The Texas Groundwater Science Advisory Committee, an initiative of the Regional Groundwater Science Partnership, released the group's report in early 2021. Their Phase 1 report was a literature review of groundwater data and subsidence studies in the greater Houston-Galveston region. [The extent of their literature search is suspect because they missed one of the principal publications on the subject (here).] The Phase 1 Report focuses on groundwater pumping and subsidence differences and impacts in Harris and Montgomery Counties. It would be beneficial to decision-makers and the public to include additional historical context such as: groundwater pumping differences in the different counties of the region, analyses of subsidence in Montgomery County as pumping changed over time, and the level of oil and gas depressurization contributing to compaction.
A SAC review of the available subsidence data indicates a correlation between groundwater pumping and subsidence. When Montgomery County converted to include water supplies other than groundwater beginning in 2015, the water-levels for wells located in Conroe and The Woodlands have generally risen after yearly declines between the time monitoring began through 2015. Correspondingly, subsidence rates have attenuated in these areas during the same period.
This adjustment is reflected in the land surface elevation changes at GPS stations P071 and TXCN (Conroe area); P013 and P069 (The Woodlands); and P047, P017, and ROD1 (northern Harris County). The remaining GPS stations in Conroe and The Woodlands do not have sufficient data prior to 2015; therefore, longer-term trends in subsidence from these stations cannot be assessed. Notably, a substantial amount of groundwater use occurs south of The
Woodlands GPS stations in lower Montgomery County; therefore, additional subsidence monitoring in this area would be beneficial as the current GPS stations may not capture the extent of subsidence.
The Phase 1 Report states that the greatest rates and amounts of subsidence occur in Harris County (p.8). On page 13, the report states, “UH [University of Houston] reports the average subsidence rate based on GPS data for the period from 2005 to 2014 to be between 17 and 19 millimeters per year (mm/yr), or 0.67 to 0.75 inches per year (in/yr).”
It should be noted that all UH reports and results regarding GPS-derived subsidence are limited to specific times and locations. Anthropogenic subsidence shows significant temporo-spatial variations, making it important to specify subsidence rates within a specific time range and area. GPS sites are more heavily weighted towards areas of known subsidence; if these sites were averaged to calculate county-wide subsidence, the result would be based on spatially biased data. Based on GPS and historical leveling data, the subsidence contours from Petersen and others (2019) show cumulative subsidence between 1 and 3 feet in southern Montgomery County. It is notable that these values are based on collected data, not groundwater model
estimates or projections.
On page 22 of the Phase 1 Report, it is stated,“…there are many ways to present the results from the HAGM [Houston Area Groundwater Model]. Examples of the various presentations include average drawdown, average subsidence, change in storage, or percent remaining available drawdown…” It is important to ensure that the modeled scenarios also include the maximum subsidence and drawdown, which are shown spatially on a figure. Maximum subsidence and drawdown results from the modeled scenarios are generally not presented together in the Phase 1 Report but are vital to landowners and public supply entities. Averaging subsidence and/or drawdown spatially may mask the effects of groundwater use at a sub-county scale.
Additionally, on page 23, the Phase 1 Report states, “…a significant portion of the simulated subsidence is due to pumping outside of the District” and cites Figure 50 as the basis for this determination. Figure 50 is titled “Calculated subsidence due to simulated pumping in the subsidence districts near Montgomery County.” This determination does not appear to be supported by the analysis approach and the way subsidence is represented in the HAGM. The report also does not include an analysis of land subsidence due to pumping in Montgomery County for comparison. It is recommended that a more complete discussion of this information be included in the report. Based on water-level data, the area’s downdip in Harris County may be affected by updip groundwater use in Montgomery County. The SAC suggests that the following represent remaining information gaps that are vital for
decision-makers:
a) What do the data show to date for Montgomery County in terms of land subsidence? What are the conclusions for the county? The Phase 1 Report appears to focus on subsidence primarily in the greater Houston-Galveston region.
b) What are the trends in historical and current groundwater use? How have these changed through time, and how are they related to subsidence?
c) Drought should be considered in terms of water usage/resource planning since those are the conditions under which water demand increases significantly. What does the historical record reflect during periods of drought for water level changes and subsidence?
[Note: The reference list shown on page 17 are shown to be available via "online" link, but they are all dead links. This is a historical reference list on the subject as of 2004 (here).].
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