Simulation of steady-state ground water and spring flow in the upper Floridan aquifer of coastal Citrus and Hernando Counties, Florida

Abstract

A digital ground-water flow model was developed to approximate steadystate, predevelopment flow conditions in the Upper Floridan aquifer of coastal west-central Florida. The aquifer is the major source of public water supply and natural spring flow in the area. The aquifer was simulated as a one-layer system with constant vertical recharge and discharge rates. A head-dependent drain function was used to simulate spring flow. Model calibration consisted of adjustments of aquifer transmiss ivit ies and recharge-discharge rates until the average absolute error per grid block was less than 3 feet and computed spring discharge was within 10 percent of measured or estimated discharges. Calibration transmissivities ranged from 8,640 feet squared per day in the northern part of the area to nearly 13,000,000 feet squared per day near large springs. Calibration inflows were about 2,700 cubic feet per second. Of this, about 2,567 cubic feet per second discharges as natural spring flow and 137 cubic feet per second discharges as upward leakage along the coast. A sensitivity analysis indicated that the model was most sensitive to changes in transmissivity and least sensitive to changes in upward leakage. The model was used to demonstrate aquifer response to large manmade stresses. Withdrawing 116 cubic feet per second from hypo the tical regional well fields resulted in potentiometric-surface drawdowns ranging from 0.1 to 1.7 feet and a drawdown of generally less than 0.2 foot along the coast. Total spring flow decreased about 5 percent, and the change to individual spring discharge varied from 0.1 to 8.0 percent of predevelopment discharge. Withdrawing 62 cubic feet per second from each of the 4-square-mile spring nodes resulted in six of the seven springs to the south of Chassahowitzka River contributing 50 percent of their flow to pumpage and three contributed 100 percent of their flow to pumpage. Springs located north of Chassahowitzka River contributed as much as 18 percent of their flow to pumpage.