Runoff Generation from Varying Land Surfaces
This is a partially developed activity description. It is included in the collection because it contains ideas useful for teaching even though it is incomplete.
This combined lab activity/lesson explores the impact of land use on the volume of runoff generation. Students experiment on simple physical models of runoff for which they are able to vary land surface vegetation, soil type, slope, volume of rainfall, intensity of rainfall,and potentially antecedent moisture conditions. The lab is compared to infiltration excess volume as calculated by the NRCS Curve Number method for direct runoff for varying land use. Finally, students determine the portion of land uses of a real location, such as their campus, using their lab results and/or the NRCS Curve Number method and explore reduction of runoff by changing land use.
- 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
- 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.
Context for Use
Description and Teaching Materials
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)
Teaching Notes and Tips
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.
References and Resources
Wiki Watershed includes a flash application that demonstrates how runoff volume changes as the land surface changes
EPA's Low Impact Development includes ideas for minimizing runoff at the source