A digital model of two-dimensional ground-water flow was used to predict changes in the potentiometric surface of the Floridan aquifer, 1976 to 2000, in a 5,938-square-mile area of west-central Florida. In 1975, ground water withdrawn from the Floridan aquifer for irrigation, phosphate mines, other industries, and municipal supplies averaged about 649 million gallons per day. Rates are projected to increase to about 840 million gallons per day by 2000. The model was calibrated under steady-state and transient conditions. Input parameters included transmissivity and storage coefficient of the Floridan aquifer; thickness, vertical hydraulic conductivity, and storage coefficient of the upper confining bed; altitudes of the water table and potentiometric surface; and ground-water withdrawals. Simulation of May 1976 to May 2000, using projected combined pumping rates for municipal supplies, irrigation, and industry (including existing and proposed phosphate mines), resulted in a rise in the potentiometric surface of about 10 feet in Polk County, and a decline of about 35 feet in parts of Manatee and Hardee Counties. The lowest predicted potentiometric level was about 30 feet below sea level. Predicted declines for November 1976 to October 2000 were generally 5 to 10 feet less than those for May 1976 to May 2000. (USGS)
