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Unit 2: Characterizing groundwater storage with well and GRACE data

Bruce Douglas, Indiana University (douglasb@indiana.edu)
Eric Small, University of Colorado (eric.small@colorado.edu)


Summary

This unit provides students with experience analyzing traditional (depth to water table measured in a well) and geodetic: GRACE (Gravity Recovery and Climate Experiment) data for monitoring changes in groundwater storage in the High Plains Aquifer. Variations across timescales are compared, from seasonal to interannual to decadal. This comparison highlights some of the challenges associated with quantifying changes in groundwater storage at the regional scale. Aquifer properties are used to consider changes in terms of both "depth to water table" and water storage. Students are asked to formulate explanations for the observed variations in the context of the water balance equation. Students compare their results to a multidecadal trend reported in the literature (Konikow, 2011).

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Learning Goals

Unit 2 Learning Outcomes

Students will be able to:

  • Calculate a change in water storage based on changes in water level and relevant physical reservoir properties.
  • Analyze well "depth to water table" and GRACE "total water storage," and distinguish between noise, seasonal variations, and secular trends.
  • Formulate explanations for the observed seasonal variations and secular trends in terms of the water balance equation.
  • Compare and contrast the strengths and limitations of traditional and geodetic techniques for monitoring groundwater storage.

Unit 2 Teaching Objectives

  • Cognitive: Facilitate the students' ability to evaluate changes in groundwater storage, given water table data and information about rock properties. Enable students to consider changes in storage in the context of the water balance equation.
  • Behavioral: Promote skills in making calculations in groundwater storage and understanding how these changes are might be recorded in various types of time-series data sets.
  • Affective: Facilitating the students' appreciation of secular changes in hydrologic stores that are critical to society and the challenges of quantifying these changes amid other variations.

Context for Use

The content in Unit 2 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 storage and its recharge and use; this typically would 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 2 can be adapted to be executed in lecture and lab settings as a series of interactive lecture activities, a lengthier in-class activity, or as part of a laboratory investigation of the use of geodesy to understand water resources. In the Measuring Water Resources module, Unit 2 is designed to follow Unit 1: Introduction to the hydrologic cycle, which serves as a preparatory exercise that provides the students with experience in basic calculation and unit conversions tools. It also establishes a set of criteria for recognizing the basic relationships between fluxes, reservoirs, and residence times. Some preparatory lecture time may be required to introduce the concept of groundwater storage so that the students are aware of the relationship between the reservoir host rock and natural variations in porosity, permeability, hydraulic conductivity, and storativity as these parameters can be modified over time, especially when there are extreme levels of change induced by groundwater pumping. Because this work is most effective when there are high quality data sets to analyze, we have compiled appropriate example data sets that may be expanded through the use of governmental resources such as the USGS or state agencies. If the entire two-week module will not be utilized, we recommend pairing Unit 2 with Unit 4: Water balance in a California drought to give students an opportunity to investigate the problems associated with imbalances between groundwater recharge and extraction. Unit 1: Introduction to the hydrologic cycle provides the motivation for undertaking the other exercises, but alternatively this context may be provided in lectures and other instructional modes such as a PowerPoint presentation or skipped if students are already familiar with hydrologic stores and fluxes. Alternatively, Unit 2 could be paired with Unit 1: Introduction to the hydrologic cycle for a robust introduction to the hydrologic cycle.

Description and Teaching Materials

Introduction

A PowerPoint presentation provides brief descriptions and definitions of the basic terms and methods that will be used in this unit. This introductory material will allow for a more engaged discussion of the different types of data collection technologies (e.g. electric tape or pressure transducer for measuring water depth in a monitoring well, gravity data based on gravitational attractions associated with water mass). This is also a good place to review the various unit conversions, calculations, and normalizations that are required in the different parts of the unit. It is very beneficial to review and practice the calculations and remind the students that dimensional analysis can be helpful in checking for internal consistency in calculations. Remind students that on the student exercise all calculations must be shown along with results and responses to the specific questions. It should be noted that it is assumed that Unit 1 will have been completed prior to undertaking Unit 2 so that the context and framework of the Hydrologic Cycle and its various reservoirs, fluxes, and resident times are familiar concepts to the students. Alternatively, this information may have been learned in another course that may be a prerequisite for the course that is using this module. In that case, brief review may be necessary.

