Editor's note: This story has been updated to clarify the Northeast Water Purification Plant in Humble is a joint project between the city of Houston, North Harris County Regional Water Authority, West Harris County Regional Water Authority, North Fort Bend Water Authority and Central Harris County Regional Water Authority.

Two reports released within the last eight months show parts of Cy-Fair have experienced land displacement, which can make flooding more severe.

Land sinking has occurred throughout the Houston region over the last century, according to a Jan. 12 report from the U.S. Geological Survey’s Oklahoma-Texas Water Science Center, with most of the land movement occurring since 1987.

This gradual, vertical decline is known as subsidence, or the sinking of the land due to movement beneath the Earth’s surface.

In Cy-Fair, as much as 7 feet of subsidence has occurred near Beltway 8 in recent decades with rates decreasing heading northwest.

John Ellis—who authored the Jan. 12 study and serves as the center’s Gulf Coast studies chief—said subsidence worsens flooding in watersheds by “altering the base flood elevations.”

“[Subsidence] is an important phenomenon to document and study. ... It can result in permanent loss of land, increased risk of flooding and damage to infrastructure,” Ellis said.

Subsidence is chiefly caused by pumping water from underground reserves, which compacts sublayers of clay and silt in aquifers beneath the Earth’s surface, according to a University of Houston geological study released in August.

The resulting sinking is a problem experienced all over the Greater Houston area.

Shuhab Khan, a UH geology professor and one of the August report’s authors, said he believes there is a balance to using groundwater sustainably.

“Groundwater is the cleanest water all over the world,” Khan said. “It is a means for drinking, for agriculture, for industry, and when we start pumping more water than the amount of water that is replenishing [aquifers], that balance is gone.”

The UH study states its findings also imply subsidence may be responsible for fault movement in the Greater Houston area based on analysis of land displacement.

Over time, subsidence can cause damage to property, pipes and roads, Khan said.

To prevent subsidence from worsening, regional organizations are working to decrease groundwater use. Efforts include a $1.4 billion expansion of the Northeast Water Purification Plant in Humble—a joint project between the city of Houston, North Harris County Regional Water Authority, West Harris County Regional Water Authority, North Fort Bend Water Authority and Central Harris County Regional Water Authority.

This plant treats Lake Houston water and turns it into drinking water for parts of Harris County and surrounding areas, including Cy-Fair.

Upon completion in 2024, officials said the plant will increase treated water capacity from 80 million gallons per day to 400 million gallons per day.

Subsidence in Cy-Fair

Experts have tracked land deformation in Houston’s growing suburbs over the past couple of decades. According to the UH report, land within Houston’s city limits is not sinking substantially, but suburbs are impacted more.

The USGS report stated some of the highest levels of subsidence in the Greater Houston area since the 1990s have been in the northwest portion of Harris County, although there is some uncertainty about the full picture due to limited historical data.

Results show areas in Cy-Fair had an average of 0.87-2.17 centimeters of displacement per year between 2016-20.

A GPS station located off FM 1960 near Windfern Road showed the highest rate of subsidence during that time period. It saw 60.25 cumulative centimeters of displacement since measuring there began in 1999. Northwest of Cy-Fair near the Waller area, however, the rate of subsidence was only 0.25 centimeters per year and had a cumulative rate of 11.34 centimeters since 1999.

The UH report also found that northwestern Harris County is experiencing “substantial negative displacement” due to population growth and development as well as the use of groundwater from aquifers. Wells can be drilled into an aquifer—a large, underground water-bearing rock—and water can then be pumped out for residential and industrial use.

According to the USGS report, which covers a new model created to study land displacement and the flow of groundwater from 1897-2018, groundwater withdrawals from the Gulf Coast Aquifer System over time have led to more than 300 feet of groundwater-level declines in the Chicot, Evangeline and Jasper aquifers.

Groundwater use across the Greater Houston region caused as much as 9 feet of subsidence.

However, once regional water authorities began to convert to surface water use instead of groundwater, Chicot and Evangeline aquifer groundwater levels began to recover.

Development leads to more groundwater pumping to meet water needs, said Mike Turco, who is the general manager for the Harris-Galveston Subsidence District, which regulates groundwater and monitors subsidence in the Houston region.

“As you add more people, you add more businesses, you’re going to have more needs for water resources, and diversifying those sources from groundwater to an alternative like surface water helps the subsidence issue and prevents subsidence for us,” Turco said.

Data from HGSD shows subsidence has contributed to flooding across the Greater Houston area.

However, Turco said a study on the impact of subsidence in the Cypress Creek watershed has not yet been done.

Experts warned excessive groundwater pumping will make flood mitigation more difficult in Greater Houston communities where it is already challenging.

“These [flooding events] are in the same areas where we are seeing subsidence rates at 1 1/2 to 3 centimeters per year,” Turco said.

In addition to worsening flooding, subsidence also contributes to changes in drainage patterns, fault movement, and damages to wells and pipelines, according to the HGSD.

Alternative sources

Although the conditions that cause subsidence can be lessened if residents cut groundwater usage and switch to alternative water sources, the effects of subsidence are permanent, said Robert Mace, water policy director at Texas State University.

“If you reduce your pumping, you can then decrease the maximum subsidence that would have occurred,” he said. “But for the most part, land subsidence is a one-way trip. Once it’s compressed, it’s not coming back up."

To prevent subsidence from worsening, HGSD set pumping requirements for all water suppliers in Harris and Galveston counties in 2013.

This includes the NHCRWA and the WHCRWA, which both serve Cy-Fair.

“We expect those subsidence rates to come down to near zero as we implement this plan,” Turco said