Unit 3: Monitoring groundwater storage with GPS vertical position
This unit shows how GPS records of surface elevation can be used to monitor groundwater changes. Students calculate secular trends in the GPS time series and then use the original and detrended records to identify sites that are dominated by the elastic response to regional groundwater changes versus those dominated by local subsidence. They then compare the magnitude and timescales of fluctuations in Earth's surface elevation that result from sediment compaction, regional groundwater extraction, and natural climatic variability. This unit provides students with hands-on experience of the challenges and advantages of using geodetic data to study the terrestrial water cycle. The case study area is in California and the GPS records include the period of the profound 2012–2016 drought.
Unit 3 Learning Outcomes
Students will be able to:
- Calculate secular trends in GPS vertical-position data, and compare these trends to variations at seasonal and interannual timescales.
- Compare and contrast records from GPS sites that are dominated by the elastic response to regional groundwater extraction versus those dominated by local subsidence.
- Quantitatively compare the magnitude of changes in Earth's surface elevation that result from sediment compaction versus those due to hydrologic (un)loading.
- Critique/evaluate the utility of GPS vertical-position data for monitoring changes in groundwater storage, compared to both traditional and other geodetic data types.
Unit 3 Teaching Objectives
- Cognitive: Facilitate the students' ability to calculate and compare secular trends versus seasonal-to-interannual fluctuations. Enable students to demonstrate the connection between changes in groundwater storage and the processes that result in changes in Earth's surface elevation.
- Behavioral: Promote skills in making and interpreting graphs that isolate secular trends and higher-frequency fluctuations. Facilitate an understanding of how numerous processes must be considered when using geodetic data to study the terrestrial hydrologic cycle.
- Affective: Facilitate the students' appreciation of the complex interactions between various hydrologic and geophysical processes, and how this complexity leads to uncertainty in our estimates of changes in water availability.
Context for Use
The content in Unit 3 is appropriate for advanced geology/geoscience courses conducted at the junior and/or senior level in which geodesy data can be introduced in conjunction with traditional presentations of material on groundwater and the water cycle. This would typically be in a course on hydrology or hydrogeology but could also be part of a course on Earth systems, environmental geology, environmental engineering, or advanced geohazards (with a connection to subsidence). Unit 3 can be adapted to lecture or lab settings as a series of interactive lecture activities, a lengthier in-class activity, or the basis of a laboratory investigation on the use of geodesy to understand water resources.
In the Measuring Water Resources module, Unit 3 is designed to follow Unit 1: Introduction to the hydrologic cycle and Unit 2: Characterizing groundwater storage, which provide the appropriate background material necessary to understand the nature of groundwater storage changes and how these changes may be monitored with geodetic tools. If the entire two-week module will not be used, we recommend pairing Unit 3 with either Unit 2: Characterizing groundwater storage or Unit 4: Water balance in a California drought, depending upon the focus of the course. It is possible to skip Unit 1: Introduction to the hydrologic cycle, as long as students are familiar with hydrologic stores and fluxes.
Description and Teaching Materials
This unit is centered around analysis of vertical-position data from nine GPS stations in California. The student exercise is formatted as a lab, but could be reformatted to be used in class or as a homework assignment. The exercise is designed to be completed in groups of three students. Each student works with data from three of the GPS sites and shares answers with other members of the group. The primary goal of the lab is for the students to identify which GPS sites are affected by regional groundwater withdrawal and which are affected by subsidence due to local pumping. Each student is given data from one site in the Sierra Nevada, one site in the Coast Range, and one in the Central Valley. The student plots the data and calculates a trend for each site. Then, they remove the trend and construct a detrended time series for each site that highlights seasonal and interannual variations. The students in each group share data to classify the sites into two very distinct categories. The mountain sites behave similarly—there is consistent uplift of several mm/yr, which accelerates over the past several years of the record due to drought. Seasonal variations are consistent with California's wet-winter climate: the lowest vertical position occurs in March. In contrast, the valley sites are subsiding at ~20 mm/yr due to groundwater extraction and compaction of sediments. On the annual timescale, the valley sites are lowest in early summer, when groundwater extraction is greatest. The final component of the lab is as follows: the students describe and sketch a conceptual model for how groundwater extraction affects local and distant sites, and compare the magnitude of regional uplift and local subsidence. The students are also asked to reflect on the strengths and limitations of monitoring groundwater storage changes using variations in vertical position recorded by GPS instruments.
