 2005) — but lets assume an annual volume of 3 million acre feet to simplify the math, that would fill the field with a 1,000,000 ft tall column of water, or in other terms, a 190 mile high column of water (for the full year). Per day, that translates into filling the 3 acre field with a little over a half mile column of water per day (or 183 skyscraper stories). As a spectator in the stands you’d watch the field fill with water at a rate of 2 ft per minute.

This year’s 100,000 acre-foot flow volume through the S79 is a 25 year low. It would be the baseball equivalent of a low scoring pitcher’s duel — but no less compeling for the baseball purist. This year’s flow rate would fill the same hypothetical playing field with a 6 mile high column of water (over the course of the full year). Per day, this year’s volume translates into filling our 3 acre field to a 91 ft depth per day (or the height of a 6-story building). As a spectator you’d watch the field fill with water at around 2 cm per minute.

This was a very simplified back-of-the-envelope calculation. It does not factor in evapotransporation, or high-water peak flows, but it hopefully scratches at the surface of translating the numbers into a scale that we can comprehend. The mind is notoriously bad at synthesizing large numbers. Unlike the American West which has vast amounts of places to store water, but where water is scarce, South Florida has vast amounts of water, but very little capacity for storing it. Later this winter — nearing Superbowl Sunday — we can make similar comparisons using a football field metrics.

Where does this leave our watersheds?

Regulatory stage in Water Conservation Area 3A is currently around a foot below the 5-year late October average, and about a half foot below late October of last year. The big news is that the S11s continue to discharge at a flow rate of over 1000 cfs into northeast 3A. That’s pushed slough water depths in northern 3A (north of I75 at Site 63) to just over a foot deep. In comparison, slough water depths in southern 3A (near US41) at Site 65 are 2.5 ft deep.

Down in the Park, slough water depths in Central Shark River Slough are just over a foot deep. That’s about a foot below the 5-year average for late October. Around 600 cfs are discharging into the Park across its northern border: 200 cfs through the S12s and 400 through the L29 culverts. An approximately equivalent 600 cfs flow rate is free-flowing under the 35-mile stretch of US41 that traverses Big Cypress National Preserve and feeds into the Park’s western arm.

The story couldn’t be any different in Loxahatchee. It’s received 14 inches of rain since the start of September. That has sent water levels in Loxahatchee to a new 5-year late October high-water mark. Loxahatchee’s regulatory stage of 17.2 ft mean sea level is 7 ft higher than Lake stage. The last time Loxahatchee was at similarly high October water levels was in October of 2000 and 2001 — which is ironic, because both years, as is this year, are associated with South Florida wide droughts. Loxahatchee regulatory stage slightly exceeded the current level in October 1999 and December 1997.

Current stage in Lake Okeechobee is 5 ft lower than the 5-year late October average, and about 2.5 ft lower than October of last year. That’s not bad considering that just a month ago in late September — with us all speculating if the Dry Season had begun — that the Lake was 4 ft lower than late September of a year ago. Somehow we climbed a foot and a half relative to last year thanks to our late season rain.

How does this compare to Big Cypress National Preserve?

Our marshes and swamp forest have been flooded with water for 5 months and running. Preserve-wide water stage has sustained its current level for the past 2 months. That means we fell about a 3-4 inches below our average-annual high water mark, but with a recent dose of October rains, water levels are currently tracking just an inch or so below our 5-year late October average.

Of interest, Turner River has recently dropped low enough to allow canoers and kayakers to paddle through the mid-reach mangrove tunnels.

One of the many mid-reach mangrove tunnels along the Turner River. River stage has to be low enough to navigate under the mangrove ceiling, but deep enough to float a boat. Those conditions are reported in the Turner River summary graphic.

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