Wet season limps into September
Week of Aug 27 – Sep 12
Summer rains were scarce in August. August District-wide rainfall totals chimed in at just below 5 inches, which is almost 3 inches below the 10-yr August average. So far this Wet Season, around 24 inches has fallen since the start of May. That’s 3 inches below the 10-yr average for the 4-month May through August span. Last year wasn’t much different, with 25 inches falling during the same period. Don’t forget that last year’s Wet Season was similarly on the dry side until dual drenchings from Ernesto (in late August) and by +4 inch weekly rainfall in early September. Otherwise last year’s Wet Season, from May through October, would have accumulated under 30 inches of Wet Season rain, instead of the 32 it ended with.
How does this year compare to the normal Wet Season rainfall? Over the past 10 years the District has averaged 39 inches of rain over our rainy half of the year (from May through October). That means we need to accumulate 15 inches over the next 9 weeks to work our way up to an average Wet Season total.
The rainiest Wet Seasons in recent memory were 46 inches in 2005, 45 inches in 2001, 47 inches in 1999, and 51 inches in 1995. The 45 inches of Wet Season rain in 2001 was especially significant since it provided a much needed boost out of the then record setting 2000-2001 drought. Our current Wet Season rainfall deficit remains an extension of the 22-month dry spell that has pervaded the region since November 2005.
Big Cypress National Preserve is leading all other South Florida basins with almost 7 inches of rain over the past 30 days. That’s actually below a below average August rainfall total for the Big Cypress area (9 inches is average) but with the District as a whole averaging 4 inches over the same 30 day period, and the Lake at 2.5 inches — 7 inches makes Big Cypress the wettest of all the watersheds. Preserve-wide stage is currently tracking around 3 inches below the 5-yr average for the start of September.
The southern half of the preserve, approximately from US41 south, remains over 6 inches deeper than to the north, where the deficit of summer rains has put the brakes on sheetflow contributions across the Tamiami Trail. Flows under the trail are typically rising to their late season peak of 3000 cfs in September, but this year flows are still floundering at 200 cfs, and have been tracking at a new 5-yr Wet Season low since early August. Surface water has yet to break above the plane in the preserve’s northwest corner. Last year that area reflooded by early July, and the year before by early June. As dry as the preserve’s NE corner is, the preserve’s southeast end is tracking at normal wet season levels.
Corkscrew Swamp Sanctuary enters September in uncharted (and below ground) Wet Season territory. One has to go all the way back to 1990 to find similarly dry conditions this late into the Wet Season, and before that to 1970 — which according to the data I’m looking at, was the year without a Wet Season at the Sanctuary. Surface water never made it out of the deepest of deep-water refugia that year, which is very similar to this year’s Wet Season to date. Last year, thanks to Ernesto, swamp forest habitat was flooded with 2 ft of water by mid September, which brought the wetting front into the mesic pineland habitat.
Regulatory stage in 3A remains at a 17-yr low for early September — at 9.1 ft mean sea level. You have to go all the way back to 1989 and 1990 to find similary low September levels. Prior to the 1990s, September regulatory stage remained at or below our current September level every 3 to 4 years. So this year’s low water levels look dry by recent standards, but is not uncommon in the decades prior to 1990. Last year’s early September stage was over a foot higher at 10.3 ft msl, and peaked at 11.2 ft msl in early October. In 2005, regulatory stage was 11.6 in early September, and stayed above 11 ft msl for a whopping 5 consecutive months from early July to early December. This year has yet to break above 9.5 ft msl. Slough water depths in southern 3A, just north of the Trail, are around 1.5 ft deep. In comparison, slough water depths in northeast 3A north of Alligator Alley are around 0.2 ft deep.
Down in the Park, absence of headwater inflows and low August rainfall levels have dropped water stage in central Shark River Slough to a new 5-yr low for early September. Keep in mind that September water stages routinely dropped at or below the current level prior to the 1990s, but relative to the past 17 years, this is a notably dry start of September in the Park. Water depth in the sloughs surrounding hydrologic monitoring station P33 (in the heart of Shark River Slough) is around 1 ft. That’s over a half foot shallower than early September of last year, and 1.5 ft shallower than September 2005.
This year’s absence of August flows through the S12s is also a departure from August of recent past. Even during Augusts of 2000, 2004, and 2006 when August flows through the S12s were low — that still equated to a rate of a few hundred cfs. In comparison, this year’s flows have largely been zero. During the low-flow Augusts of recent memory, the S12s peaked at around 1500 cfs, +4000, and 2000 cfs in 2000, 2004, and 2006 respectively. It will be interesting to see if, and how much, water peaks through the S12s this year. Headwater flows into the park from Big Cypress, L29 Culverts, and the S332s (Taylor Slough Headwaters) have also been down for the month of August. ENP is in the same boat as its headwater preserve this year, relying more or less completely on rainfall.
The S11s and S10s also remain closed, and are both pooling up around 3.5 ft of water. Regulatory stage in both WCA2 and Loxahatchee have dropped around 5 inches below the 5-yr average for early September. Current slough water depth in central WCA at Site 2-17 is currently just under a foot, and about a foot lower than early September of last year (in the wake of Ernesto). For 15 year period from 1965 to 1980, regulatory stage in WCA2 was maintained at a much higher level. Early September slough water depths routinely topped above 2 ft in that era. But the regulation schedule was modified to better sustain ecological and water management conditions in the basin. That makes this year’s start of September for WCA2 dry in comparison to the last 5 years and in comparison to 1965 to 1980, but more or less an average September for the 20-year span from 1980 to 2000.
Around 500 cfs continue to flow into Lake Okeechobee from the Kissimmee River (S65E), but keep in mind that early September flow rates through the S65E have averaged around 5000 cfs over the past 5 years. That average is skewed however by high-flow spikes of +10000 cfs flows in Septembers of 2003 and 2004. Last year flows through the S65 peaked at around 3500 cfs. 2006 was the first year since 2000 that weekly flow rates through the S65E didn’t peak above 5000 cfs. This year’s weekly discharge peak to date has been 1500 cfs. Flows down the Caloosahatchee through the S79 have yet to peak above a weekly discharge rate of 500 cfs, and if that trend continues this will be the first year since 2000 that S79 did not exceed a 5000 cfs weekly discharge rate.
In summary, September typically our high-water point of the water cycle. This years record low start of September levels is the accumulation of a rainfall deficit that’s been adding (or this case subtracting) since November 2005, ironically right after Wilma put an exclamation point at the end of a very wet and prolonged 2005 Wet Season. Who knew at the time that would also mark the beginning of a drought.
Our current situation heightens the importance of rainfalls in September and October. 11 inches of September rain fell in 2004, and 12 inches of September rain fell in 2001. 8 inches of October rain fell in 2005 (Wilma) and 1999 (the lesser known Katrina, Irene, Harvey), and a whopping 11 inches of October rain fell in 1995. On the other hand, the District received a combined total of 7 inches of rain in September and October of last year.
Hopefully we’ll end this Wet Season on a rainier (but not windier) note.