Water Quality - Nutrient Management and Cropping Systems - Lesson 12 - Terracing
Target Grade Level / Age Range:
Students will understand how terracing fields and other landscapes can slow the flow of surface water and minimize undesirable soil erosion in fields.
Students will practice engineering skills and innovation to solve soil erosion caused by slope and water.
Students will determine what slope a field may have and recommend a terracing system that would be best suited to that field.
- Computers with internet access, tablets, or other similar devices. One for every two students.
- Tape measures (optional)
- String (optional)
- String levels (optional)
- Scratch paper
Suggested Companion Resources (books and websites)
- Terraces: https://youtu.be/9EGzCA1y2Ks
- Terraces and Tile lines: https://youtu.be/A-GfMPWLOMY
- Terraces from Iowa NRCS: https://youtu.be/BBLgvN9jA8g
- Issue 3, Iowa Ag Today: http://www.iowaagliteracy.org/resources/publications/publications.aspx
- Why do they do that? https://iowaagliteracy.wordpress.com/2016/11/21/why-do-they-do-that-terraces-and-tile-lines/
- Iowa Agricultural Practices and the Environment: https://store.extension.iastate.edu/Product/ian104-pdf
- New Terrace Design for Iowa's Gentle Slopes: https://www.nrcs.usda.gov/wps/portal/nrcs/detail/ia/home/?cid=nrcs142p2_008697
- Terraces: Iowa Job Sheet: https://www.nrcs.usda.gov/wps/portal/nrcs/detail/ia/technical/?cid=nrcs142p2_008537
- Conservation Choices: Terraces - https://www.nrcs.usda.gov/wps/portal/nrcs/detail/ia/newsroom/factsheets/?cid=nrcseprd416089
- Wildlife use of terraces in Iowa row crop fields - http://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1127&context=leopold_grantreports
- More than 100 miles of terraces http://www.iowaagriculture.gov/waterResources/WatershedProjects/WestTarkio.asp
- Field and Slope Erosion: http://www.watersheds.org/earth/field.htm
- Farm Basics Terraces: https://youtu.be/UxHYwrBIyZ8
- Farm Basics Terraces and Field Contours: https://youtu.be/cx2NFVH3loM
- Calculating Slope: http://www.co.cowlitz.wa.us/DocumentCenter/View/1268
- How do I calculate slope? http://serc.carleton.edu/mathyouneed/slope/slopes.html
- How to calculate slope: https://youtu.be/3HU-sh9eui4
Vocabulary (with definitions)
- Terraces: wide man-made steps installed on fields that have a slope or grade. They create flat strips of land between barriers.
- Contour farming: farmers till fields along natural contour lines of the landscape topography
- Slope: slope on a landscape is the change in height over a specific horizontal distance. Slope can be used to help determine the amount of erosion that may occur during a rainstorm
Background – Agricultural Connections (what would a teacher need to know to be able to teach this content)
One thing that Iowa farmers struggle with is soil loss and erosion from water running across the field. When water after a rainstorm flows across the field it can pick up soil particles and carry those particles downstream. Loosing that soil off the field might make the field less fertile. The steeper the slope or grade of the land (like a hillside) the faster the water will move. The faster the water moves, the more soil it might pick up and carry away with it.
So, farmers need to try and slow down the movement of water. Hence, terraces. Terraces are man-made earthen structures that intercept runoff on slopes. They change long slopes into a series of shorter slopes. At each level of the terrace, water has a chance to slow down and the soil has a chance to settle out which keeps it on the field. The result is that cleaner water leaves the field and not as much erosion occurs.
Farmers mound up soil on the hillside creating a somewhat level area with a short steep backslope down to the next level. The top, flat area can still be farmed with crops. The short steep backslopes are seeded with perennial grasses. The roots of these perennial grasses help hold the slope in place.
Sometimes terraces can also include a tile line and drain. In some cases, and if there is considerable water build up, farmers can install a tile line and drain. This will allow the soil to settle out and the water to be siphoned off into an underground pipe. This allows the water to run through the pipe down the slope without collecting any soil. The water is discharged at the end of the pipe. This also reduces soil compaction and enables good root development.
In Iowa, terraces are a fairly common practice. In fact, hundreds of miles of terraces help cut soil loss. In the West Tarkio Watershed Project Priority Area terraces reduce soil loss by as much as 13 tons! New terraces might be installed in the fall of the year after growing crops have been harvested or in the spring of the year before crops are planted. In addition to reducing soil erosion, terraces can help retain moisture for growing crops and water conservation purposes. Terraces can even help create nesting habitat in the grassy back slopes that are largely untouched.
Interest Approach or Motivator
Farmers want to maximize land use. To do this they might decide to plant land that isn’t perfectly flat. When rain falls on land that isn’t perfectly flat, not all of it will be immediately absorbed. Some water might run downhill. As water runs downhill it picks up speed. As speed increases, there is a greater chance that it might pick up soil particles and carry them away. This causes soil erosion taking the best topsoil from the field and depositing it in rivers and further downstream. Farmers want to keep soil on their field.
