Lake Okeechobee’s level continued to recede last week, primarily due to evapotranspiration.
On Friday, Sept. 2, the lake level was 12.56 feet above sea level, compared to 12.62 feet seven days prior. The average lake level for Sept. 2, based on the 1965-2007 levels, was 14.22 ft. The top of the Water Shortage Management Band for this date, as set in the Lake Okeechobee Regulation Schedule in 2008 (LORS08) is 12.4 feet.
No lake water was released east or west last week.
To the east, no lake water has been released at Port Mayaca since April 2021.
To the west, no lake water was released because local basin runoff was sufficient to exceed the minimum flow at the Franklin Lock. For the seven day period ending Sept. 2 (Aug. 27-Sept. 2), flow through the Julian Keen Jr. Lock at Moore Haven was 0 cubic feet per second. Flow through the Franklin Lock, which is 43 miles from Moore Haven, was 3,445 cfs, all from local basin runoff. Most of the rain fell west of the Ortona Lock. Flow at the Ortona Lock was just 895 cfs. Some permitted water users draw water from the river for irrigation. Thanks to runoff from local rainfall, water levels in the river were sufficient that no water was needed from Lake O.
The South Florida Water Management District has set the minimum target flow at the Franklin Lock at 457 cfs. Flows below 450 cfs (measured at the Franklin Lock) are considered harmful as the salinity levels in the estuaries are too high. Flows higher than 2,800 cfs are also considered harmful because the salinity levels drop too low. The ideal beneficial freshwater flow at the Franklin Lock advocated by estuary scientists is around 1,000 cfs, but the ideal salinity level varies for different parts of the estuaries.
Flow south for water supply and environmental uses averaged 352 cfs. (for a total of 4,887 acre feet or 1.6 billion gallons of water.
South of the lake, the Water Conservation Areas (WCAs) are all below schedule for this time of year.
According to the South Florida Water Management District data, for the period of Aug. 22-Aug. 28, surface water inflows to Lake Okeechobee totaled 6,470 acre feet (2.1 billion gallons of water). Direct rainfall into the lake was 31,960 acre feet (10.4 billion gallons). Flows south totaled 12,470 acre feet (4 billion gallons). Flows west to the Caloosahatchee River totalled 1,580 acre feet (0.5 billion gallons). Evapotranspiration (a combination of evaporation and plant transpiration) removed 30,760 acre feet of water (10 billion gallons) from the lake.
For that same period, the WCA 2A and 3A had 4,470 acre feet (about 1.5 billion gallons) of total inflows from the stormwater treatment areas and 89,560 acre feet of direct rainfall (29.2 billion gallons). Flows south to Everglades National Park totaled 20,600 acre feet (6.7 billion gallons). Evapotranspiration accounted for 50,830 acre feet of water loss (16.6 billion gallons).
Lake Okeechobee, at 730 square miles, is the second largest freshwater lake within the continental United States. On average, most of the water that leaves the lake does so through evapotranspiration. One inch on Lake Okeechobee is about 12 billion gallons of water.
Likewise, the STAs and WCAs south of the lake also lose most of their water through evapotranspiration. Evapotranspiration is even more prevalent in the STAs and WCAs because they are shallow and filled with plants.
The STAs cover about 61,600 acres or 96 square miles.
The A-1 Flow Equalization Basin (FEB) is about 15,000 acres or 23 square miles. The FEB optimizes STA treatment performance.
WCA-1 is approximately 134,000 acres or about 210 square miles.
WCA-3 is approximately 586,000 acres or 915 square miles.
“Evapotranspiration from Lake Okeechobee, the Stormwater Treatment Area (STAs)s, the Water Conservation Areas or any other water body in South Florida is impacted by several factors,” explained Jason Schultz of SFWMD. “One factor is the amount and type of vegetation present in the water. Evapotranspiration is a combination of evaporation into the air and transpiration of water being absorbed by plants. The WCAs and STAs typically have much more vegetation and much more emergent vegetation above the water than Lake Okeechobee, which tends to have more submerged aquatic vegetation. Therefore evapotranspiration rates are typically higher from WCAs and STAs than from Lake Okeechobee.
“Water temperature impacts evapotranspiration rates and the depth of water bodies impacts how much sunlight can penetrate and heat water,” he added. “WCAs and STAs are typically much shallower and the water is typically warmer than Lake Okeechobee, therefore they typically have higher evapotranspiration rates than the lake.”