MODFLOW-NWT data sets for simulation of Effects of Surface-Water and Groundwater Inflows and Outflows on the Hydrology of the Tsala Apopka Lake Basin in Citrus County, Florida

The U.S. Geological Survey, in cooperation with the Southwest Florida Water Management District, calibrated a model to quantify the inflows and outflows in the Floral City, Inverness, and Hernando pools of the Tsala Apopka Lake basin in Citrus County, Florida. The calibrated model, which uses MODFLOW-NWT version 1.1.2, simulates hydrologic changes in pool stages, groundwater levels, spring flows, and streamflows caused by the diversion of streamflow from the Withlacoochee River to the Tsala Apopka Lake basin through water-control structures. A surface-water/groundwater flow model was developed using hydraulic parameters for lakes, streams, the unsaturated zone, and the underlying surficial and Upper Floridan aquifers estimated with an inverse modeling calibration technique. After calibration, the model was used to assess the relation between inflows and outflows in the Tsala Apopka Lake basin and changes in pool stages. Simulation results using the calibrated surface-water/groundwater flow model showed that the largest leakage rates from the pools to the Upper Floridan aquifer occurred at the deep lake cells and that these leakage rates were higher than any recharge rates to the Upper Floridan aquifer in the model area. There was downward leakage to the Upper Floridan aquifer in most of the Floral City, Inverness, and Hernando pools. The calibrated surface-water and groundwater flow model was used to simulate hydrologic scenarios that included: small changes in rainfall rates, projected increases in groundwater pumping rates for 2025 and 2035, no flow for the entire period through the eight water-control structures in the Tsala Apopka Lake basin, and the removal of the Inglis Dam and the Inglis Bypass Spillway on Lake Rousseau. This USGS data release contains all of the input and output files for the simulations described in the associated model documentation report (https://doi.org/10.3133/sir20185055).