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Cities need separate measurements for groundwater and ground movement: monitoring wells show water level or pressure head, while tools such as satellite InSAR, GNSS, geodetic surveys, and borehole extensometers track different aspects of land motion. Combining these records can reveal patterns consistent with groundwater-related subsidence, but no single measurement proves the cause.
What cities need to measure
Groundwater monitoring and subsidence monitoring answer different questions. A well records the water level in an unconfined aquifer or the potentiometric surface—the pressure level water would reach—in a confined aquifer. It does not measure whether the land above it has moved.
Ground-motion instruments measure surface displacement or aquifer-system compaction. Comparing those observations with groundwater records helps investigators evaluate whether changes in groundwater conditions may be contributing to deformation. Other surface or subsurface processes, including deeper earth processes, can also move the ground, so correlation alone does not establish cause. The U.S. Geological Survey (USGS) describes these complementary methods in its Methods of Measuring Land Motion.
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Manual measurements in monitoring wells
A water-level meter, often an electric tape, provides a direct reading of the water level in an accessible well. Repeated measurements are useful when they are made from a consistent reference point and recorded systematically. The instrument measures water in the well, not land movement. USGS describes accuracy testing for electric groundwater-level measurement tapes in its 2014 technical report.
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- High Precision Measurement: 0.04inmm minimum scale with ±0.07in repeatability ensures accurate water level readings in wells, boreholes, tanks, and pipes.
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Pressure transducers and data loggers
A pressure transducer in a well can record water-level changes continuously or at frequent intervals, showing what happens between field visits. Its record needs quality checks: compare transducer readings with manual measurements, and follow appropriate calibration and data-quality procedures. USGS provides a procedure for measuring water levels in wells and piezometers with a submersible pressure transducer in GWPD 16, version 2010.1.
How cities measure ground movement
Satellite InSAR: map movement across an area
Interferometric synthetic aperture radar (InSAR) compares radar observations from different dates to map relative movement of the ground surface over a broad area. It can help identify where deformation is occurring and how its pattern changes. It does not measure groundwater levels or identify the cause of movement on its own.
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Signal quality and interpretation depend on local conditions. Dense vegetation and high humidity can reduce signal quality, and mapped displacement needs to be assessed alongside site information and ground-based observations. USGS outlines the method and its limitations in Methods of Measuring Land Motion and Measuring human-induced land subsidence from space. The 2003 USGS fact sheet says InSAR could measure and map surface changes as small as a few millimeters in the context described there; that older statement is not a performance guarantee for every current satellite product or site.
Continuous GNSS: track fixed points over time
Continuous Global Navigation Satellite System (GNSS) stations record movement at fixed locations. They provide a time series of total surface movement at those points, but the network is spatially limited and does not isolate aquifer compaction from other causes.
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- Precision and Accuracy: The piezometer provides high-precision data on groundwater levels, which is essential for making informed decisions in water resource management, construction, and environmental protection.
- Ease of Use: Whether used manually or with automated sensors, the piezometer is easy to operate and provides fast, accurate readings without the need for complex setup or maintenance procedures.
- Real-Time Data: When equipped with automated systems, piezometers can offer real-time data, making them a valuable tool for remote monitoring, early warning systems, and large-scale groundwater management projects.
- Cost-Effective: The piezometer offers an affordable solution for accurate groundwater monitoring and water table assessment, especially in comparison to more expensive, high-tech instruments.
- Low Maintenance: Built with durable, corrosion-resistant components, the piezometer requires minimal upkeep, ensuring long-lasting reliability in both short-term and extended monitoring projects.
Repeated geodetic surveys: revisit benchmarks
Surveying established benchmarks on repeated visits measures changes in surface elevation at those points. Surveys can cover more locations than a single borehole extensometer, but they are intermittent and do not distinguish aquifer compaction from other sources of movement.
Borehole extensometers: measure compaction at depth
An extensometer measures expansion or compaction of the aquifer system at a particular location, providing precise, depth-specific information. Its coverage is local, and drilling and installation can be substantial. USGS describes these ground-based methods in Methods of Measuring Land Motion and Methods of subsidence and sea level rise monitoring.
