Spatial and Temporal Variability of Subsidence: Fort Bend County, Texas

This study presents new geodetic data revealing the evolution and magnitude of land subsidence in Fort Bend County (FBC) through 2014. Subsidence in the greater Houston metropolitan area has been monitored since 1926 and several generations of maps have portrayed more than three meters of subsidence. Recent studies indicate that the land surface has stabilized or rebounded up to 2.0 centimeters in southeast Houston, coincident with aquifer levels recovering to approximately 30 meters below land surface.

FBC, located west of Houston, was chosen to study the early drivers of subsidence. GPS stations co-located with monitoring wells across the county indicate that periodic subsidence can occur when the potentiometric surface is more than 30 meters below land surface, whether groundwater levels are rising, falling or remain stable. Nine out of the fifteen GPS stations in FBC recorded more than one centimeter of subsidence during the drought of 2011.

The potentiometric surface across FBC shows a strong correlation with areas of subsidence where the hydraulic head is more than 30 meters below land surface. In 2011, the Rosenberg and Simonton GPS stations experienced 3.0 and 4.0 centimeters of subsidence, respectively. Farther to the west, where hydraulic head is less than 30 meters BLS, GPS stations are stable.

Further investigation of Rosenberg indicates that groundwater levels and the rate of groundwater withdrawal controls subsidence, affecting all three-directional components. The TXRO and PA10 GPS stations show 5.0 centimeters inelastic deformation occurred from 2005 through 2013 in both the horizontal and vertical directions. This deformation correlates with periods of increased rates of groundwater withdrawal, indicating that localized pumping can create independent subsidence.

Observational data were acquired from sixteen high-precision Global Positioning System (GPS) stations, with three or more years of data, in FBC. Data were processed using the GIPSY/OASIS v. 6.3 software package developed by NASA’s Jet Propulsion Laboratory (JPL). Global position coordinates were transformed from the IGS08 reference frame to the Stable Houston Reference Frame (SHRF) using a 14-parameter Helmert transformation. Groundwater-level changes were analyzed for the period from 2005 to 2014 utilizing sixty-five USGS monitoring wells.