Break students into groups of three and have them take on the role of a farmer, an engineer, and a soil scientist. Hand out the accompanying lesson worksheet. The farmer has a gently sloping field 5% slope, but still experiences some soil erosion during heavy rainstorms. Have students brainstorm possible solutions to reduce, minimize, or eliminate soil loss from water erosion. Students should sketch out their ideas on a piece of paper and be prepared share and explain their solutions to the rest of the class.
Objective 1: Understanding how to calculate slope
- Have students work in pairs and log on to http://serc.carleton.edu/mathyouneed/slope/slopes.html
- In pairs, students should work through the example. When complete they should go on to the practice problems and finally to the assessment.
Objective 1 (alternative):
- Make copies of the worksheet http://www.co.cowlitz.wa.us/DocumentCenter/View/1268 and distribute one to every student.
- Have students work in pairs following the directions on the worksheet to calculate various slopes around the classroom (top of desk to base of classroom door, i.e.) or if possible slopes outside.
Objective 2: Terraces
- Present the content from slides 3-10 from the accompanying PowerPoint. Have students capture notes and important information into their notebooks. Farmers in Iowa and across the Midwest use terraces to minimize soil erosion from water on fields with a slope or on a hillside.
- Why terraces?
- Reduce rate of runoff and allow soil particles to settle, cleaner water is carried off in a non-erosive manner
- Used on farmland with uniform, moderate slope
- Terraces collect water and store it until it can infiltrate into the ground or be released through a stable outlet like an underground pipe
- Terraces channel and slow runoff water and carry it to a stable outlet like a grassed waterway
- Terraces must fit the contour of the land
- Grassed terraces can provide increased habitat for birds and other wildlife
- Types of Terraces are:
Grassed backslope terraces: have a farmable front slope with a 2:1 backslope (2 feet horizontal to every 1 foot of vertical drop).
- Grass slope on one side, farmed on one side
- Benefit: used on slopes greater than 6%
- Narrow base terraces: have 2:1 slopes on both the front slope and backslope.
- Grass slopes on both sides
- Benefit: used on slopes greater than 6%
- Grass Front Farmable Back Slope terraces: good on gently sloping cropland (1-6 percent slopes).
- Should not be built on slopes greater than 6%
- Benefit: less ground out of production
- Benefit: less maintenance
- Benefit: less expensive than broad base
- Broadbase terraces: are flatter looking terraces that are farmed on both slopes. They should not be built on land slopes greater than 8 percent. Farmable slopes should not be steeper than 5:1
- Flatter looking, farmed on both sides
- Should not be built on slopes greater than 6%
- Benefit: Entire terrace can be farmed
- Why terraces?
Wrap-Up: Have students go back to their original groups and review their plans to deal with the farm and slope. Students should work in their group (farmer, engineer, and soil scientist) and revise their plans. Be prepared to provide additional scratch paper or extra worksheets for the students. Did they choose a terrace system to manage the slope? If so, which one? Why?
Students can present their plans to the class or turn in their ideas and papers to the instructor.
Essential Files (maps, charts, pictures, or documents)
Did you know? (Ag facts)
- In the West Tarkiko Watershed Project, more than 113 miles of new terraces have been installed, cutting potential soil loss by up to 13 tons per year.
Extension Activities (how students can carry this beyond the classroom)
- Have students watch the video ‘How to Calculate Slope’. https://youtu.be/3HU-sh9eui4 Using a notebook or pad of paper, have students work in pairs outside to estimate slopes around school property, at nearby locations, or at their homes.
- Interview a farmer that has installed terraces or a representative from the department of agriculture or NRCS that works with terraces. Write a news story based on the interview explaining terraces to be published in a local or statewide newspaper.
Lesson plan development was funded by the Resource Enhancement and Protection Conservation Education Program (REAP CEP). Resource Enhancement and Protection Program (REAP): Invest in Iowa our outdoors, our heritage, our people. REAP is supported by the state of Iowa, providing funding to public and private partners for natural and cultural resources projects, including water quality, wildlife habitat, soil conservation, parks, trails, historic preservation and more.
Iowa Agriculture Literacy Foundation
National Agriculture Literacy Outcomes
- T1.9-12.f. Evaluate the various definitions of “sustainable agriculture,” considering population growth, carbon footprint, environmental systems, land and water resources, and economics
Iowa Core Standards
- HS-PS2-3. Apply scientific and engineering ideas to design, evaluate, and refine a device that minimizes the force on a macroscopic object during a collision. * (installation of terraces)
- HS-ESS3-4. Evaluate or refine a technological solution that reduces impacts of human activities on natural systems. * (bioreactors, buffer strips, terraces, cover crops)
This work is licensed under a Creative Commons Attribution 4.0 International License.