U.S. Geological Survey Scientific Investigations Report 2004-5073, 49 pages(Published August 2004)—ONLINE ONLY
Lake Seminole is a 37,600-acre impoundment formed at the confluence of the Flint and Chattahoochee Rivers along the Georgia–Florida State line. Outflow from Lake Seminole through Jim Woodruff Lock and Dam provides headwater to the Apalachicola River, which is a major supply of freshwater, nutrients, and detritus to ecosystems downstream. These rivers,together with their tributaries, are hydraulically connected to karst limestone units that constitute most of the Upper Floridan aquifer and to a chemically weathered residuum of undifferentiated overburden.
The ground-water flow system near Lake Seminole consists of the Upper Floridan aquifer and undifferentiated overburden. The aquifer is confined below by low-permeability sediments of the Lisbon Formation and, generally, is semiconfined above by undifferentiated overburden. Ground-water flow within the Upper Floridan aquifer is unconfined or semiconfined and discharges at discrete points by springflow or diffuse leakage into streams and other surface-water bodies. The high degree of connectivity between the Upper Floridan aquifer and surface-water bodies is limited to the upper Eocene Ocala Limestone and younger units that are in contact with streams in the Lake Seminole area. The impoundment of Lake Seminole inundated natural stream channels and other low-lying areas near streams and raised the water-level altitude of the Upper Floridan aquifer near the lake to nearly that of the lake, about 77 feet.
Surface-water inflow from the Chattahoochee and Flint Rivers and Spring Creek and outflow to the Apalachicola River through Jim Woodruff Lock and Dam dominate the water budget for Lake Seminole. About 81 percent of the total water-budget inflow consists of surface water; about 18 percent is ground water, and the remaining 1 percent is lake precipitation. Similarly, lake outflow consists of about 89 percent surface water, as flow to the Apalachicola River through Jim Woodruff Lock and Dam, about 4 percent ground water, and about 2 percent lake evaporation. Measurement error and uncertainty in flux calculations cause a flow imbalance of about 4 percent between inflow and outflow water-budget components. Most of this error can be attributed to errors in estimating ground-water discharge from the lake, which was calculated using a ground-water model calibrated to October 1986 conditions for the entire Apalachicola–Chattahoochee–Flint River Basin and not just the area around Lake Seminole.
Evaporation rates were determined using the preferred, but mathematically complex, energy budget and five empirical equations: Priestley-Taylor, Penman, DeBruin-Keijman, Papadakis, and the Priestley-Taylor used by the Georgia Automated Environmental Monitoring Network. Empirical equations require a significant amount of data but are relatively easy to calculate and compare well to long-term average annual (April 2000–March 2001) pan evaporation, which is 65 inches. Calculated annual lake evaporation, for the study period, using the energy-budget method was 67.2 inches, which overestimated long-term average annual pan evaporation by 2.2 inches. The empirical equations did not compare well with the energy-budget method during the 18-month study period, with average differences in computed evaporation using each equation ranging from 8 to 26 percent. The empirical equations also compared poorly with long-term average annual pan evaporation, with average differences in evaporation ranging from 3 to 23 percent. Energy budget and long-term average annual pan evaporation estimates did compare well, with only a 3-percent difference between estimates. Monthly evaporation estimates using all methods ranged from 0.7 to 9.5 inches and were lowest during December 2000 and highest during May 2000. Although the energy budget is generally the preferred method, the dominance of surface water in the Lake Seminole water budget makes the method inaccurate and difficult to use, because surface water makes up more than two-thirds of the energy budget and errors in measured streamflow can be substantial.
Abstract
Introduction
Purpose and Scope
Study Area
Physiography
Climate
Previous Studies
Well and Surface-Water-Station Numbering System
Acknowledgments
Ground Water
Hydrogeologic Setting
Hydraulic Characteristics
Overlying Semiconfining Units
Upper Floridan Aquifer
Lisbon Formation
Ground-Water Levels
Seasonal Fluctuations
Effects of Drought and Pumping
Surface-Water Influence
Surface Water
Drainage
Streamflow
Dams and Navigational Improvements
Water-Budget Calculations for Lake Seminole
Precipitation
Surface Water
Ungaged Inflow to Lake Seminole
Error Analysis
Lake Storage
Ground-Water Inflow and Lake Leakage
Estimation of Flow Rates
Error Analysis
Lake Evaporation
Energy-Budget Method
Empirical Equations
Priestley-Taylor Equation
Penman Equation
DeBruin-Keijman Equation
Papadakis Equation
Georgia Automated Environmental Monitoring Network
Evaluation of Evaporation Estimates and Methods
Water-Budget Summary
Water-Budget Error
Sensitivity Analysis
Summary
References Cited
Appendix —Methods and Instrumentation
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