The hydrogeology of Hardee and De Soto Counties in west-central Florida was evaluated, and a ground-water flow model was developed to simulate the effects of expected increases in ground-water withdrawals for citrus irrigation on the potentiometric surfaces of the intermediate aquifer system and the Upper Floridan aquifer. In 1988, total citrus acreage in Hardee and De Soto Counties was 89,041 acres. By the year 2020, citrus acreage is projected to increase to 130,000 acres. Ground water is the major source of water supply in the study area, and 94 percent of the ground-water withdrawn in the area is used for irrigation purposes. The principal sources of ground water in the study area are the surficial aquifer, the intermediate aquifer system, and upper water-yielding units of the Floridan aquifer system, commonly referred to as the Upper Floridan aquifer. The surficial aquifer is a permeable hydrogeo1ogic unit contiguous with land surface that is comprised predominately of surficial quartz sand deposits that generally are less than 100 feet thick. The intermediate aquifer system is a somewhat less permeable hydrogeologic unit that lies between and retards the exchange of water between the overlying surficial aquifer and the underlying Upper Floridan aquifer. Thickness of the intermediate aquifer system ranges from about 200 to 500 feet and transmissivity ranges from 400 to 7,000 feet squared per day. The highly productive Upper Floridan aquifer consists of 1,200 to 1,400 feet of solution-riddled and fractured limestone and dolomite. Transmissivity values for this aquifer range from 71,000 to 850,000 feet squared per day. Wells open to the Upper Floridan aquifer. the major source of water in the area, can yield as much as 2,500 gallons of water per minute. The potential effects of projected increases in water withdrawals for citrus irrigation on groundwater heads were evaluated by the use of a quasi-three-dimensional, finite-difference, ground-water flow model. The model was calibrated under steady-state conditions to simulate September 1988 heads and under transient conditions to simulate head fluctuations between September 1988 and September 1989. The calibrated model was then used to simulate hydraulic heads for the years 2000 and 2020 that might result from projected increases in pumpage for citrus irrigation. The model simulation indicated that increased pumpage might be expected to result in: A maximum decline of more than 10 feet in theintermediate aquifer system at a proposed grove in eastern De Soto County and an average decline of more than 2 feet in much of the study area. An increase in downward leakage to the intermediate aquifer system from the overlying surficial aquifer system from 178 to 183 million gallons per day. A decrease in upward leakage from the intermediate aquifer system to the surficial aquifer from 1.58 to 1.47 million gallons per day. A maximum decline of about 5 feet in the Upper Floridan aquifer at a proposed grove in eastern De Soto County and a decline of more than 2 feet in much of the model area. An increase in downward leakage to the Upper Floridan aquifer from the intermediate aquifer system from 180 to 183 million gallons per day. A decrease in upward leakage from the Upper Floridan aquifer to the intermediate aquifer system from 4.32 million gallons per day in 1989 to 3.89 million gallons per day in the year 2,000. but an increase in upward leakage to 5.10 million gallons per day by the year 2020, reflecting a change in hydraulic gradient.