Hydrogeology, land-surface subsidence, and documentation of the Gulf Coast Land Subsidence and Groundwater-Flow (GULF) model, southeast Texas, 1897–2018

First posted January 13, 2023 For additional information, contact: Director, Oklahoma-Texas Water Science Center U.S. Geological Survey 1505 Ferguson Lane Austin, TX 78754-4501 https://www.usgs.gov/centers/ot-waterContact Pubs Warehouse As a part of the Texas Development Board groundwater availability modeling program, developed Gulf Coast Land Subsidence and Groundwater-Flow model (hereinafter, “GULF model”) ensemble to simulate flow land-surface subsidence in northern aquifer system (the study area) from predevelopment (1897) through 2018. Since publication previous for greater Houston area 2012, there have been changes distribution withdrawals advances tools. To reflect these more recent conditions, GULF was cooperation with Harris-Galveston Fort Bend Districts provide an updated Groundwater Availability Model.Since early 1900s, most three hydrogeologic units that compose system—the Chicot, Evangeline, Jasper aquifers and, recently, Catahoula confining unit. Withdrawals are used municipal supply, commercial industrial use, irrigation purposes. large quantities caused widespread groundwater-level declines than 300 feet (ft). Early development system, which began before 1900, resulted nearly 50 percent eventual historical minimums having reached as 1946 some areas. These led 9 ft subsidence—historically central southeastern Harris County Galveston County, but recently northern, northwestern, western Montgomery County—from depressurization compaction clay silt layers interbedded sediments.In generalized conceptual water enters topographically high outcrops northwestern system. does not discharge streams flows intermediate deep zones southeastward outcrop areas where it is discharged by wells upward leakage low near coast. The uppermost parts include areas, under water-table (unconfined) conditions confined pressure. depth increases accumulate, evolve into pressure.Groundwater were simulated using MODFLOW 6 Skeletal Storage, Compaction, package. consists six layers, one each five surficial top layer includes Transient during 1897–2018 combination multiyear, annual, monthly stress periods. An initial steady-state period configured represent mean annual inflows outflows. package uses head-based formulation simulates delayed drainage response sediment levels.The history matched observations at selected wells, benchmarks, borehole extensometers, vertical displacement Global Positioning System stations. A Bayesian framework uncertainty modeled parameters outputs interest. History matching quantification performed Monte Carlo approach enabled iterative smoother software produce models fit data. substantially reduced computational demand parameter estimation approximating first-order relation between inputs outputs, thereby allowing 183,207 adjustable be relatively time cost.The history-matched values within ranges previously published agree current understanding spatial temporal patterns good agreement observed (or estimated) levels, subsidence, compaction, obtained across based on qualitative quantitative comparisons. Ensemble groundwater-flow rates unit 0.0–0.49 inch (in.), 0.09–0.33 in., 0.01–0.07 0.01–0.05 respectively. 0.31 0.19 0.03 respectively.The GULF-model-simulated recharge largest inflow (75 percent), other 25 inflow. outflows included (1) net surface-water/groundwater exchange (50 (2) use (49 (3) Mexico (1 percent). sum (1,041,973 acre-feet per year [acre-ft/yr]) elastic expansion fine-grained numerical solver error (339 acre-ft/yr) minus (654,172 represents reduction storage (388,140 acre-ft/yr). Most depletion long-term primarily inelastic compaction.The estimate benchmarks primary locations use. Simulated 0.2 0.5 ft, or about 5 16 benchmark locations. 0.8 1.2 33 57