Current Land Subsidence in the Houston Metropolitan Area, Texas, Derived from GPS Observations (1993-2012)

This article summarizes recent land subsidence that has occurred in the Houston metropolitan area. Subsidence measurements derived from observations at 11 borehole extensometers and 90 GPS sites during the past 20 years (1993-2012) were investigated in this study. Precise Point Positioning with Single Receiver Phase Ambiguity (PPP-SRPA) resolution employed by the GIPSY-OASIS software package (V6.2) was applied to calculate daily positions of GPS antennas. GPS and extensometer observations indicate that the overall subsidence rate in the Houston metropolitan area has been decreasing since the 1970s, which was when groundwater withdrawal regulations started to be enforced by the Harris-Galveston Subsidence District (HGSD). Currently, the subsidence in the southeast of the Houston metropolitan area has almost ceased. Slight rebound has been observed at several sites along the Houston Ship Channel area since 2005. Nonetheless, a relatively small area within the Houston Ship Channel area that runs northwest from approximately Texas City to League City has continued to subside. There is some evidence that suggest that this subsidence is the result of local oil and gas withdrawal rather than groundwater withdrawal. Subsidence also continues in the west and northwest of the Houston metropolitan area, where groundwater regulations have only recently been implemented. The maximum rate is 2.5 cm/year. It is evident that the groundwater withdrawal regulations enforced by HGSD have successfully reduced the subsidence in the Houston metropolitan area. Long-term GPS observations also indicate that subsidence rates vary spatially and temporally depending on local groundwater withdrawals and the clay-to-sand ratio in subsurface sediments. The ground water and aquifer systems respond slowly to human actions. It took almost two decades (1980s and 1990s) to halt the subsidence in the east part of the Houston metropolitan area after groundwater regulations were implemented in the late 1970s. Therefore, a long-term perspective is needed to manage groundwater resources and control land subsidence. Long-term subsidence monitoring and mapping are critical to the scientific understanding and management of our land and groundwater resources.