Comments on Unit 2 Student Exercise

Depending on the course, students may or may not have the background to tackle the student exercise without first receiving instruction in the specifics of groundwater well and satellite gravity data. Once this instruction has been provided, either through classroom lectures or assigned reading, the individual parts can be assigned to be completed during class time, as a homework assignment, or in a laboratory meeting at a subsequent time. The ideal setting would be a combination of lecture time and time for the students to engage with the materials so that they can ask questions as they run into problems, which would also allow for general discussions. This can be done as small-group work, but because the goal is to have the students familiar and able to perform the calculations and construct appropriate displays of the data in tables for graphs, active participation by all students is best. This suggests that at least students should be required to turn in the exercise individually.

Part 1: This first section makes use of "traditional data"" depth to groundwater well data from the USGS. Until fairly recently, these were the only data available. They have a long period of record, which makes them valuable for looking at both long- and short-term trends. Data from three wells located in the northwestern portion of the High Plains Aquifer (HPA) are provided. Students examine the seasonal variations and long-term trends in the data. The three wells were selected to demonstrate the range of behavior that exists. Students are asked to formulate explanations for these fluctuations in the context of the water balance equation. These three wells were selected because of their long record, but they stopped being monitored in 2010–2012. If you wish to use wells that come up to the current year, you can look for ones of interest in the High Plains Aquifer Groundwater Network website.

Part 2: This section makes use of a relatively new technique for determining changes in terrestrial water storage (TWS) through the use of data from the Gravity Recovery and Climate Experiment (GRACE). It is expected that the students will need to be introduced to both the technology behind the data as well as how the data is derived so that it can be used to directly compare to the traditional data used to study groundwater. This may require additional preparation using some of the resources provided as links (see References and Resources section below). Again, the students compare seasonal and long-term trends. However, the GRACE period of record is relatively short (starting 2002). It is expected that the students will realize it is challenging to quantify trends from such a short record, given the variability that exists on seasonal and internal timescales. Students compare results from the GRACE and water table analysis, and generate explanations for the observed differences.

Part 3: This final portion of Unit 2 asks the students to reflect and comment on what knowledge they had with regard to the subject and the methods for collecting the data used in groundwater storage before and after working on this unit. This section is intended to encourage the students to reflect on what they have learned in completing this unit in terms of both the scientific methods and analyses they completed as well as how much their views have changed of the new techniques that can be adopted to solve problems that society is facing, by virtue of the spacial and temporal scales available through the use of geodetic-based data sets.

Teaching Materials

  • Presentation: Unit 2 Measuring Groundwater Background Information (PowerPoint 2007 (.pptx) 3.5MB Dec11 18)

    Presentation: Unit 2 Measuring Groundwater Background Information
    Click to view

  • Student exercise
  • Student exercise data files
    • Unit 2 student exercise data files (Zip Archive 4.8MB May15 18)
      Data set includes:
      • Depth-to-groundwater Excel tables and Google Earth map for three USGS wells in Wyoming
      • GRACE Terrestrial water storage anomaly (TWSa) Excel table and Google Earth and pdf maps showing spatial extent of GRACE cells that were used to generate TWSa across the High Great Plains Aquifer. The map files includes an example of a wetter period (May 2011), drier period (November 2012), and the difference between the two. This helps us see the spatial variability of TWSa.
  • Background info on GRACE satellite data (Gravity Recovery and Climate Experiment)
    • How GRACE works (Acrobat (PDF) 244kB Jun2 15)
    • Video: GRACE: Tracking Water From Space
    • Note: GRACE satellites were launched in 2002 and had an originally planned mission length of five years, which was far exceeded. By late 2016, the satellites were still functioning but operations were adjusted to only run normally during times of direct sunlight, to compensate for reduced battery capacity (http://www2.csr.utexas.edu/grace/). The GRACE Follow-on (GRACE-FO) mission launched in mid 2018 to carry on this important work.

