Baseflow recession

Students will gain skills in data manipulation and filtering, charting and curve fitting (model building) and model comparison, and in summarizing the results of the investigation in a report which contains charts, tables, and discussion.

Learning Outcomes
Students will:

1. Build procedural skills in working with data. The skills range from moving data from an online server into a spreadsheet, to filtering data and summarizing by some attribute of the data (averaging by month, year, etc), to writing equations in cells, to charting and curve fitting;
2. Gain insight into how Earth works based on their own interactions with the data, that is, they learn how to discover relationships; and
3. Develop transferable analytical skills

This activity:

• Is part of an introductory level course in hydrogeology. It has been administered to sophomore and higher students at a 4 year liberal arts college.
• Is usually administered as a computer laboratory exercise.
• Requires some familiarity with spreadsheets, including copy-pasting data, inserting formulas and charts, fitting curves, etc. Students are expected to learn these skills as part of the exercise.
• Requires that students have some knowledge of streamflow (discharge, gage station data) and the water cycle;
• Is at the cusp of a transition in hydrogeology, where course material moves from surface hydrology to groundwater, and thus serves as a significant introduction into stream and groundwater interactions.
• Has been used as an extra credit exercise in a 100 level non-science majors course (Sustaining Water). Video tutorials linked from the assignment provide training on how to perform the spreadsheet operations.

The activity file below includes the instructions for how to model baseflow recession. It also contains guidelines for a report, and how the students will be evaluated.
Baseflow Recession Project Description (Acrobat (PDF) 212kB May17 21)
Baseflow Recession Modeling Data (Excel 2007 (.xlsx) 58kB Apr30 19)

Students routinely encounter this project without having any experience with a spreadsheet. I like to introduce them to Google Sheets, which has fewer data format issues in calculations, and a simpler interface. But Excel or Sheets or any other spreadsheet which create scatter charts and fit an exponential equation to data is suitable. Students have to learn how to insert a formula and "copy it down", how to reference cell locations in a formula, how to make a chart, how to control chart appearance through its properties, and how to fit a trendline to a data series. All of these operations are hurdles. Most students can make it over the hurdles. Video tutorials help. Feel free to use the tutorials linked from the project description (they're mine).
Once students get the charts, they will often find that they did not select a continuously declining part of the stream flow record. They should make every effort to find simple declining flows, but even when the curves appear continuous, exponential functions don't always apply. Obvious disturbances to a steady decline include precipitation and temperature (think freezing the river!), or less obvious ones such as multiple aquifers supplying baseflow which have different characteristics (thickness, hydraulic conductivity, areal extent), and extraction of water by humans.
Part of the lesson is that students learn how to think about time series as responses to a number of possible variables, both intrinsic to the system (stream networks, infiltration characteristics, transpiration, aquifer characteristics), as well as those that force the signal (climate, humans, etc). It's rich, and somewhat of a tangled snarl of interactions, but I think the exposure to this richness is valuable for training geoscientists.

Students hand in a report, with charts with baseflow recessions and trendlines, a summary table which encapsulates trendline parameters, an estimate of how long it will take for the stream to go dry if no rain falls, and analysis and discussion of streamflow recession results and implications. Each of these report components are evaluated by a rubric. Each component receives a weight. An ensemble score is based on the sum of each score times the weight. A rubric is included in the exercise guidelines.

The following documents provide background information on major influences on baseflow recessions, as well as descriptions of how working hydrologists have performed baseflow analyses in the past.

Bingham R. H, Regionalization of low-flow characteristics of Tennessee streams U.S. Geological Survey, Water-Resources Investigations Report 85-4191, 1986. https://pubs.usgs.gov/wri/wri85-4191/pdf/wrir_85-4191_a.pdf

Rorabaugh M.I., and W. D. Simons, Exploration of methods of relating ground water to surface water, Columbia River basin-Second phase, USGS Open-File Report 66-117, https://doi.org/10.3133/ofr66117.

Tallaksen, L.M. 1995. "A Review Of Baseflow Recession Analysis". Journal Of Hydrology 165 (1-4): 349-370. Elsevier BV. doi:10.1016/0022-1694(94)02540-r. https://www.sciencedirect.com/science/article/pii/002216949402540R