Every month, the District will publish a monthly feature, an in-depth look at a component of the District’s mission to prevent subsidence in Harris and Galveston counties. July’s feature explores why the District monitors subsidence.
In the 1970s, subsidence damages to private property and infrastructure were so severe that the Texas Legislature stepped in to create the Harris-Galveston Subsidence District to end subsidence within the District.
The only way to stop subsidence? Curb groundwater use. Through years of study, the District found that subsidence rates increased in direct proportion to how much groundwater was withdrawn in that immediate area. That conclusion, along with mounting evidence of subsidence occurring in the region, informed the District’s Regulatory Plan and guided efforts to reduce rates.
Continuing to closely study the phenomenon is key to repeating the success of the District’s efforts along the coast – effectively stopping subsidence – and preventing future damages further inland.
3 Benefits of Monitoring Subsidence
In order to prevent future subsidence, the community needs to know where it’s happening. Measuring subsidence and aquifer water levels over time does three things:
- Provides data to predict where future subsidence will occur. Documenting the region’s population trends, corresponding groundwater usage, aquifer water level changes, and current subsidence rates allow the District to use mathematical models to predict where subsidence will likely occur.
- Informs the District’s regulations. The District monitors subsidence to help balance regulations with people’s access to alternative water. Coastal areas were the initial focus of the District’s regulations, and subsidence rates there have been greatly reduced to essentially zero. While subsidence is still occurring in some parts of the District, it is expected that the complete implementation of the regulatory plan in 2035 will result in the same success observed in the coastal areas.
- Alerts the community and general public. Since subsidence occurs slowly, measuring rates as the District’s Regulatory Plan is implemented over the next 14 years will provide local municipalities, other state and local agencies, and residents with the knowledge on how their groundwater use affects their homes and properties. The data provides context and highlights the importance of investing in alternative water infrastructure to reduce their reliance on groundwater.
A permanent GPS stations located in Humble, Texas.
How Subsidence is Monitored
Quick reminder: subsidence, the sinking of land, is caused by excessive groundwater withdrawal which results in the lowering of the aquifer water-level (an indication of depressurization of the aquifer) that causes the aquifers to compact. This compaction is seen at the surface as subsidence, and can cause damage roads, infrastructure and contribute to flooding.
There are two primary measurement methods used to monitor the movement beneath the ground (compaction) and at the surface (subsidence): extensometers and GPS stations.
Extensometers
Deeply anchored wells that measure compaction between the well anchor and the ground level. Measuring aquifer compaction is key to identifying where sinking at the surface will occur. In cooperation with the District, The United States Geological Survey (USGS) operates and maintains 14 extensometers in the Houston region.
GPS Stations
Global Positioning Systems (GPS) instruments are positioned at over 230 locations throughout Harris, Galveston, Fort Bend, and the surrounding counties where each site collects accurate position data (x,y,z) every 30 seconds. There are two types of GPS stations – continuous and periodic. Although the District employs many methods to measure subsidence, the GPS network currently serves as the primary measurement method.
Other monitoring methods include:
InSAR
In addition to the data collected by the GPS stations, the District is working with Southern Methodist University to integrate data obtained from Interferometric Synthetic Aperture Radar (InSAR), a remote sensing technique using data collected from orbiting satellites.
Because this novel technique scans the land surface during different times and generates an image of the changes between the two datasets, the density of data will provide more information about subsidence that may be occurring between GPS monitoring stations.
Traditional Level Surveying
The District, and other state and federal agencies, have utilized traditional surveying using National Geodetic Survey (NGS) benchmarks to document the subsidence in the Houston region beginning in 1906.
This time-intensive process for capturing data is less used with the advent of GPS technology, but still useful when identifying areas where precise information on changes in land surface are beneficial. The last regional survey for this purpose was completed by the District and the NGS in 2000.
Satellites scan the land surface at regular intervals on defined orbits around the earth producing “scenes” over time for InSAR data.
How Subsidence Data is Used
Annual Groundwater Report: The University of Houston processes all of the raw data collected at the GPS monitoring network sites utilizing the latest technology and analytical methods. Each year, all data collected at each GPS site is reprocessed to assure the most up to date information is available. The District publishes the calculated subsidence rates each Spring as part of the Annual Groundwater Report, which also includes data on aquifer water levels and groundwater usage from the District’s permittees.
Interactive Subsidence Map: a visualization of the Annual Groundwater Report, this interactive map show five year subsidence rates throughout the region.
New Research: Not only does the District rely on this data to inform its Regulatory Plan, but also uses the indications of future or ongoing subsidence rates in areas to explore further study, such as subsidence impact on flooding along the Spring Creek Watershed.