Drought continues but rain clouds beckon
SFL Weekly Watersheds Summary: May 7 – May 13

WEATHER.

The severity of this year’s drought is most pronounced in the northern reaches of the greater Kissimmee-Okeechobee-Everglades (KOE) watersheds. The northern stretches of the KOE system — including the Kissimmee, Okeechobee, Upper East Coast, and Everglades Agricultural Area have all received less than 35 inches over the past 365 days. That’s very similar in severity to the 2000-2001 drought. Rains have been more abundant to the south. Several major basins in the southern end of the peninsula — including the Southwest Coast, Big Cypress NP, and Miami-Dade — have received slightly over 50 inches over the past 365 days, with Water Conservation Area 3 receiving around 45 inches over the same period. That makes this year noticeably rainier than the drought of 2000-2001 when Big Cypress NP only received a paltry 40 inches of rain over a similar 365 day time span. That’s a full foot less of water compared to 2000-2001. Interestingly, Miami-Dade received a relatively robust 50 inches of annual rain during both this year’s drought year and the 2000-2001 drought year.

That goes to show that rainfall distribution is just as important as the lump sum amount. And don’t forget the role that big storms like Ernesto can locally and sub-regionally boost annual totals, but in some cases those short-duration deluges may not reflect what is truly soaked into the landscape since big portions are quickly swept out to sea. Also at work in adding to this year’s drought severity was the early end of this year’s rainy season. This year’s dry season got a 4-5 week head start right out of the gate by merit of the rainy season fizzling to an early end in late September and the absense of late-season tropical rains in the Fall. Hurricane forcasters mid-year reversal in the tropical season outlook was a welcome surprise for Florida residents, but proved to be a double edged sword for water planners who had strategically lowered the Lake in Spring of 2006 in preparation for the early-season outlook of yet another hyper-active hurricane season. By mid-summer, however, a burgeoning El Nino raised just enough higher altitude westerly wind sheer to repress tropical storms from forming. That was the good news.

The bad news was that the El Nino died on the vine by mid winter — and never brought the above-average winter season rains that could have reversed the southern peninsula’s 18-month drying trend. That brings us to our current condition. Forecasters are calling for an especially active hurricane season this year (producing up to 17 tropical storms and hurricanes), and formation of a La Nina condition that could stiffle summer rains. It’ll be a wait-and-see game as usual.

BIG CYPRESS.

With all the fires raging in the Big Cypress, there’s been some discussion on when the wet season will finally kick in.

In terms of cloud formations, the meteorological wet season starts around the third week of May when night-time low temperatures finally break above 70° F. That’s marks the official return of regular afternoon showers. The weekly average night-time low at Naples Airport has climbed up to 68° F as of Sunday.

In terms of rainfall, May has averaged 4 inches of rain over the past 10 years. In 2003 over 8 inches of rain fell in May. That ushered in an early start to the wet season (in fact their was no drydown below the swamp forest and marsh habitats for that year). June is really the difference maker in terms of flipping the wet season switch. June has averaged a whopping 10.2 inches of rain over the past 10 years. That’s 1.5 inches higher than the average of the other 3 core months of the rain season (July, August, September). A whopping 20 inches of rain fell on the Big Cypress in June of 2005. Last June only 8.3 inches fell.

Probably the most direct way to define the start of the wet season is by simply looking at the ground. Last year (2006) it took until the very end of June for the surface water wetting front to rise into the wet prairies. The year before (in 2005) heavy June rains flushed water into the wet prairies by early June. Dates of wet prairies flooding for years prior to 2005 are as follows: late July (2004), late May (2003), mid June (2002), mid July (2001), mid August (2000), mid July (1999), mid July (1998), mid June (1997), late May (1996), never dropped below wet prairie level (1995), late June (1994).

Of course the above dates are a simplification. On many of those years (1999, 2000, 2001, 2004) surface water hovered in the swamp forest and tall cypress swales for several weeks before the wetting front finally broke into the wet prairies (think really bad mosquito seasons). During other years (2005, 2006) heavy deluges rapidly shot water from the underlying aquifer into the wet prairies within a week’s period (they’ve been among our lighter mosquito seasons). Based on past history, it will probably be somewhere between late May and late July when surface waters flush across the preserve at the wet prairie level.

EVERGLADES.

Regulatory stage in WCA is about 1 ft below its mid-May 5-year average, and about a half foot lower than mid May of last year. Slough water depths in southern 3A have now shallowed to 1 ft deep. That’s about the same depth that they bottomed out between mid-May and mid-June of last year. The lowest water mark in recent history for southern 3A was in May 2001 when slough water depths (at Site 65) dropped to around a half foot deep. Sloughs in southern 3A went completely dry in May and June of 1989. That translates into an 18-year hydroperiod. In comparsion, sloughs in northern 3A have been dry for 2 months now — which translates into an 8 month hydroperiod for the previous year.
Up in Loxahatchee, water depths in its central sloughs are approaching zero. In comparison, central sloughs of WCA2 have been dry for 2.5 months now (since early March). Interestingly, water levels down the Park have stablized since the end of March due to some abundant April rains. Central Taylor and Shark River Slough are still holding slough water depths of about a half foot.

LAKE O.

Lake O stage is about 4 ft below the 5-year average for mid May, and about 3.5 ft below mid-May of last year. The Lake bottomed out last year in late June at 12 ft msl. This year the Lake dropped below 12 ft msl in early January. So its now been 4 months that the Lake has dropped below last year’s low water mark (and running). Current Lake O stage (as of Sunday) is 9.32 ft msl, which is virtually identical to its mid-May level during the drought of 2000-2001. The Lake bottomed out at 8.97 ft msl in late May of 2001, and stayed below 10 ft msl for 3 months from mid April to mid July of 2001. Currently the lake has been below 10 ft msl for a little over a month.

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