GroMoPo Metadata for Northern Gulf Coast aquifer model

In cooperation with the Harris-Galveston Subsidence District, Fort Bend Subsidence District, and Lone Star Groundwater Conservation District, the U.S. Geological Survey developed and calibrated the Houston Area Groundwater Model (HAGM), which simulates groundwater flow and land-surface subsidence in the northern part of the Gulf Coast aquifer system in Texas from predevelopment (before 1891) through 2009. Withdrawal of groundwater since development of the aquifer system has resulted in potentiometric surface (hydraulic head, or head) declines in the Gulf Coast aquifer system and land-surface subsidence (primarily in the Houston area) from depressurization and compaction of clay layers interbedded in the aquifer sediments. The MODFLOW-2000 groundwater flow model described in this report comprises four layers, one for each of the hydrogeologic units of the aquifer system except the Catahoula confining system, the assumed no-flow base of the system. The HAGM is composed of 137 rows and 245 columns of 1-square-mile grid cells with lateral no-flow boundaries at the extent of each hydrogeologic unit to the northwest, at groundwater divides associated with large rivers to the southwest and northeast, and at the downdip limit of freshwater to the southeast. The model was calibrated within the specified criteria by using trial-and-error adjustment of selected model-input data in a series of transient simulations until the model output (potentiometric surfaces, land-surface subsidence, and selected water-budget components) acceptably reproduced field measured (or estimated) aquifer responses including water level and subsidence. The HAGM-simulated subsidence generally compared well to 26 Predictions Relating Effective Stress to Subsidence (PRESS) models in Harris, Galveston, and Fort Bend Counties. Simulated HAGM results indicate that as much as 10 feet (ft) of subsidence has occurred in southeastern Harris County. Measured subsidence and model results indicate that a larger geographic area encompassing this area of maximum subsidence and much of central to southeastern Harris County has subsided at least 6 ft. For the western part of the study area, the HAGM simulated as much as 3 ft of subsidence in Wharton, Jackson, and Matagorda Counties. For the eastern part of the study area, the HAGM simulated as much as 3 ft of subsidence at the boundary of Hardin and Jasper Counties. Additionally, in the southeastern part of the study area in Orange County, the HAGM simulated as much as 3 ft of subsidence. Measured subsidence for these areas in the western and eastern parts of the HAGM has not been documented.美国地质调查局（U.S. Geological Survey）与哈里斯-加尔维斯顿沉降区（Harris-Galveston Subsidence District）、本德堡沉降区（Fort Bend Subsidence District）及孤星地下水保护区（Lone Star Groundwater Conservation District）合作，开发并校准了休斯顿地区地下水模型（Houston Area Groundwater Model, HAGM）。该模型模拟了得克萨斯州墨西哥湾沿岸含水层系统北部自开发前（1891年以前）至2009年的地下水流动与地面沉降过程。 自含水层系统开发以来的地下水抽取活动，导致墨西哥湾沿岸含水层系统的测压水位面（水力水头，简称水头）出现下降，同时因含水层沉积物中夹存黏土层的卸压与压密，引发地面沉降（主要发生在休斯顿地区）。 本报告中描述的MODFLOW-2000地下水流动模型共包含4层，分别对应该含水层系统除卡塔胡拉隔水系统（Catahoula confining system，即该系统的假设不流动基底）外的各水文地质单元。HAGM采用1平方英里的网格单元，划分为137行、245列，其侧向不流动边界设置为：西北方向为各水文地质单元的分布范围，西南与东北方向为与大型河流相关的地下水分水岭，东南方向为淡水区的下倾边界。 模型校准遵循既定标准，通过对选定的模型输入数据开展试错调整，并进行一系列瞬态模拟，直至模型输出的测压水位面、地面沉降及选定的水均衡组分等结果，能够较好复现野外实测（或估算）的含水层响应（包括水位与沉降数据）。 HAGM模拟得到的沉降结果，与哈里斯县、加尔维斯顿县及本德堡县的26个有效应力与沉降关联预测（Predictions Relating Effective Stress to Subsidence, PRESS）模型结果总体匹配良好。模拟结果显示，哈里斯县东南部地区最大累计沉降量可达10英尺（ft）。实测沉降数据与模型结果均表明，涵盖该最大沉降区域以及哈里斯县中部至东南部大部分区域的更大地理范围，其累计沉降量至少达到6英尺。 针对研究区西部，HAGM模拟显示沃顿县、杰克逊县及马塔哥达县的最大累计沉降量可达3英尺；研究区东部的哈丁县与贾斯珀县交界区域，模拟最大累计沉降量同样为3英尺；此外在研究区东南部的奥兰治县，模拟最大累计沉降量也达到3英尺。目前尚未有针对研究区西部与东部上述区域的实测沉降资料记录。