New Subsidence District Report Shows Improvements, Challenges

5/22/25 – The Harris-Galveston Subsidence District released its 2024 Annual Groundwater Report last week. The document represents a sort of scientifically graded report card for the District.

It shows that serious subsidence problems remain in fast growing areas where regulations have yet to be fully phased in. But it also shows that subsidence has decreased dramatically where regulations have been in place the longest.

Finally, it explains what the region’s leaders are doing to get people off groundwater in those fast-growing areas. Groundwater is the major cause of subsidence or sinking of the land and can damage infrastructure such as streets, sewers, and pipelines. It can even trigger geologic faults.

Subsidence has also been linked to flooding in several ways. The effect is obvious near the coast in areas susceptible to storm surge. But it’s less intuitive farther inland at higher elevations. It has to do with creating bowls in the landscape that alter the gradient of rivers and streams. But I’ll save that discussion for a subsequent post.

The 2024 report starts with four main topics:

Climate
Water Use
Groundwater Levels
Subsidence

Each affects the next. Let’s look at these topics briefly, then look at what the District is doing to get fast-growing areas off groundwater.

Climate Impact Negligible Last Year

Last year, precipitation was above normal for all National Weather Service reporting stations throughout the region except for Katy. Above-normal precipitation typically reduces the demand for groundwater to irrigate lawns and crops. However, irrigation usually comprises only a small percentage of total demand.

Groundwater Use Increased in Fast Growing Areas

The District monitored groundwater use in its three different regulatory areas and then averaged all three.

Regulatory Area:

#1 is closest to the coast where the District first implemented groundwater regulations 50 years ago.
#2 is farther inland. Think of Central Harris County.
#3 is the most recently regulated and the farthest inland. Regulations won’t fully phase in until 2035 there.

Here’s what happened to groundwater demand in each:

#1 showed a small uptick compared to recent years, linked primarily to industrial use.
#2 showed a drop compared to the previous two years.
#3 also showed a drop compared to the previous two years, but an increase compared to the previous 10 years. The District attributed this to rapid population growth in Area #3.

Regulatory Area #3 accounted for a whopping 82% of all groundwater used in all three areas combined.

Looking at all three areas combined, the average declined compared to the previous two years. But growth in Area #3 water demand pulled the average up compared to the previous 10 years.

The gradual phase in of groundwater restrictions in Area #3 gives new developments time to migrate to water from alternative sources.

Groundwater Usage Decreasing as Alternative Water Usage Increases

The District defines “alternative sources” as anything other than groundwater. It usually refers to surface water from various sources such as the Trinity, San Jacinto or Brazos Rivers.

Overall, the use of alternative water is growing and reached a new high last year. Conversely, groundwater use declined despite growth in total water demand.

But growth in surface water supplies met that demand and then some.

All things considered, that’s good news.

Impact on Groundwater Levels

Restricting groundwater use reduces subsidence. To see its impact, the District measured increases and decreases in water well levels. The District can provide comparisons of hundreds of well levels today with the previous year, five years and 48 years. Predictably, the highest decreases clustered within the fastest growing areas to the north and west.

But well levels also actually rose in some areas. Where that happened, aquifers are recharging faster than they are depleting – another good sign that the regulatory regime is working.

Impact on Subsidence

The Subsidence District maintains an extensive network of GPS stations that monitor subsidence across 12 counties.

Two things stood out:

The fast-growing Area #3 subsided much faster than the other areas that already reduced their use of groundwater.
Katy showed the greatest subsidence. It received the least rain and is also among the fastest growing areas in the region.

Filling Observational Gaps Between Wells

The District also used “interferograms” to render high-resolution subsidence maps. Radio waves bounced off earth form the basis for interferograms. By measuring the timing of return signals as the satellite circles the globe endlessly, and comparing the differences, the technique can achieve an accuracy of millimeters.

Interferograms produced by the Subsidence District clearly show bands of subsidence that correspond to the length of time regulations have been in effect.

But there’s even better hope for the future.

Projects Enabling Groundwater Conversion

Area leaders have collaborated on four large projects to move people off groundwater.

Luce Bayou Interbasin Transfer Project completed last year. It moves water from the Trinity River to Lake Houston. (Red line below.)
Northeast Water Purification Plant Expansion designed to treat 400 million gallons per day. (Green star below.)
Northeast Transmission Line Project, which will pipe treated surface water from Lake Houston into central and northern Harris County. (Purple line below.)
Surface Water Supply Project which will take water from the new plant to western Harris and northeastern Fort Bend Counties. (Orange Line Below)