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In the karst terrain of Citrus and Hernando Counties in Florida, spatial variations in hydraulic connectivity between the stream or lake beds and the Floridan aquifer affect surface and subsurface water-level and flow conditions. The Tsala Apopka Lake pools at Floral City, Inverness, and Hernando in Citrus County must be maintained within a range of specified stages to meet human and environmental needs. Changes in rainfall patterns also affect the fraction of the flow in the Withlacoochee River that could be used for diversion through water-control structures to these pools. Available simulation tools that can analyze the temporal changes in pool stages due to flows of various magnitudes through the structures vary in complexity and a rather simple surface-water/groundwater simulation is proposed based on the lake, streamflow routing, and the unsaturated zone packages of MODFLOW-NWT. Simulated stages at the three pools for 2004 were within 0.5 ft of the measured stages and simulated infiltration rates between the stream and lake beds and the aquifer were calculated to be about 25 percent of annual rainfall. The conceptual flow model, the inverse modeling calibration for 2004 hydrologic conditions, and the application of the calibrated parameters to simulate hydrologic conditions from 2004 to 2012 for these three pools are presented in this study. Flows through the structures are specified as multiples of the estimated flows using existing rating curves for the structures. The capability of the MODFLOW-NWT packages to simulate complex surface-water/groundwater interactions such as the effects of the flows through structures on the pool stages in Citrus County, makes it an effective simulation tool. The merits and limitations of this modeling approach are recognized and were appropriately considered to attain a more comprehensive understanding of the hydrologic system and to optimize the effectiveness of surface-water management practices. |