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- Precision and Accuracy: The piezometer provides high-precision data on groundwater levels, which is essential for making informed decisions in water resource management, construction, and environmental protection.
- Ease of Use: Whether used manually or with automated sensors, the piezometer is easy to operate and provides fast, accurate readings without the need for complex setup or maintenance procedures.
- Real-Time Data: When equipped with automated systems, piezometers can offer real-time data, making them a valuable tool for remote monitoring, early warning systems, and large-scale groundwater management projects.
- Cost-Effective: The piezometer offers an affordable solution for accurate groundwater monitoring and water table assessment, especially in comparison to more expensive, high-tech instruments.
- Low Maintenance: Built with durable, corrosion-resistant components, the piezometer requires minimal upkeep, ensuring long-lasting reliability in both short-term and extended monitoring projects.
Compare methods by what they can establish
| Method | What it measures | Useful for | Limitation or check |
|---|---|---|---|
| Monitoring well with manual tape | Water level or potentiometric surface at the well | Direct, repeatable groundwater-level observations | Does not measure land motion; use a consistent reference point and records. USGS: 2014 tape accuracy report. |
| Pressure transducer and data logger | Continuous or frequent well-level series | Tracking changes between site visits | Compare against manual readings and maintain calibration and data-quality procedures. USGS: GWPD 16, version 2010.1. |
| InSAR | Relative surface displacement across a mapped area | Locating and following broad deformation patterns | Signal and interpretation depend on site conditions; it does not identify cause alone. USGS: Methods of Measuring Land Motion. |
| Continuous GNSS | Total surface movement at fixed stations | Continuous point records | Spatially limited; does not isolate cause. USGS: Methods of Measuring Land Motion. |
| Repeated benchmark survey | Elevation change at surveyed points | Checking surface elevation over a larger area than one extensometer may cover | Intermittent observations; does not separate aquifer compaction from other causes. USGS: Methods of subsidence and sea level rise monitoring. |
| Borehole extensometer | Aquifer-system expansion or compaction at a location | Precise, depth-specific compaction information | Local coverage; drilling and installation can be substantial. USGS: Methods of Measuring Land Motion. |
How to build a useful municipal monitoring approach
There is no single network layout established here as right for every city. A practical design uses the methods according to the questions they can answer: broad-area mapping to find patterns, wells to track aquifer conditions, and selected ground instruments to validate or investigate local movement. The choices depend on coverage, continuity, site conditions, precision, the need for depth-specific compaction data, installation and maintenance burden, and availability of independent checks.
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- Define the questions. Decide whether the city needs to track water-level trends, locate surface deformation, measure compaction at depth, or investigate links among them.
- Establish groundwater records. Use monitoring wells for repeatable water-level observations; where a continuous record is useful, add transducers and compare their readings with manual measurements.
- Map and verify movement. Use InSAR for broad spatial coverage where conditions permit, then consider continuous GNSS or repeated surveys for ground-based checks. Use extensometers where depth-specific compaction information is needed.
- Compare records over time. Evaluate groundwater levels alongside movement observations and compaction data. Look for consistent patterns, while accounting for other possible causes and the limitations of each method.
- Maintain data quality. Keep measurement references and records consistent, check automated water-level records against manual measurements, and document instrument calibration and quality procedures.
Can groundwater pumping make land sink?
Yes. Groundwater use can contribute to land subsidence, but a falling water level is a risk signal, not proof that subsidence has occurred at a particular site. The USGS states that more than 80 percent of known land subsidence in the United States is a consequence of groundwater use; this is a national-level summary published on its Land Subsidence page, not a city-specific rate.
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A city investigating a possible connection needs paired evidence: groundwater-level observations and independent land-motion measurements, with compaction data where relevant. A local cooperative study described by USGS involved the City of San Diego and Sweetwater Authority using InSAR to understand coastal aquifer response. USGS notes that combining InSAR with ground-based geodetic and monitoring-well networks can inform water management; the example illustrates a multi-method approach, not a network specification for every city. See Aquifer-Storage Change and Land-Surface Elevation Change Monitoring in the Tucson Active Management Area.
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