Runoff Generation from Varying Land Surfaces

After completing this activity, students should understand that

• precipitation can either infiltrate into the ground or become runoff
• the amount of water that can infiltrate is primarily dependent upon the rainfall volume, the rainfall intensity, the soil type, the slope, and the land surface
• runoff generally decreases surface water quality as this water carries ground contaminants directly into rivers and streams
• changes in the ways that land is used (e.g. garden versus lawn) have a large impact on runoff generation

After this activity students should be able to

• estimate the amount of runoff generated from a given area based on the land surface
• propose methods for decreasing the amount of runoff by changing or adapting the land surface

The full activity allows students to participate in experimental design within a constrained set of variables. After an initial prescribed experiment, students should be able to develop a hypothesis using the other experimental variables and design their own experiment to test it. The lab results should work in tandem with a standard engineering approach to estimating runoff (NRCS Curve Number) to deepen the students' understanding of how runoff is generated. In the evaluation of an existing area, students are able to apply their new academic knowledge to an real example from their own lives. Finally, the open-ended design activity at the end allows students to evaluate the environmental impact of their own campus and imagine real changes that could be made to lessen this impact.

This activity could be completed as part of an introductory geology, hydrogeology, or engineering hydrology course. The author has used this lesson, in modified form, with 5th grade students studying watershed quality. The time needed depends on the depth of exploration of the topics and how much time is allowed for the design components. The physical models require materials easily purchased at a hardware store. No previous skills are required, although the mapping component of the exercise could be made more sophisticated by use of a GIS.

Materials for this lab/activity are still under development, but the general activities have been tested on students. Detailed descriptions of the tray construction, applications using GIS, sample calculations using the Curve Number method, and student worksheets will be added in the future.

Depending upon the background of the students, this activity should begin with a review or introduction to the hydrologic cycle, with an emphasis on runoff and infiltration.

Part I: Runoff lab experiments
This lab answers the question: how does the type of land surface affect the percentage of runoff versus infiltration from a given precipitation event? Each team of students will have three experimental setups with different land surfaces. The initial version of this activity uses 1)glazed ceramic tile for an impermeable surface like pavement 2)sod/grass on local soil 3)herbs or other garden plants in a standard planting mixture. Other surfaces could be used. The experimental models are built with items from a hardware store: deep heavy-duty paint trays and wire mesh "hardware cloth". See the attached file for details on how to put together the trays. In addition, each group will need a measuring cup/graduated cylinder, a piece of PVC pipe with small holes drilled in it to distribute the precipitation (or a sprinkling type watering can), a timer, and blocks or books used to increase the slope of the trays.

In the initial experiment, all groups will apply the same volume of water at the same rate/intensity to each of the three surfaces. The runoff will drain to the basin portion of the tray and can then be collected in a measuring cup. Student should be able to directly compute the percentage of rain that becomes runoff and compare the three surfaces. In this case they will find that, not surprisingly, the ceramic tile produces the highest percentage of runoff. Students may be surprised to find that the grass surface also produces a relatively large volume of runoff.

A note about quantitative analysis: Depending upon the students, this can become more involved. At a minimum, students should calculate the percentage of precipitation that becomes runoff (e.g., 50 ml of water applied and 25 ml runoff = 50% runoff). Student who are more comfortable with computation should calculate the precipitation as a depth (volume applied divided by the area of the land surface) as well as an intensity (depth divided by time of application.) Finally, students can calculate the slope of the experimental set up, rise over run expressed as a percent grade, in engineering style.

After the first set of experiments, students are asked to think about how other factors will affect the amount of runoff generated. The available variables are 1) the total volume of precipitation 2) the intensity of precipitation 3) the slope of the tray and 4) potentially other surfaces/soils. With younger students this was phrased as "How high, how fast, how much!" a description that would likely work well with undergraduates too. This part of the activity should be run as a student-led inquiry. Each team should identify one or more experimental questions, a hypothesis, and a description of the experiment that will test their hypothesis. After completing their experiment(s), the teams should write up their results to share with the class. This part of the lab can be as simple as one additional experiment or a series of experiments.

Finally, the class comes back together to share results. Depending upon the size of the class, there should be enough data from the first experiment to perform some simple statistical analysis. Each team should also present their hypothesis, experiment, and results.

Part II: Calculate Runoff Using an Engineering Method
In this part the instructor introduces a simple empirically-based method of calculating runoff based solely on land cover, the USDA/NRCS Curve Number Method. See the attached presentation file for a sample application/calculation using this method. This exercise will be most useful if different land surfaces are compared under the same soil and hydrologic conditions. A more quantitatively adept group of students should be asked to determine the weighted runoff from a mixed use area.

Part III: Estimate Runoff from an Actual Site
In this part of the activity, students will evaluate the land use of an actual site such as their campus or school grounds. Either graphically or by using more sophisticated methods, such as GIS, the percentage of land area for each type is calculated. This can be accomplished with something as simple as a Google Earth satellite image or more involved such as GIS data/shapefiles. After the total area is categorized, students can calculate the total runoff generated from 1" of precipitation over the entire area using the NRCS Curve Number method, the simple relationships they developed in Part I, or both. In the presentation file, there is an example from Rolling Hills Elementary School. The students calculated the portion of their campus that was impermeable (pavement or buildings), turfgrass, and garden. The garden area is highly permeable soil that is frequently worked combined with decomposed granite. After determining the total runoff generation from the site, students can propose ways to decrease the runoff by altering the land use. In the case of the elementary school in the presentation, areas of turfgrass are being converted to garden space, which will significantly reduce runoff. Other ideas that students could explore are alternative surfaces to traditional pavement and use of rain barrels to capture rainfall on building roofs. Students should quantify the reduction in runoff from their designs and present their concepts to the rest of the class.

Special thanks to Marcy Fry and her Rolling Hills Elementary 4/5 grade class for trying out the runoff lab and producing a wonderful presentation of their results for the Ocean Institute (Jan 2013)

Presentation on Runoff Generation for Different Land Surfaces (Acrobat (PDF) 808kB Mar6 13)

Depending on the student audience, review or introduction of the hydrologic cycle may be required to understand why we are interested in minimizing the amount of precipitation that becomes runoff and encouraging infiltration. This could be a complete 1-hr lesson in itself.

If students complete many experimental replicates on the same trays, the soil will become saturated and the trays will begin producing Dunne runoff. To prevent this, additional drainage holes will be required as described in the detailed instructions. This will make the activity much messier.

As this activity can include a high degree of student creativity, having the students present their results works well as an assessment method. Students can design their own experiments with the trays and present those results, explore the quantitative relationship among variables (e.g. quantify slope versus percent runoff), and present their creative designs of changing the land surface to reduce runoff.