Water Log 8/19

Lowest August in 17 years for many watersheds

Week of August 13 – 19

Here we are in the middle of the wet season, just a few weeks shy of our traditional high-water mark month of September, but several of our watersheds remain perched down at unusually low levels.

Water Conservation Area 3A is at a 17-yr low-water mark for August. At first glance that doesn’t seem to square with upstream WCAs 1 and 2, which are both tracking in tune with 5-yr August averages, but both the S10s and S11s remain closed, with over 3.5 ft of water stacking behind the structures. That and subpar rains have dropped WCA3 to its lowest level since the 1989-1990 drought years. As dry as 3A is, its current regulatory stage is still around 1 ft higher than August 1989 when 3A’s regulatory stage was at 8 ft msl. Interestingly, not even spring drydown’s (May) rarely have dropped down that low in 3A over the past 15 years. Suffice it to say that 1989 was a very dry year.

Not surprisingly, downstream Everglades National Park (at P33) has also dropped to a 17-year low for the month of August. Slough water depth at P33 is currently around 1.25 ft deep. That’s only a few inches below last August, but a full foot shallower than August 2005. One has to go all the way back to 1989 and 1990 when August slough depths at the Park’s P33 were shallower. 1989 and 1990 were anomolously low-water years for the Park. Slough water depth at P33 didn’t exceed a foot deep for the entirety of the summer for both years. In comparison, last year, also a dry summer by historical standards, P33 stage peaked at a mid-October 1.75 ft water depth.

The common denominator for 1989, 1990, and 2007 is an absence of headwater flows through the S12s. 1989 was an especially low year. Combined S12 flows that year peaked at a miniscule 250 cfs for only a few week period at the end of October. Compare that to last year, which was by no means a chart-topper, when combined S12 flows peaked at over 2000 cfs, and exceeded 500 cfs for a 3-month span on either side of that peak. 2005 was the biggest year in recent memory, peaking at 4000 cfs flows, and sustaining 500+ flows for a 7-month continuous span. This year seems to be shaping up like 1989. So far this summer, flows through the S12s have yet to exceed 50 cfs.

Corkscrew Swamp is also at its lowest August level since 1990. For the past couple years, August was already splashing a foot deep of water in the swamp forest habitat along the boardwalk. This year, water levels are over a foot lower, which means that water remains fairly limited in the low-water refugia pools along the boardwalk and in the central marsh.

That brings us to Lake Okeechobee. 1989 and 1990 were on the dry end of the spectrum for the Lake, but August Lake stage in 1989 and 1990 was 1.8 and 2.5 ft higher than this year’s August. This year’s Lake stage has rebounded around 7-8 inches from its late June low-water mark, but at 9.5 ft msl the Lake is still at the same stage it was in middle April. Lake stage has now been below 10 ft msl for 4 consecutive months. In 2001 it stayed below 10 ft msl for a 3-month span, but rose to 12 ft msl by the end of August, and peaked at 15 ft msl in late November of that year, thanks in part to heavy (10 inch) 2001 September rainfalls and subsequent inflows from the Kissimmee Basin which spiked above 6,000 cfs and flowed at 1000+ cfs through November of that year. So far this year, 500-1500 cfs flows have been flowing into the Lake for 4 consecutive weeks now, but the Lake has only received a paltry sub-4 inches of rain over the past 30 days.

This is the driest August we’ve seen in the Big Cypress National Preserve since 2000. Preserve-wide stage is currently 4 inches below the 5-yr August average, and 6 inches below our average September high-water mark. Flows under US41 are at a new 5-yr August low, currently flowing at around 500 cfs in comparison to the 5-yr 2000 cfs August average. The southern half of the preserve south of US41 and Loop Road is our wettest area, which is currently holding water depths around 4-6 inches deeper than near I75. The northeast corner of the preserve near Okaloacoochee Slough continues to remains unusually dry. Wetland water depths have yet to break across the marsh plane other than in the deeper pools of low-water refugia. That places stage around 1.5-2 ft lower than the 5-yr August average, and about 1.5 ft lower than the central preserve.
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