Teaching Notes and Tips

  • The student handout is written assuming that the work is being done as part of a course that would be providing the appropriate background information so the student would be familiar with the various concepts, equations, and assumptions that are being used. If this is not the case, special attention needs to be paid to providing sufficient background information so that the students can proceed with the mechanics involved in each part of the unit. The expectation is that through working through the individual questions the student will develop an understanding of both the concepts and the data.
  • The units 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.
  • The Google Earth files provided here are reference maps so students can see where the USGS wells are located and the extent of the GRACE data coverage for this assignment. They are not part of the data analysis itself. Alternatively, the instructor could provide printout images of the Google Earth files in a handout.
  • Unit 2 is where extensive manipulation of time-series data sets become a primary component of the workload. If students are unfamiliar with Excel, you may need to walk them through the process of doing Excel calculations and graphs; for example, converting well data from feet to meters in the next adjacent column or making clean, easy-to-interpret plots. Examples associated with the depth to ground water data sets, assuming that Excel software will be used, include changing the default time labeling, inverting the plotting scheme for positive numbers for depth to water for the groundwater well data, and techniques for selecting data ranges for time series that include 5000 data pairs. For the GRACE data, similar plotting/display manipulations (e.g. x-axis cross of the y-axis, highlighting positive from negative anomalies with different color symbols) improve the quality of the data plots making interpretation more straightforward.
  • 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.
  • Students seem to be surprised by the difference between seasonal cycles and long term trends. They seem to need help understanding how the season cycles related to irrigation use.
  • If this unit is being used in a lower-level course it can be very effective as a learning tool, but some modifications need to be made. One is to reduce the number of plots the students need to make by supplying both the raw data and the data with the plot already constructed. The students can then go directly to interpretation and analysis of the time series and not spend extra time and potentially getting frustrated trying just to plot the data.
  • Make sure that students are familiar terms and variables being used (ex. difference between porosity and specific yield). Understanding can be enhanced by paying close attention to units and dimensional analysis while the students are performing the required calculations. One analog that almost all students can understand is the role ice plays in a glass with a beverage in it and the relative amount of solid (ice) versus pore space (filled with liquid) that can be directly observed as the beverage is consumed. If the unit is to be used in lower-level courses, a vocabulary sheet and examples of calculations with dimensional analysis would need to be included with the student handout.
  • Again, it is assumed that upper-level students should be able to sort through the various data sets provided to find the one needed to answer the various questions. It might be wise to help the students with some sort of organization of the data and/or to include the data file name with each section or question so the students know what file to look for. It should be noted that the data sets provided could be substituted for data from other regions, if that is desirable; as more GRACE data becomes available, the inclusion of longer time series will be possible. One question that may be considered is whether to have the students find the data from the original sources so they become familiar with how this is done and learn to conduct their own quality control.

Assessment

Observation of student activity and conversations, individual questioning, and group discussion are excellent ways to conduct formative assessment as the students complete this exercise.

The Unit 2 Student Exercise is the summative assessment for Unit 2. Instructors can use it to evaluate how well the students have mastered the various concepts and required data manipulations to create either the results of a calculation, the conversation of units, or the display of data in table or graphical format. 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 2 Student Exercise 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 2 Example Assessment Rubric (Microsoft Word 2007 (.docx) 84kB May21 17)

References and Resources

Additional Resources for Instructors:


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This module is part of a growing collection of classroom-tested materials developed by GETSI. The materials engage students in understanding the earth system as it intertwines with key societal issues. The collection is freely available and ready to be adapted by undergraduate educators across a range of courses including: general education or majors courses in Earth-focused disciplines such as geoscience or environmental science, social science, engineering, and other sciences, as well as courses for interdisciplinary programs.
Explore the Collection »