Unit 1.3: Understanding Perturbations to Hydrologic Systems
Overall learning objective for this activity: Students will use rainfall and runoff data to describe the hydrologic cycle and construct a water balance on a watershed scale.
Specific learning objectives for this activity:
- Students will analyze rainfall and runoff data from two small watersheds, one agricultural and one urban, and use them to construct a water balance equation.
- Students will compare and contrast, quantifying the difference in the water balance between the two different land uses.
- Students will evaluate the role of ecosystems services in watershed hydrology and the water balance.
Context for Use
This activity may be used alone as an introduction to watershed rainfall and runoff data, from which the impact of land use on runoff can be analyzed quantitatively. It can be used in combination with Units 1.1 and 1.2 to explore the role of ecosystem services within a watershed that help regulate the hydrologic cycle and provide for freshwater resources. This activity would be appropriate in a range of introductory courses, including courses in water resources, sustainability, ecology, Earth science and geology, land-use planning, anthropology, and landscape design.Class Size: This activity can be adapted for a variety of class sizes.
Class Format: This activity is designed for individual lecture sessions, but it is suitable for use in a lab setting or as a homework assignment as well. Students can work together, in groups of 2–4, but each student should complete his/her own assignment.
Time Required: This activity is designed to be completed in a 50-minute class period.
Special Equipment: Student groups should have a computer with Google Earth installed. If that is not possible, the instructor can print out information before class. Not every student needs a computer for this activity, but at least one computer must be available per group. From the instructors' experiences, having at least one computer per three students is ideal.
Skills or concepts that students should have already mastered before encountering the activity: This activity is self-contained and assumes no familiarity with basic concepts of ecosystem services or the hydrologic cycle.
Description and Teaching Materials
Rainfall-runoff data for both an agricultural watershed (Rock Creek) and an urban watershed (Big Creek) in Ohio are provided in table form (Rainfall-Runoff Data for Big Creek and Rock Creek Watersheds (Microsoft Word 2007 (.docx) 47kB Aug15 16)). An X-Y scatter plot for the agricultural watershed (Rock Creek) is also provided on which the urban watershed data (Big Creek) can be plotted by the students. Google Earth files of the boundary and drainage network (Rock Creek & Big Creek watersheds (KMZ File 232kB May29 15)) and land cover for each watershed (Big Creek and Rock Creek Land Cover (KMZ File 101kB Dec2 16)) are also included for use presentation or discussion. These materials are used to examine changes in land use and their impact on the water balance equation.
Teaching Notes and Tips
Below is a lesson plan for a 50-minute class period.
Examine the rainfall-runoff data (10 min)
Data for Big Creek watershed should have been plotted for homework. Ask students to summarize the average rainfall-runoff data from Big Creek for comparison with Rock Creek using the water balance equation.
For the Rock Creek watershed, recall that the average values for P and Q for the period of record were
P = Q + ET + ΔS
37.37 in = 13.02 in + ET + ΔS
The remainder, to balance the equation, is 24.25 in, the amount of water that is stored within the watershed as soil moisture and eventually leaves the watershed through evaporation and transpiration or percolates more deeply and becomes groundwater.
Similar to Unit 1.2, write the water balance equation on the board (P = Q + ET + ΔS). Ask students again what terms can be quantified from the Big Creek watershed data. This can be done for any given year, or it can be done for an average over the years of record as well, as was done for Rock Creek watershed. For the Big Creek watershed, the average for the period of record would be
P = Q + ET + ΔS
38.15 in = 20.29 in + ET + ΔS
The remainder, to balance the equation, is 17.86 in, the amount of water that is stored within the water as soil moisture and eventually leaves the watershed through evaporation and transpiration or percolates more deeply and becomes groundwater.
ET +ΔS (in)
Brainstorming reasons for the differences in the two data sets (10 min)
The data sets are similar in that the range of annual rainfall for the Big Creek watershed is within the range of rainfall for the Rock Creek watershed. The difference is in runoff. Have students pair up to brainstorm the reasons for the difference. Follow this with a general session focused on the watershed as a system and around the water balance equation. Reasons should be attributable to either a change in evapotranspiration or in storage within the watershed and the system variables that affect it. It is certainly a way to organize the ideas from the brainstorming session.
Examining land use (15 min)
Review the Rock Creek and Big Creek watersheds. Make available to students the Google Earth file that shows the boundaries and drainage network for the two watersheds (Rock Creek & Big Creek watersheds (KMZ File 232kB May29 15)). Alternatively, slides in the PowerPoint presentation contain the same information.
Individually or in pairs, have students tie land use differences to their respective impacts on the hydrologic cycle. They should be as specific as possible and use the language introduced in Units 1.1 and 1.2. Open the discussion up in the last 5 minutes so that ideas are shared and a uniform sense of the impact is available to the class as a whole.
Following deforestation for agricultural purposes at the time of Euro-American settlement, the change from agricultural land uses to urban land uses is the most pervasive. Many of your students will have come from homes that were built in the past 30–50 years in suburbia. It is valuable to point this out. If you are using Google Earth, the "Show historical imagery" function could be used for an area with which you are familiar that shows urban or suburban development. This was also covered in Unit 1.1 with some examples, so you could simply use those if you prefer. The main point here is that impervious surfaces cause the perturbation or disturbance of the system, creating a positive feedback between other system variables that is deleterious.
Ecosystem services (15 min)
The remainder of the class should be spent discussing the impacts to ecosystem services caused by urbanization and the ways to mitigate/remedy those impacts. Critical questions include the following:
- What ecosystem services are impacted during conversion of land use from agricultural uses to urban uses?
- In what ways is the impact mitigated in urban watersheds?
- Are there ways in which natural ecosystem services can be restored or mimicked in the urban watershed to restore some of the services more natural areas provide?
- How does the systems perspective allow for identification of the services a watershed provides?
The critical questions suggested at the very end of the Teaching Notes and Tips can be used for a written assignment for assessment. In that case, a summary of the water balance for watersheds dominated by different land uses should be included to preface the questions. A Unit 1.3 Assessment (Microsoft Word 2007 (.docx) 20kB Sep3 16) is included. A key is also included:
References and Resources
- A good review of the hydrologic or water cycle is available at http://water.usgs.gov/edu/watercyclesummary.html.
- If Unit 1.3 is being used alone, students should read the "Watershed Hydrology Literacy" handout (Watershed Hydrology Literacy (Microsoft Word 2007 (.docx) 1.5MB Aug25 16)) as before-class preparation material. This handout defines key terms and concepts associated with the hydrologic cycle, watersheds, and water balance. A watershed literacy quiz is included (Watershed Hydrology Literacy Assessment (Microsoft Word 2007 (.docx) 14kB Dec1 16)) to focus students on local aspects of the hydrologic cycle and watersheds.
- A Power Point presentation (Unit 1.3 Presentation (PowerPoint 2007 (.pptx) 8.9MB Dec2 16)) is included with images associated with the hydrologic cycle and watershed characteristics, statistics related to the watershed for which the rainfall and runoff data were determined, and a plot of the rainfall-runoff data.
- Rock Creek and Big Creek data are included in table form, with the Rock Creek data plotted (Rainfall-Runoff Data and Plot (Microsoft Word 2007 (.docx) 46kB Aug28 15)).