The Lake Wales Ridge is an uplands recharge area in central Florida that contains many sinkhole lakes. Below-normal rainfall and increased pumping of ground water have resulted in declines both in ground-water levels and in the water levels of many of the ridge lakes. A digital flow model was developed for a 3,526 square-mile area to help understand the current (1990) ground-water flow system and its response to future ground-water withdrawals.

The ground-water flow system in the Lake Wales Ridge and adjacent area of central Florida consists of a sequence of sedimentary aquifers and confining units. The uppermost water-bearing unit of the study area is the surficial aquifer. This aquifer is generally unconfined and is composed primarily of clastic deposits. The surficial aquifer is underlain by the confined intermediate aquifer and confining units which consists of up to three water-bearing units composed of interbedded clastics and carbonate rocks. The lowermost unit of the ground- water flow system, the confined Upper Floridan aquifer, consists of a thick, hydraulically connected sequence of carbonate rocks. The Upper Floridan aquifer is about 1,200 to 1,400 feet thick and is the primary source for ground-water withdrawals in the study area.

The generalized ground-water flow system of the Lake Wales Ridge is that water moves downward from the surficial aquifer to the intermediate aquifer and the Upper Floridan aquifer in the central area, primarily under the ridges, with minor amounts of water flow under the flatlands. The water flows laterally away from the central area, downgradient to discharge areas to the west, east, and south, and locally along valleys of major streams. Upward leakage occurs along valleys of major streams.

The model was initially calibrated to the steady-state conditions representing September 1989. The resulting calibrated hydrologic parameters were then tested by simulating transient conditions for the period October 1989 through 1990. A final test of model calibration was conducted by successfully simulating transient conditions for the period October 1988 through September 1989. Altitudes of the water table, base of the surficial aquifer, riverbed conductances, confining-unit leakances, aquifer transmissivities, and net recharge and discharge rates were determine during calibration.

Steady-state and transient simulations reasonably approximated measured aquifer heads and lake levels. Residuals were within the established calibration criteria that required 68 percent of all simulated heads to be within + - 2 feet of observed surficial aquifer heads and lake levels and + - 5 feet of observed intermediate and Upper Floridan aquifer heads. Simulation of streamflow was poor, probably due to the scale of the model and regulated streamflow conditions. Simulation indicates a marked difference between the ground-water flow rates of September 1989 (steady-state conditions, end of wet season) and May 1990 (large pumpage, end of dry season) in million gallons per day: September May 1989 1990 Pumping rate 126 486 Donward leakage (into 367 564 Upper Floridan aquifer) Streamflow 67 13 Net lateral boundary flow 218 115 Total discharge (excluding 479 626 evapotranspiration.

The calibrated flow model was used to simulate the short-term (one year) effects of 1990 water year pumpage (349 Mgal/d) on the September 1989 ground- water flow system in response to five different pumping schemes: (2) no pumpage, (2) no public supply pumpage, (3) no industrial pumpage, (4) no agricultural pumpage, and (5) no regional pumping outside the Water Use Caution Area. Simulation of no pumpage indicated maximum aquifer head rises of about 2 feet in the surficial aquifer and lakes, about 12 feet in the intermediate aquifer and about 16 feet in the Upper Floridan aquifer. The high rate recharge areas along the Lake Wales Ridge are most affected by pumping. Simulation of no agricultural pumpage resulted in a maximum recovery of about 2 feet in each aquifer. Simulation of no industrial or mining pumpage resulted in a maximum of less than one foot in the surficial aquifer and lakes, about 10 feet in the intermediate aquifer, and about 14 feet in the Upper Floridan aquifer. Simulation of no public supply pumpage indicated a maximum recovery of less than one foot in the surficial aquifer and lakes, about 4 feet in the intermediate aquifer, and about 10 feet in the Upper Floridan aquifer. Simulation of no regional pumping outside the Water Use Caution Area indicated recoveries of less than 2 feet within the Water Use Caution Area.

Simulations were used to investigate long-term aquifer changes in response to two development alternatives: (1) continuation of 1990 water year hydrologic conditions and pumping rates (349 Mgal/d), and (2) increased pumpage (506 Mgal/d). Simulation of continued 1990 water year hydrologic conditions and pumping for 20 years indicated that head decline of more than 10 feet might be expected in each aquifer in the northern part of the Water Use Caution Area. Simulation of increased pumpage (an additional 45 percent) for 20 years indicated head declines of more than 20 feet in each aquifer in the northern part of the Water Use Caution Area. Because lakes are hydraulically connected to the surficial aquifer, lake levels within the Water Use Caution Area could decline substantially as a result of present and future pumping and a continuation of 1990 hydrologic conditions. These relatively large head declines were accompanied by decreased simulated lateral boundary outflow of about 40 percent and decreased simulated streamflow of about 32 percent. Equilibrium conditions at the end of the two 20-year simulations had not been attained.