Unit 1.2: Exploring the Hydrologic Cycle
Overall learning objective for this activity: Students 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 characterize the different components and processes of the hydrologic cycle as they relate to a small watershed.
- Students analyze rainfall and runoff data from a small watershed and use the data to construct a water balance.
- Students evaluate the role of ecosystems services in watershed hydrology and the water balance.
Context for Use
This activity may be used alone or in combination with Unit 1.1 as introduction to the hydrologic cycle, using rainfall and runoff data from a small watershed in terms of a water balance, in a quantitative manner and at a scale relevant to and understandable by students. It can be used in combination with Unit 1.3 to assess differences in the rainfall-runoff relationship between agricultural watersheds and more urban built-up watersheds, or as an application within the Ecosystem Services Approach to Water Resources Module. This activity would be appropriate in a range of introductory courses, including courses in water resources, sustainability, ecology, environmental science, 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
In preparation for this activity, students should read the "Watershed Hydrology Literacy" handout (Watershed Hydrology Literacy (Microsoft Word 2007 (.docx) 1.5MB Aug25 16)). This handout defines key terms and concepts associated with the hydrologic cycle, watersheds, and water balance. A watershed literacy quiz can be used prior to the class as homework (Watershed Hydrology Literacy Assessment (Microsoft Word 2007 (.docx) 14kB Dec1 16)).
Rainfall-runoff data for an agricultural watershed in Ohio (Rock Creek) are provided, both as raw data and plotted as an X-Y scatter plot (Rainfall-Runoff Data and Plot (Microsoft Word 2007 (.docx) 46kB Aug28 15)). A Google Earth file of the watershed boundary and drainage network for the watershed is also included (Rock Creek watershed (KMZ File 178kB May28 15) ). These materials are used to examine the hydrologic cycle in terms of a water balance in this watershed.
Teaching Notes and Tips
Introduction to the hydrologic cycle (5 min)
Use one of the following resources to introduce the hydrologic cycle to your class. The students already have an image in mind of the typical water cycle, illustrating rain falling over a landscape with a stream flowing into an ocean. Your intent here should be to provide a concrete sense of the hydrologic cycle, placing it within the context of watersheds, ideally your local watershed. Several tools are available to start this discussion relative to your campus location:
- Surf Your Watershedâ€”through the EPA, for a general location within a watershed. Good for accessing other sites relative to environmental issues such as impaired watersheds or stream reaches.
- EDNA Derived Watersheds for Major Named Riversâ€”if you wish to use a drainage network as a discussion starter, the USGS site provides the most detailed network. Selecting a watershed from the list spawns a Google Earth session. Expand Watershed Layers, turn off the Land Cover layer, turn on the Streams, and zoom in to your area of interest.
One of the EarthLabs activities, titled What's a Watershed?, may provide a useful, extra activity associated with this unit.
- Streamerâ€”from the USGS, this is an especially good way to illustrate upstream contributing networks if you reside along a larger stream, or the downstream destination of flow using the Trace Upstream and Trace Downstream tools. The Trace Upstream tool is nice because it shows the drainage network upstream and the basic outline of the watershed. The Trace Downstream tool can be used to illustrate where runoff (and hence sediment, pollutants, flood discharge) ultimately ends up. The drainage networks area not detailed (i.e., first-, second- and third-order streams, and maybe higher, are not shown), but the tracer tools and Trace Report functions are neat.
Use one of these, or Google Earth, to review important terminology associated with the hydrologic cycle, specifically: precipitation, runoff, infiltration, evaporation, and transpiration. You might include a review of the hydrologic data associated with your specific campus location. For example, you could guide a discussion on the pathway a rainfall drop would follow depending on where it fell on campus: it may infiltrate and transpire through plants, it may simply evaporate, or it may run off from sidewalks and roadwaysâ€”and if it does, what path does it take? Where does it ultimately flow?
Refocus the discussion relative to value of the hydrologic cycle (5 min)
Ask the following questions of your class: What is the value of the hydrologic cycle? What is its purpose? What are its strengths and weaknesses? Wait for their responses and write them on the overhead or the board.
- Strengths: simplified, easy to understand, a teaching tool, global view of the main processes and pathways for water movement
- Weaknesses: not very detailed, may or may not be applicable locally, not very functional except as a learning tool
This will provide a good transition to a watershed-based hydrologic cycle and, specifically, the water balance equation. The water balance equation describes the flow of water in and out of a system. It is represented by the equation:
P = Q + ET + Î”S,
where P is precipitation, Q is runoff, ET is evaporation, and Î”S is the change in storage (all these figures are expressed in inches). In this module, the system is a watershed and the boundary of the system is the watershed divide.
Strengths of examining the hydrologic cycle in terms of a watershed water balance (10 min)
Begin by having students brainstorm on the strengths of a watershed water balance, asking them to go beyond general statements. They might start with the strengths discussed for the hydrologic cycle, but encourage discussion that builds on the weaknesses previously identified. It is detailed, it is local but useful, and it is functional. If they have completed the watershed hydrology literacy quiz, they can incorporate that information in their analysis.