After the students complete the exercise, there is an animation that can be used to help bring together the ideas from the lab into a straightforward visualization. Students also appreciate seeing the findings from a wider range of stations than just the nine that they investigate. It can also help them appreciate the severity of the 2012–2015 California Drought in preparation for dealing with it more directly in Unit 4: Water balance in a California drought.
Youtube: Measuring Drought: A GPS Network Offers A New Perspective
MP4: Measuring Drought: A GPS Network Offers A New Perspective (MP4 Video 16MB Jun3 17)
- Presentation: Unit 3 background information (PowerPoint 2007 (.pptx) 6.2MB Jun22 17)
- Additional background information can be found in the document USGS Publication Circ. 1182: Land Subsidence in the United States, a section of which is given below.
- USGS Cir 1182, Part I - Mining Ground Water (Acrobat (PDF) 944kB Oct4 15)
- This document can be assigned to the students as a reading or just used by the instructor as a reference.
- Student exercise and supporting files
- Unit 3 student exercise (Microsoft Word 2007 (.docx) 391kB Dec5 17)
Unit 3 student exercise PDF (Acrobat (PDF) 416kB Dec5 17)
- The student exercise is formatted as a lab, appropriate for use in a typical 2.5-hour lab period. Much of the data analysis and interpretation can be completed during the lab period. The summary and conceptual drawing will likely require additional time beyond the lab period.
- Unit 3_Student Data File (Excel 2007 (.xlsx) 1.9MB Feb14 18)
- Student exercise data: GPS position time series from nine sites in California. Includes the Summary Plot table that students are asked to copy into their exercise write up.
- Unit 3 GPS Station locations and basin boundaries Google Earth files (KMZ File 109kB Feb5 18)
- Google Earth station locations and watershed boundaries. Students use this to become familiar with their GPS station locations and regional physiography.
- Sacramento-San Joaquin Basin Map (Acrobat (PDF) 4.2MB Jun3 17)
- Alternative map to see the watershed and basin boundaries.
- California Geologic Map (Acrobat (PDF) 1.5MB Oct29 15)
California Geologic Map w/ GPS stations marked (Acrobat (PDF) 1.7MB Nov29 17)
- We provide versions of the map both with and without the GPS stations marked - depending on whether you wish the students to locate the stations themselves or not.
- California Geologic Map Legend (Acrobat (PDF) 967kB Oct30 15)
- Answer keys
- Includes completed calculations and graphs
- Youtube: Measuring Drought: A GPS Network Offers A New Perspective (also available in Spanish - change subtitles to Spanish in the Youtube settings)
- MP4: Measuring Drought: A GPS Network Offers A New Perspective (MP4 Video 16MB Jun3 17)
- Detrending Data
- Detrending Data Instruction Sheet (Microsoft Word 2.9MB Nov11 19)
Detrending Data Instruction Sheet_PDF (Acrobat (PDF) 1.9MB Nov11 19)
- Provides information on detrending geodetic data in Excel. Could be given to the students as a reference or instructor can overview the process in a demonstration/lecture.
- Updating Data
- Updating GPS data via UNAVCO website (Microsoft Word 2007 (.docx) 75kB Jan17 19)
- Instructions for how to update the GPS data used in this unit. Intended for instructor reference or for student use if the instructor wishes the students to download the data themselves.
Teaching Notes and Tips
- Students may need help with specific Excel functions such as "slope" to calculate the trend. They may also need help thinking through how to "detrend" the position data.
- Likely unit-conversion issues for students: When a trend is fit to data (e.g., in Excel), the slope of the best fit line has units of mm/day (because the data are daily). In the exercise, trends are considered in terms of mm/yr. Remind students they will need to do a unit conversion.
