DOI: 10.5066/p9xm8a1p
title: MODFLOW 6 model and ensemble used in the simulation of groundwater flow and land subsidence in the northern part of the Gulf Coast aquifer, 1897-2018 (ver. 2.0, September 2023)

description: The U.S. Geological Survey (USGS), in cooperation with the Harris-Galveston Subsidence District and Fort Bend Subsidence District, constructed a finite-difference numerical groundwater-flow model of the northern Gulf Coast aquifer region for 1897 through 2018 using MODFLOW 6 with the Newton formulation solver to simulate groundwater flow and land-surface subsidence. Model parameter estimation and uncertainty analysis were conducted with PEST++ Iterative Ensemble Smoother software. The simulated results are described in the associated U.S. Geological Survey Professional Paper 1877. The model archive includes all the necessary files to run the MODFLOW 6 model and process the results for the posterior base realization as displayed in the accompanying report. The readme file describes selected files and directories in the archive and provides instructions for running the model. This U.S. Geological Survey data release contains all of the input and output files for the simulations described in the associated model documentation report (https://doi.org/10.3133/pp1877). Since the early 1900s, most of the groundwater withdrawals in the Houston-Galveston region, Texas, have been from the four primary aquifers that compose the Gulf Coast aquifer system&amp;amp;amp;amp;mdash;the Chicot, Evangeline, Jasper aquifers and, more recently, the Catahoula confining unit. Withdrawals from these aquifers are used for municipal supply, commercial and industrial use, and irrigation. Withdrawals of large quantities of ground water have resulted in potentiometric surface declines in the Chicot, Evangeline, and Jasper aquifers and land-surface subsidence&amp;amp;amp;amp;mdash;primarily in the Houston area&amp;amp;amp;amp;mdash;from depressurization and compaction of clay layers interbedded in the aquifer sediments. In a generalized conceptual model of the aquifer system, water enters the ground-waterflow system in topographically high outcrops of the hydrogeologic units in the northwestern part of the study area. Water that does not discharge to streams flows to intermediate and deep zones of the system southeastward of the outcrop areas where it is discharged by wells and by upward leakage in topographically low areas near the coast. The uppermost parts of the aquifer system, which include outcrop areas, are under water-table conditions. As depth increases in the aquifer system and as interbedded sand and clay accumulate, water-table conditions evolve into confined conditions. A numerical groundwater-flow model simulating groundwater flow and land subsidence for the northern part of the Texas Gulf Coast aquifer system was constructed by using MODFLOW6 coupled with the Skeletal Storage, Compaction, and Subsidence package. Groundwater flow was simulated for 1986&amp;amp;amp;amp;ndash;December 2015 by using a combination of multi-year, annual, and monthly stress periods, and an initial steady-state stress period was configured to represent mean annual inflow and outflows. The model was calibrated to groundwater-level observations at selected wells, and land-surface subsidence.