Indicator Water Levels in Lake Okeechobee

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1 Indicator. - Water Levels in Lake Okeechobee What is the desired restoration condition? The desired restoration condition for water levels in Lake Okeechobee is to eliminate harmful high and low water levels in the lake that have been documented to cause adverse impacts to plants, fish, and wildlife, while still providing for a seasonal range of water levels that has documented benefits. This multi-faceted goal addresses both extreme water levels that cause acute harm (above 17 feet or below 11 feet above mean sea level [msl]) and prolonged moderate water levels (above 15 feet or below 1 feet above msl for 1 months or more). The desired restoration condition also defines a desired gradual recession of water from a winter high near 15.5 feet to a spring low of near 1.5 feet above msl. The scientific basis for these goals is provided in Aumen and Gray (1995), Smith et al. (1995), and Havens (). Why is this indicator important and why is it a good indicator of CERP restoration? This indicator is important for the following reasons: Water levels in Lake Okeechobee determine suitability of that resource as habitat for fish, wading birds, snail kites, and a variety of other biota. Water levels in the lake are highly responsive to changes in regional hydrology. The current state is one where extreme high and low water levels occur at a frequency that has caused harm to the plant and animal communities. Spring recessions in water level rarely occur as desired, most often because water is too deep to allow recession to successfully dry out the lake s littoral zone. Water levels in the lake are expected to respond strongly to the Comprehensive Everglades Restoration Plan (CERP), as indicated by earlier model runs during restoration planning. Spatial variation is not a factor here because despite its large size, the lake is a single impounded system in which water levels vary simultaneously in all of the regions (pelagic, nearshore, and littoral) with only a minimal time lag. How is the interim goal for this indicator predicted? The five specific attributes of this performance indicator are scored using the standard methods established during the Restudy and currently in use during the Restoration Coordination and Verification (RECOVER) Evaluation Team s process for evaluating alternatives. These methods are described in detail in the Lake Okeechobee Conceptual Ecological Model (Appendix A, RECOVER ) and in a peer-reviewed journal article (Havens ). In brief, the South Florida Water Management Model (SFWMM) is run at 53

2 each five-year increment, taking into consideration the CERP projects anticipated to be on line at those time steps. A 3-year hydrograph is provided for Lake Okeechobee at each time step, and these hydrographs are used to score the performance indicators. The indicators are 1) extreme high water level lake stage in excess of 17 feet; ) prolonged moderate high water level stage in excess of 15 feet for more than 1 months; 3) extreme low water level stage below 11 feet; ) prolonged moderate low water level stage below 1 feet for more than 1 months; and 5) spring recession a desired gradual decline in stage from near 15.5 feet in January to near 1.5 feet in June. The CERP goal for the first four indicators is to have no such events. The goal for spring recession is to have events occurring in nearly every year of the 36-year simulation period (i.e., in >3 years). Counts are made of the number of times that specified extremes are exceeded (or spring recession goals are met). The uncertainty for this set of indicators includes uncertainty of water level estimates for the lake from the SFWMM, and uncertainty regarding how certain variations in water level affect the biological components of the ecosystem. What are the predictions for five-year increments? Predictions were made for the 1995 baseline (95Base), two interim periods during CERP implementation (1 and 15), and for full CERP implementation (D13R). Lake Levels in Excess of 17 Feet The predicted occurrence of lake levels in excess of 17 feet declines by 5 percent in 15 and in the completed D13R plan compared to the 95Base (Figure..1). The occurrence is not reduced to the desired restoration condition of zero. 5 Stage >17 ft (Target = ) Base 1 15 D13R Figure..1. Predictions of lake levels in excess of 17 feet 5

3 Lake Levels in Excess of 15 Feet The predicted occurrence of lake levels in excess of 15 feet for 1 months or more duration drops by over 7 percent in 1, 15, and in the completed D13R plan compared to the 95Base (Figure..). The occurrence is not reduced to the desired restoration condition of 8 Stage >15 ft / 1 mos. (Target = ) 6 95Base 1 15 D13R zero. Figure... Predictions of lake levels in excess of 15 feet Lake Levels Below 1 Feet The predicted occurrence of lake levels below 1 feet for 1 months or more duration drops progressively from 1 to 15 to the completed D13R plan, when the frequency is just 5 percent of that occurring under the 95Base (Figure..3). The occurrence is not reduced to the desired restoration condition of zero. 5 Stage <1 ft / 1 mos. (Target = ) Base 1 15 D13R Figure..3. Predictions of lake levels below 1 feet 55

4 Lake Levels Below 11 Feet The predicted occurrence of lake levels below 11 feet increases in 1, and then declines to less than 5 percent of the 95Base, with the completed D13R plan (Figure..). The occurrence is not reduced to the desired restoration condition of zero Stage <11 ft (Target = ) 95Base 1 15 D13R Figure... Predictions of lake levels below 11 feet Spring Recession Events The predicted occurrence of desired spring recession events increases in 15 and under the completed D13R plan, but never approaches the goal of having 3 or more events in the 36- year model run (Figure..5) Spring Recession (Target >3) 95Base 1 15 D13R Figure..5. Predictions of spring recessions 56

5 Hydrologic Summary The frequency of occurrence of harmful high and low water events is predicted to improve over time with implementation of CERP projects, but is not reduced to the desired restoration condition. The frequency of desired seasonal variation remains much lower than is considered necessary to restore a healthy ecosystem. Optimization of the Lake Okeechobee Regulation Schedule, in concert with construction of D13R projects, might provide better overall hydrologic performance for the lake than is indicated by these model results. How will we track whether the interim goals established for the indicator have been achieved? The stage monitoring network for Lake Okeechobee is detailed in the CERP Monitoring and Assessment Plan: Part 1, Monitoring and Supporting Research (RECOVER ). What additional work is needed to improve this interim goal? At this time, no additional work is necessary to improve this interim goal. References Aumen, N.G. and S. Gray Research synthesis and management recommendations from a five year, ecosystem-level study of Lake Okeechobee, Florida (USA). Archiv fur Hydrobiologie, Advances in Limnology 5: Havens, K.E.. Development and application of hydrologic restoration goals for a large subtropical lake. Lake and Reservoir Management 18: RECOVER.. CERP Monitoring and Assessment Plan: Part 1, Monitoring and Supporting Research. Restoration Coordination and Verification Team (RECOVER), c/o United States Army Corps of Engineers, Jacksonville District, Jacksonville, FL, and South Florida Water Management District, West Palm Beach, FL. Smith, J.P., J.R. Richardson, and M.W. Callopy Foraging habitat selection among wading birds (Ciconiiformes) at Lake Okeechobee, Florida, in relation to hydrology and vegetative cover. Archiv fur Hydrobiologie, Advances in Limnology 5:

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