Calibration and sensitivity analysis of a ground-water flow model of the coastal lowlands aquifer system in parts of Louisiana, Mississippi, Alabama, and Florida

Abstract

Listen

Translate article

The coastal lowlands aquifer system, consisting of aquifers in sediments of Miocene age and younger in southern Louisiana, Mississippi, and Alabama and in western Florida, is being studied as part of the U.S. Geological Survey's Gulf Coast Regional Aquifer-System Analysis program. This report describes the calibration and sensitivity analysis of a multilayer, finite-difference ground-water flow model developed to quantify flow in the aquifer system. Initial calibration of the model by trial-and-error was followed by use of a parameter estimation program. Transmissivities of permeable zones within the aquifer system, vertical leakances between the zones, and the storage coefficient of the aquifer system were varied to obtain the best match between model-simulated and measured water levels for the period 1958-82. The mean error, root-mean-square error (RMSE), and standard deviation of the residuals between model-simulated and measured water levels were used to evaluate the progress of calibration, with greatest weight given to minimizing the RMSE. Best calibration results were obtained in the model layer that represents the uppermost part of the aquifer system. Good results also were obtained in the subsurface part of the rest of the aquifer system where waterlevel gradients are relatively low and uniform. Calibration of the model is relatively poor in the outcrop areas of the lower part of the aquifer system and near some major pumping centers, where steep and irregular water-level gradients are difficult to simulate at the scale of the model. Sensitivity analysis of the calibrated steady-state and transient models was performed by varying values of transmissivity, vertical leakance, and storage coefficient; the same parameters were varied during calibration. Changes in RMSE were used as the primary indicator of sensitivity. Near the calibrated values, the model is most sensitive to changes in transmissivity and almost as sensitive to changes in vertical leakance. By layer, the model is most sensitive to changes in the transmissivity of layer 2, which represents the upper part of the aquifer system, and in the vertical leakance between layers 1 and 2, which represents flow between a constant-head upper boundary and the top of the aquifer system. If transmissivity or vertical leakance is changed throughout the model, however, the effects are accentuated in the lower layers because much of the water flowing in these layers passes through and is affected by the overlying layers. The model is relatively insensitive to changes in the coefficient of storage because only a small part of the total flow is derived from storage. INTRODUCTION The coastal lowlands aquifer system is Geological Survey's Gulf Coast Regional program (Grubb, 1984). The GC RASA program tions that present a regional overview of the conditions in the principal aquifers of the objective of this study is to describe lowlands aquifer system. A digital flow model was the principal tool used to investigate Aquif cesses have been dominant during deposition of these sediments. Advanc:.ng deltaic fronts pushed the shoreline and its associated beach, dune, and lagoonal deposits seaward while blankets of fluvial sediments were deposited areas that is not discharged locally to streams or by evapotranspiration moves downward to the regional flow system and then toward discharge areas eit lower altitudes in the coastal plain and along major stream valleys. In places, pumping of ground water has altered the natural predevelopment gradients much of the natural discharge area. and has initiated recharge in Saltwater occurs downdip in the marine and deltaic parts of the aquifer system. Freshwater moving downdip from recharge areas tends to push the saltwater ahead of it, but the downdip movement of saltwater is blocked where the sand beds in the aquifer system pinch-out or are displaced by faulting. Water can move out of the downdip part of the sand beds only by upward leakage through overlying sediments (Martin and Whitoman, 1989, p. 4). DESIGN OF THE GROUND-WATER FLOW MODEL The coastal lowlands aquifer system was divided into five permeable zones, A-E as shown in figure 2, in order tol use a digital ground-water flow model to investigate the lateral and vertical distribution of flow (Weiss and Williamson, 1985). The massive coastward-thLckening wedge of sediments was first divided into zones in intensively-pumpad areas (Baton Rouge, Louisiana,