Strengthsâ€”simplified, easy to understand, inputs are measured locally (e.g., the daily weather report, especially reference to depths of rainfall in the viewer region following a rainfall event), outputs are more evident (e.g., flooding) or significant (e.g., water supply), quantitative using simple math (addition and subtraction), specific to a watershed, functional (i.e., if streamflow from a watershed is the basis for a public water supply or for irrigation, the amount available as output cannot exceed input and is in fact much less), illustrates variability (e.g., seasonal, year-to-year).
Introduce the Rock Creek watershed (10 min)
Open Rock Creek watershed (KMZ File 178kB May28 15) to spawn a session of Google Earth. This will provide an opportunity for clearing up any misconceptions about watersheds and inputs/outputs of the watershed system; it also address the ET and Î”S terms in the water balance equation. The following questions might help guide further discussion:
Where might precipitation fall in the map area during a given rainfall event? Where does it flow to in general? In what direction? To where? What area defines the water balance?
- How is precipitation, or the input to this watershed system, measured? Where? Rainfall is generally measured at a rain gage and reported as a depth, but it is really a volume when considered over the area (depth x area) of the watershed.
- What are the potential areas, or reservoirs, where water is stored? Ask them to visualize a rainfall event. What happens to rainfall, most of it in fact, after rainfall ends? Using prompts like "Why (or when) do we water our lawns?" can get these conversations going depending on the level of the course.
- When and where does evaporation occur? Transpiration?
- Where is output, or runoff, measured in the watershed? Where does it come from? The stream network? But where does streamflow come from? Watershed output is the measured streamflow at the outlet, generally as a volume or volume per time (i.e., streamflow discharge or total discharge). It is converted to a depth, referred to as runoff when distributed over the entire watershed area (volume/area).
Assess comprehension and application using rainfall-runoff data from Rock Creek watershed (15 min)
- How is scientific data plotted? By convention, the independent variable is plotted on the X-axis and the dependent variable is plotted on the Y-axis. Which is the independent variable? Which is the dependent variable?
- If runoff equaled rainfall, what would the relations look like on an X-Y scatterplot? Does runoff equal rainfall? Does output equal input? Go back and forth between input/rainfall and output/runoff. Use them interchangeably in your presentation and discussion.
- If runoff is not equal to rainfall, is there a consistent ratio between them? Students can calculate this for several data pairs or visualize this on the plot.
- The discussion should end with a sense of why this is not the case. Where is the missing water? And in what proportion or range of proportions?
Summarize the average rainfall-runoff data from Rock Creek watershed by illustrating it in the water balance equation (5 min)
Write the water balance equation on the board (P = Q + ET + Î”S). Ask students what terms can be quantified from the Rock Creek watershed data. This can be done for any given year. It can be done for an average over the years of record as well. For the Rock Creek watershed, the average for the period of record would be
P = Q + ET + Î”S
37.37 in = 13.02 in + ET + Î”S
The remainder, to balance the equation, is 24.25 in. This is the amount of water that is stored within the watershed, either temporarily (as soil moisture that eventually leaves the watershed through evaporation and transpiration), or more permanently (by percolating more deeply and becoming groundwater).
Assign homework for the following activity:
Work to be assigned for the following class should include plotting the rainfall-runoff data for Big Creek watershed. It is to be plotted on the previously supplied plot of Rock Creek data using a different symbol so that a direct comparison of rainfall-runoff data can be made for the two watersheds. To facilitate work in the next activity, students should also calculate values for average annual rainfall and average annual runoff for both watersheds. A Google Earth file of the boundary and drainage network for each watershed is included for either instructor or student use (Rock Creek & Big Creek watersheds (KMZ File 232kB May29 15)). These materials are used to examine changes in land use and their impact on the water balance equation.
In a one-minute paper, ask students to answer either of the following questions or design one of your own:
- Describe one ecosystem service the Rock Creek watershed provides relative to the water balance of this watershed.
- How does the watershed ecosystem regulate the water balance of the Rock Creek watershed?
Homework assigned for the following class should include plotting the rainfall-runoff data for Big Creek watershed. Students should plot the rainfall-runoff data for Big Creek watershed on the previously supplied plot of Rock Creek data using a different symbol. This can be graded for completeness and attention to detail (i.e., correct plotting positions). A key is included:
Use a one-minute paper to assess application to Unit 1.2. Use either of the following questions or design one of your own. Describe one ecosystem service the Rock Creek watershed provides relative to the water balance of this watershed. How does the watershed ecosystem regulate the water balance of the Rock Creek watershed? An assessment, based on a one-minute paper, and rubric are included: (Unit 1.2 Assessment (Microsoft Word 2007 (.docx) PRIVATE FILE 18kB Sep3 16)). A key is included:
References and Resources
- A good review of the hydrologic or water cycle is available at http://water.usgs.gov/edu/watercyclesummary.html.
- In preparation for this activity, students should read the "Watershed Hydrology Literacy" handout (Watershed Hydrology Literacy (Microsoft Word 2007 (.docx) 1.5MB Aug25 16)). This handout defines key terms and concepts associated with the hydrologic cycle, watersheds, and water balance. A watershed literacy assessment can be assigned as homework prior to or following the class (Watershed Hydrology Literacy Assessment (Microsoft Word 2007 (.docx) 14kB Dec1 16)).
- A PowerPoint presentation (Unit 1.2 Presentation (PowerPoint 2007 (.pptx) 3.6MB Dec1 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)).