Friendswood, TX, June 10, 2020

The Harris- Galveston Subsidence District (District) has been monitoring water use, groundwater levels and subsidence in Harris, Galveston and adjacent counties since 1975. Subsidence, the lowering of land-surface elevation, is caused by the depressurization of aquifers due to widespread use of groundwater as a primary water source. The mission of the District is to cease ongoing subsidence and prevent the occurrence of future subsidence.

As part of this effort, it is important for the District to provide consistent, high-quality information to the public regarding groundwater use, aquifer water-levels, and subsidence. This report is the compilation of the largest multi-agency effort in the State of Texas by leveraging the resources of both the Harris-Galveston and Fort Bend Subsidence Districts with the City of Houston, the Lone Star Groundwater Conservation District, the Brazoria County Groundwater Conservation District, the United States Geological Survey, and the University of Houston.

“Each year, the District collects groundwater and alternative water supply use from our permittees. This information provides the foundation for understanding how much groundwater is being used within the District, how our permittees are using groundwater and perspective on the conversion from groundwater to surface water for our three regulatory areas,” said Dr. Tina Petersen, Deputy General Manager.

In 2019, the total amount of groundwater that was pumped within the District is 228.1 million gallons per day (MGD), with 92 percent being used for public supply. Groundwater use has dropped from about 60 percent in 1976 to about 23 percent in 2019. Even still, groundwater remains the second largest source of water supply for the District.

Since 1976, water users in the District have been working to change their source water from primarily groundwater to alternative sources of water that will not contribute to subsidence, primarily treated surface water. The District’s Regulatory Plan requires permittees to convert to alternative water supplies in order to reduce their reliance on groundwater sources. The primary alternative water supply used in the region is surface water sourced from three river basins: the Brazos River Basin, the San Jacinto River Basin and the Trinity River Basin. Total alternative water use for 2019 was 746.7 MGD. Total water demand, which includes groundwater, surface water, and reclaimed water, was determined to be 974.8 MGD in 2019, which is 3 percent lower than the total water demand reported in 2018.

Aquifer water-level is the best measure of the pressure in the aquifer and provides vital information to understand the impact that water use changes has on subsidence. The change in water-level in the Chicot and Evangeline aquifers since 1977 clearly shows the impact of District regulation on the aquifers. Generally, Regulatory Areas One and Two (see interactive map on District website, www.hgsubsidence.org)  have seen a significant rise in the potentiometric water-level up to 200 feet and 240 feet in the Chicot and Evangeline aquifers, respectively. The area of rise is a result of the reduction of groundwater use as required by the District’s Regulatory Plan. Conversely, in Regulatory Area Three and nearby northern Fort Bend and southern Montgomery Counties, water-levels continue to be significantly lower than the historical benchmark with declines of nearly 280 feet in the Evangeline and Jasper aquifers. Although these areas are growing rapidly, the required conversion to alternative sources of water will not be mandated in the District until 2035 and in the Fort Bend Subsidence District until 2025.

Since the late 1990s, the District has been utilizing global positioning system (GPS) to monitor subsidence in the Houston-Galveston area. The average annual rate of subsidence over the last 5-years is a useful measure to show the current subsidence activity at a GPS monitoring site. The annual rates of subsidence observed in Regulatory Areas One and Two are generally stable, since both areas have reached their full regulatory conversion level (1990 and 1995, respectively) and aquifer potentiometric water-levels have risen. Subsidence rates are consistently above about 0.5 centimeters per year (cm/yr) throughout Regulatory Area Three, where groundwater remains a primary source of water.  Subsidence rates in Montgomery County are generally stable in areas where alternative water supplies have been implemented and groundwater use has been reduced.

Ashley Greuter, G.I.T., Subsidence Monitoring Project Chief for the District states “An essential part of the District’s mission is to monitor subsidence in Harris, Galveston, and adjacent counties through the use of GPS monitoring sites. Currently, the District, in cooperation with the University of Houston, measure subsidence from over 230 sites throughout the region to understand the distribution and magnitude of current subsidence and historical trends. The District continues to expand and refine the subsidence monitoring network.”

About the Harris-Galveston Subsidence District

The District was created in 1975 by the Texas Legislature with the mandate to cease and prevent subsidence within Harris and Galveston Counties by regulating the use of groundwater. Subsidence has been a long-standing problem in Harris, Galveston, and surrounding counties, with total subsidence near Galveston Bay reaching over 10 feet. Subsidence can contribute to flooding, infrastructure damage, and faulting.     

The District’s mission is to regulate the use of groundwater in Harris, Galveston and Fort Bend counties, to cease ongoing and prevent future subsidence that can lead to infrastructure damage and contribute to flooding.  

For Additional information and to view an interactive subsidence rate map visit our website at www.hgsubsidence.org