Georgia Water Science Center
This report is available online in pdf format (18.4 MB): USGS SIR 2006-5234 ()
L. Elliott Jones and Lynn J. Torak
U.S. Geological Survey Scientific Investigations Report 2006-5234, 83 pages (Published December 2006)
To determine the effects of seasonal ground-water pumpage for irrigation, a finite-element ground-water flow model was developed for the Upper Floridan aquifer in the lower Flint River Basin area, including adjacent parts of the Chattahoochee and Apalachicola River Basins. The model simulates withdrawal from the aquifer at 3,280 irrigation, municipal, and industrial wells; stream-aquifer flow between the aquifer and 36 area streams; leakage to and from the overlying upper semiconfining unit; regional ground-water flow at the lateral boundaries of the model; and water-table recharge in areas where the aquifer is at or near land surface. Steady-state calibration to drought conditions of October 1999 indicated that the model could adequately simulate measured groundwater levels at 275 well locations and streamflow gains and losses along 53 reaches of area streams. A transient simulation having 12 monthly stress periods from March 2001 to February 2002 incorporated time-varying stress from irrigation pumpage, stream and lake stage, head in the overlying upper semiconfining unit, and infiltration rates.
Analysis of simulated water budgets of the Upper Floridan aquifer provides estimates of the source of water pumped for irrigation. During October 1999, an estimated 127 million gallons per day (Mgal/d) of irrigation pumpage from the Upper Floridan aquifer in the model area were simulated to be derived from changes in: stream-aquifer flux (about 56 Mgal/d, or 44 percent); leakage to or from the upper semiconfining unit (about 49 Mgal/d, or 39 percent); regional flow (about 18 Mgal/d, or 14 percent); leakage to or from Lakes Seminole and Blackshear (about 2.7 Mgal/d, or 2 percent); and flux at the Upper Floridan aquifer updip boundary (about 1.8 Mgal/d, or 1 percent). During the 2001 growing season (May–August), estimated irrigation pumpage ranged from about 310 to 830 Mgal/ d, about 79 percent of the 12-month total. During the growing season, irrigation pumpage was derived from decreased discharge or increased recharge of stream-aquifer flux (from about 23 to 39 percent), leakage to or from the upper semiconfining unit (from about 30 to 36 percent), regional flow (from about 8 to 11 percent), Lakes Seminole and Blackshear (about 2 percent), and flux at the Upper Floridan aquifer updip boundary (about 1 percent). Storage effects (decreased storage gain or increased storage loss) contributed from about 11 to 36 percent of irrigation pumpage during the growing season.
Water managers can use the model to determine where and how much additional ground-water pumpage for irrigation should be permitted based on a variety of hydrologic constraints. For example, the model results may indicate that in some critical locations, additional ground-water pumpage during a prolonged drought might reduce stream-aquifer flux enough to cause noncompliance of established minimum instream flow conditions.
Purpose and Scope
Physiography and Drainage
Well and Surface-Water Station Numbering System
Simulated Effects of Seasonal Ground-Water Pumpage
Ground-Water Flow Model Development
Conceptualization of Geohydrologic Flow System
Governing Equations and Simulation Approach
Specified-Head Boundary: Upper Floridan Aquifer Updip Limit
Industrial and Municipal Pumpage and Off-Channel Springflow
Head-Dependent Flux Boundaries
Regional Ground-Water Flow
Flow Across Streambeds
Upper Semiconfining Unit and Lake-Bed Leakage
Upper Floridan Aquifer Characteristics
Ground-Water Flow Model Performance and Analysis
Steady-State Calibration, October 1999
Ground-Water Level Residuals
Ground-Water Flow Direction
Transient Simulation, March 2001—February 2002
Source of Seasonal Ground-Water Pumpage
Summary and Conclusions
This report is available online in pdf format (50 MB): USGS SIR 2006-5234 ()
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