- The Teaching with Spreadsheets across the Curriculum site provides support for teaching with programs such as Excel. If your students need supporting math practice, The Math You Need site provides an opportunity to brush up on skills such as graphing and unit conversion. Teaching with Google Earth provides a variety of resources for using this powerful program during teaching.
- Misconceptions: Most undergraduate geology majors have heard of isostasy, so they might think the vertical displacement due to hydrologic loading is an isostatic adjustment. It needs to be clarified that what is being studied is an elastic response of the rocks themselves, not an isostatic adjustment. Elastic response is instantaneous and ~20 times smaller; whereas isostatic adjustment has time scales of hundreds to thousands of years but ultimately the adjustment will be much greater.
- The unit requires use of computers. Ideally each student will have their own computer or laptop. At least there should be one computer per work group. If it is not feasible to use computers during the class/lab period, the instructor should take extra steps to make sure that students understand what is required of them to do the exercise outside of class.
- It is important to stress that the lab is based on real GPS data, which makes the data analysis more challenging (but more interesting) than if an "idealized" data set was used.
- Instructions for downloading data from UNAVCO: Updated GPS position data can be downloaded from UNAVCO. For example, data for site P307 can be found at the P307 GPS station webpage: http://www.unavco.org/instrumentation/networks/status/pbo/overview/P307. Then click on the link under Time Series Data: NAM08 CSV. Students should not be directed to this site, as they would find the time series plots they are supposed to make in this assignment.
- If you wish to greatly streamline the unit or do the unit with less advanced students, the instructor may produce the GPS graphs and just have the students interpret them for the exercise.
The Unit 3 Student Exercise is the summative assessment for Unit 3. Instructors can use it to evaluate how well the students have mastered the various concepts and required data manipulations, and how this information is used in the summary section where students conceptualize the different responses observed in the groups of GPS stations. The student responses should be graded using the rubric that is given to the students. Instructors can modify this rubric to assign point values in a manner that is consistent with their course-grading scheme. The various descriptions for performance on each of the numbered tasks have been written in an attempt to make them applicable for the range of responses asked for in each of the numbered tasks. As such there may need to be some degree of flexibility in how the rubric is applied.
Student metacognition is an important part of the learning process. The Unit 3 student handout includes a final section to encourage students to reflect on their own learning during this unit and its personal significance. Students' ideas should not be graded, but responses can be scored using the rubric included in the handout to judge the level of engagement of the student.Unit 3 Example Assessment Rubric (Microsoft Word 2007 (.docx) 62kB Jun10 17)
References and Resources
- Introduction to GPS presentation (PowerPoint 2007 (.pptx) 30.6MB Nov11 19) – Presentation that can be used if an instructor wishes to provide more background information on how GPS works.
- An alternative to the pdf geological map is the interactive online Geologic Map of California.
- Konikow, 2011, Contribution of global groundwater depletion since 1900 to sea‐level rise, Geophysical Research Letters, v 38, L17401, doi:10.1029/2011GL048604
- Famiglietti, 2014, Commentary: The global groundwater crisis, Nature Geoscience, v 4, p 945–48.
- A synopsis was written by NASA's Earth Observatory
- Amos, C. B., Audet, P., Hammond, W. C., Bürgmann, R., Johanson, I. A., & Blewitt, G. (2014). Uplift and seismicity driven by groundwater depletion in central California. Nature.
- Borsa, A. A., Agnew, D. C., & Cayan, D. R. (2014). Ongoing drought-induced uplift in the western United States. Science, 345(6204), 1587–1590.
- Chew, C. C., & Small, E. E. (2014). Terrestrial water storage response to the 2012 drought estimated from GPS vertical position anomalies. Geophysical Research Letters, 41(17), 6145–6151.
- Faunt, C.C., ed., 2009, Groundwater Availability of the Central Valley Aquifer, California: U.S. Geological Survey Professional Paper 1766, 225 p.
- Galloway, D., D. R. Jones, and S. E. Ingebritsen (1999), Land Subsidence in the United States: U.S. Geological Survey Circular 1182, Reston, VA.
GPS data from around the USA can be downloaded at the Plate Boundary Observatory website.