Measuring Subsidence

Why Monitor Subsidence?

Subsidence, the sinking of land, occurs when excessive groundwater withdrawal causes aquifer water levels to decline, which leads to the aquifer compacting or sinking in on itself. This compaction is seen at the surface as subsidence and can cause damage to roads, infrastructure and contribute to flooding.

How is Subsidence Measured?

Multiple factors are measured when examining subsidence. To measure the actual compaction and sinking of the land surface, these are a few methods utilized in the Houston region:

Extensometers are deep wells (ranging from 500 feet to more than 3,000 feet below the ground surface) that measure aquifer compaction between the well anchor and the ground level. Although an extensometer does not measure the land surface’s movement, it provides accurate information on the compaction of the aquifer materials. The U.S. Geological Survey (USGS) maintains 14 extensometers in the greater Houston-Galveston region.

GPS Monitoring Stations use satellites, which orbit the earth, to accurately determine the station’s position on the ground. The Harris-Galveston and Fort Bend Subsidence Districts maintain 95 permanent global positioning system (GPS) monitoring stations and work with other local entities and the University of Houston to gather data from more than 230 sites throughout the region.

InSAR, another satellite technology known as Interferometric Synthetic Aperture Radar (InSAR), satellites scan the land surface at regular intervals on defined orbits around the earth producing “scenes” over time. These scenes are compared and any differences show whether the land surface is moving toward the satellite (uplift) or away from the satellite (subsidence). This method provides data on any subsidence that may be occurring between GPS stations, and together they provide a comprehensive view of subsidence.

Traditional Surveying uses National Geodetic Survey (NGS) benchmarks to periodically document any changes to land surface.

Cross-sectional view of a borehole extensometer.

Example of a permanent GPS station, P023 located in Bayou Vista.

GPS stations installed by the District and designated by periodic or continuous monitoring.

How GPS monitoring data is processed?

Working with the University of Houston (UH), quality checks are performed for each GPS station in the region. The Districts’ raw data collected at each site are processed by UH using GipsyX, a software package developed by NASA’s Jet Propulsion Laboratory, and transformed to a stable reference frame developed and published by UH for our region. The reference frame provides more accurate measurements of localized changes in the land surface and minimizes other movement factors, such as plate tectonics. For detailed information on the processing methodology, please refer to this site.

Published Data

Interactive Map: The District compiles the processed data provided by UH into an interactive map for all GPS stations in the subsidence monitoring network. The District strongly encourages users, who download GPS data from the monitoring network, to include a citation to the District’s Annual Groundwater Report as the source of the data.

Annual Groundwater Reports: The District includes the subsidence data provided by UH, aquifer water levels measured and published by the USGS, and reported groundwater use data collected by the District in its Annual Groundwater Report.