Subsidence is the gradual sinking of the land caused by subsurface movement. The greater Houston area experiences subsidence due to groundwater withdrawals from the Gulf Coast Aquifer System, a multilayered groundwater reservoir made up of layers of sand, silt, and clay that compact when too much groundwater is removed.
Photo of Brownwood Subdivision home in Baytown, TX (1976)
The process is slower than that of other hazards; however, once this subsidence occurs, it is irreversible and has significant impacts over time, including:
- permanent loss of land elevation
- damage to important infrastructure like homes, buildings, pipelines, roads, and bridges
- disruption to natural drainage patterns and reduced effectiveness of drainage systems; and
- contributing to increased flood risks
One of the best-known and most widely studied examples of the impacts of excessive groundwater withdrawal and subsidence occurred at the Brownwood Subdivision in Baytown, Texas, where a prestigious neighborhood sank nearly ten feet over a few decades and was ultimately abandoned, now serving as a nature preserve.
Our region’s first recorded subsidence occurred back in 1918 near Baytown, Texas. As the area grew, so did water demand, and subsidence became more of a concern; some areas sank over 9 feet within four decades, prompting the creation of the Harris-Galveston Subsidence District in 1975 to regulate groundwater near the coast. By the 90s, this area worked hard to diversify its water supplies, bringing in alternative water sources like treated surface water to meet its water demand and reduce reliance on groundwater, which resulted in stabilized subsidence rates. However, subsidence in our region has shifted farther inland, as groundwater remains a primary water source until alternative water supplies become available.
The District publishes an Annual Groundwater Report each year that provides the latest information on subsidence in our region, including climatic conditions, water-use data, aquifer data, and measured subsidence for the previous calendar year. The results of the Annual Groundwater Report ultimately tell the story about our region’s subsidence mitigation efforts and help to better inform local decision-makers with data that can be used to build a more resilient community. In areas that have reduced their reliance on groundwater, little to no subsidence occurs, whereas in areas that still rely on groundwater as their primary water source, subsidence continues.
To see where subsidence is currently happening, check out our latest interactive subsidence map!
Our region’s subsidence problem is a direct result of our reliance on groundwater from the Gulf Coast Aquifer System, which is simply not built to support large volumes of withdrawal. However, now that we know the cause, we can work together to prevent further subsidence by reducing reliance on groundwater through:
- Science-driven groundwater regulations that allow groundwater to supply water demand at a volume that does not further cause subsidence
- Regional water planning efforts to meet growing water demands through alternative water supplies. In areas that have transitioned from groundwater to alternative water supplies, like treated surface water, subsidence rates have significantly improved, whereas in areas that rely heavily on groundwater to meet their water demands, subsidence continues.
- Water conservation efforts that protect our natural resources and help us use our water supplies more efficiently.
The goal is to shift our region’s primary water supply from groundwater to sustainable alternative water supplies that do not cause further land subsidence. However, the transition from groundwater to alternative water supplies (surface water, reclaimed water, etc.) is complex and requires regional water planning, which is why the Texas Legislature created Regional Water Authorities to provide local control for these supply projects and are responsible for facilitating the conversion from groundwater to alternative water supplies to meet HGSD’s Regulatory Plan requirements.
- Central Harris County Regional Water Authority
- North Harris County Regional Water Authority
- West Harris County Regional Water Authority
- North Fort Bend Water Authority
Local and regional water providers have worked diligently to deliver sustainable solutions for their communities. For example, the West Harris County Regional Water Authority, the North Fort Bend Water Authority, and the City of Houston have joined efforts to construct the Surface Water Supply Project. Additional efforts were made at the Northeast Water Purification Plant Expansion near Lake Houston to increase treated water production from 80 million gallons per day to 400 million gallons per day.
The Harris-Galveston Subsidence District has a dedicated Science & Research Program that utilizes several scientific methods, technologies, and techniques to accurately measure subsidence, including:
- Extensometers – deep wells that the U.S. Geological Survey (USGS) uses to measure aquifer compaction between the well anchor and the ground level. Currently, there are 14 extensometers in the greater Houston area.
- Global Positioning Systems (GPS) – satellites that track and detect movement in land elevation for a specific GPS station location. HGSD maintains nearly 100 GPS monitoring stations and works with other local entities to gather data from over 200 stations throughout the region. Each year, an interactive subsidence map is published using data collected from GPS monitoring stations to show where subsidence is occurring.
- Interferometric Synthetic Aperture Radar (InSAR) – satellites that scan the land surface at regular intervals to determine whether the land surface is moving toward the satellite (uplift) or away from the satellite (subsidence). This method provides additional coverage and data to map subsidence beyond site-specific GPS stations.
- Benchmark Surveying – using permanent marks or reference points with a known surface elevation from National Geodetic Survey (NGS) benchmarks to document changes in land elevation.