ABSTRACT

In November 1997, the U.S. Geological Survey, in cooperation with the City of Houston Utilities Planning Section and the City of Houston Department of Public Works & Engineering, began development of a numerical finite-difference model (MODFLOW) to better understand the hydrology, flow, and land-surface subsidence in the Chicot and Evangeline aquifers.

The study area covers 18,100 square miles. The model was vertically discretized into three 103-row by 109-column layers resulting in a total of 33,681 grid cells. Layer 1 represents the water table using a specified head, layer 2 represents the Chicot aquifer, and layer 3 represents the Evangeline aquifer.

The years 1977 and 1996 were chosen as potentiometric-surface calibration periods during transient model calibration. The simulated and measured potentiometric surfaces of the Chicot and Evangeline aquifers for these years match closely.

Simulation of land-surface subsidence and water released from storage in the clay layers was accomplished using the Interbed-Storage Package of the MODFLOW model. Land-surface subsidence was calibrated by comparing simulated long-term (1891-1995) and short-term (1978-95) land-surface subsidence with published maps of land-surface subsidence for about the same period until acceptable matches were achieved.

Simulated 1996 aquifer flow rates indicate that water simulated from clay storage is about 19 percent of the total water withdrawn from the Chicot aquifer, but only about 10 percent of the total water withdrawn from the Evangeline aquifer is simulated from clay storage.