Water Log 8/5

SFL rainfall above average for July

Week of July 30 – August 5

District-wide, around 8.5 inches of rain fell for the month of July. That’s 1.25 inches above July’s 10-yr average. The Lake received the least amount of July rainfall (around 6.2 inches), with the east Caloosahatchee and Broward Basins leading the way with around 11 inches of July rainfall. Lower summer rains over the Lake are nothing new. Water on a clear summer day is cooler than land — a property of water that stiffles convectional heating, cloud formation, and rainfall over Florida’s larger lakes, with Lake Okeechobee being the biggest and most notable example. In the winter, when rain storms are primarily frontal (not convectional) rainfall over the lake is similar in magnitude to adjacent watersheds.
Interestingly, the Lake’s 6.2 inches may seem low in comparison to other basins, but it was almost a half-inch above the Lake’s 10-yr July rainfall average. In comparison, the 7.7 inches of July rain in Big Cypress National Preserve, 1.5 inches greater than what fell on the Lake, was a half-inch lower than the preserve’s 10-year July average. That just goes to show that the degree to which our rainfall stacks up against the historical picture has just as much to do with where it lands as to how much falls from the sky.

Two things to keep in mind now that we’ve entered into August, the 3rd of our four core Wet Season months (June through September).

First, over the long-term water levels in our watersheds tend to crest towards the end of September or early October. It varies from year to year, some years have crested as early as June (as occurred in 2005) or as late as January (as occurred in January 1995 in the Everglades). In any event, we’re closing in the 2-3 month envelope when high water conditions generally occur.

Second, we’re also entering the second and more active half of the hurricane season. Hurricane researcher William Gray recently revised this year’s tropical storm outlook to include 15 named tropical storms, 8 hurricanes, with 4 developing into intense hurricanes. Only 3 named storms have takens shape so far this year. The big question for coastal communities and water managers is if and where the storms will make landfall. Historically, hurricane-strength storms only infrequently make Florida Landfall during the months of June and July. Both June and July combined account for less than 10 percent of hurricane-strength storms to make landfall in Florida. Our current month (August) accounts for 10 percent of storms. September and October are the big months. Historically, over 50 percent of the hurricane-stength storms to make a Florida landfall have occurred in the months of September and October. September is the highest at 30 percent. The chance at a hurricane-strength storm making Florida landfall diminishes abruptly in November (3 percent) and December (under 1 percent). But keep in mind that late-season tropical storms can contribute significant rainfall (such as Mitch in November 1998). Big Cypress: Big Cypress National Preserve ,Corkscrew and OK Slough . Preserve-wide stage has been tracking 1-4 inches below the 5-year average since the end of June. If you’ll recall, Wet Season rains started heavy right out of the gate (with the help of TS Barry) during the first week of June, causing a relatively quick reflooding of the preserve’s wetlands up above the cypress and into our wet prairies. But its been slow going since the middle of June. The wetting front has inched to the edges of the wet prairies at the base of the hydric pines. Our current wetland depth is around 5 inches below the preserve’s average-annual September high water mark, during which the wetting front floods up into our hydric pines to the base of the mesic pine communities. The northwest corner of the preserve fed by Okaloacoochee Slough remains perplexingly dry compared to rest of the preserve. Wetland stage has yet to break above ground surface even in the deeper slough communities.
Everglades: Everglades National Park ,WCA3,WCA12. Regulatory stage in Water Conservation Area 3A is 9.1 ft mean sea level. That’s about 1.5 ft lower than the 5-year early August average, and over a half foot lower than early August of last year. This is the lowest 3A’s regulatory stage has been this late in the Wet Season (early August) since the drought-years of the late 1980s (1989, 1990) when it hovered below 9 ft msl for most of the Wet Season. In any event, the low upstream stages in 3A have contributed to below normal flows into downstream Everglades National Park, with the S12s currently discharging at under 25 cfs. In comparison to recent history, the S12s has averaged +1,500 cfs over the past 5 Augusts, and has peaked at an average flow rate of just under 3,000 cfs over the past 5 Octobers. Last year, discharge through the S12s peaked at 2,500 cfs in early October, and the year prior peaked at over 4,000 cfs in early September.

Down in the Park, wetland water depth at Shark River Tower is about a half-foot deep. Farther east, in central Shark River Slough (at P33), water depths are 1.25 ft deep which brings the water surface up above the slough and ridge landscape types; but that’s still about 4-5 inches below the 5-yr early August average, and about 11 inches below the Park’s 5-yr early October high-water mark. Down in the Park’s southeast corner, structural discharges into the Taylor Slough headwaters through the S332s have been occurring since late June at a few hundred cfs average discharge rate.

Upstream at the northern Everglades, water is flowing into the Conservation Areas through the S7 pump station (see weekly photo), S6 pump station, and S5A pump station. The connecting gates between Conservation Areas 1, 2, and 3 are still closed. The S11s are stacking up 3.5 ft of water on their headwater side, flooding wetlands on their upstream side with 2.5 ft of water depth. The S10s are currently holding back 4 ft of water. Wetland water depths in central Loxahatchee are currently around a foot deep, which matches its early August 5-yr average, and about 5 inches below its average 5-yr early October high-water mark. In comparison, regulatory stage in Water Conservation Area 2 is currently over a foot below its average 5-yr late September high-water mark.
Lake Okeechobee: Lake O ,Kissimmee. Lake stage has risen to 9.4 ft mean sea level. That puts it about a 1.7 ft lower than early August of the 2001 drought year. A steady weekly inflow of 1,500 cfs from the Kissimmee has helped boost Lake stage, but other contributions from Lake Istokpoga and its downstream canal network, Fisheating Creek, and direct rainfall combined for a weekly average of around 8,000 cfs entering the Lake, not even taking into account backflows throught L8 Canal Point, Port Mayaca, or the S77 Moore Haven Lock. Over the past 5 wet seasons, the Lake has reached an average early October high water mark of 16 ft msl, which is around 6.5 ft higher than the Lake’s current level. The Kissimmee River is actually flowing into the Lake at its 5-yr average for early August, but still down from its 5-yr average September crest discharge of +5,000 cfs. But keep in mind that high September average was boosted by +10,000 cfs flows in September 2003 and 2004.
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