IYS Soils Activity

Where Does All the Rainfall Go?

Summary (per student, group etc.) (may be more than one) Participants will explore various fac- Materials Type of Lesson tors that influence the amount of run- All materials are readily available in retail 1. Hands-on (participants touch the stuff) off that occurs when rain falls on the outlets. Substitutions are easy to make if nec- 2. Outdoor landscape by comparing the amount essary. This demo used three runs total, using 3. Indoor of runoff generated from a given area a single pan that was refilled with fresh, dry of bare soil, the same area with a por- soil prior to each run. Additional pans could 4. Demonstration (scientist or teacher tion covered by an impervious surface be purchased to fill with soil in advance. demo, outside professional) (simulating pavement, roofs, etc.), — Set of two foil disposable rectangular 5. Experiment (follow procedure, get and the same area with a portion of cake pans, 12 1/4 by 8 ¼ by 1 3/32 results, interpret results) the impervious area converted to a inches. One pan is used to hold the soil simulated “rain garden”. The activity and the other is used to capture runoff. Time Needed can easily to modified to explore the — 1000 mL measuring cup 1. Scientist prep time + cleanup time effects of slope, soil texture, anteced- — 3 pieces of 2 by 4 lumber, each 9 Most of the prep time will be acquiring the ent moisture content, soil bulk density, inches long the amount of impervious area, or soil supplies. The actual setup of the experi- — Watering can, spray bottle, or sprayer cover (e.g., mulch, vegetation, erosion ment can be participatory, so no advance control fabrics) on runoff volume. — Gridded template plastic (available prep time is necessary. Cleanup time will where quilting supplies are sold), 12 × depend on the location. If done outdoors Learning Objectives/ 18 inches on grass, no cleanup will be necessary Outcomes — Compost or small bag of potting soil. other than disposal of the soil used in the Less than 1 pound is required. runoff pans. If done outside on cement or 1. To investigate the influence of — Approximately 30-40 pounds of mineral indoors on a floor that can get dirty and impervious surfaces on runoff soil (can use topsoil sold at a garden soil wet, minimal time will be required to volume, which can be directly (40 lb bag) or collect local soil) clean up spilled soil and water. related to how urban and suburban — Scissors 2. 30 to 60 min depending on how many development can greatly influence — Small binder clip variables are tested. hydrology and the need for storm- water management. — Plastic spoon 2. To investigate how rain gardens Methods/Procedures can reduce runoff volume and be 1. Cut two, 3 by 12.5 inch pieces of the grid- a critical part of stormwater Ages of Audience ded template plastic using the scissors. management. 1. Elementary Cut a rectangular section, 2 by 6 inches, 3. Related topics include the ability of 2. Middle School High School out of the center of one of these pieces. soil or composts in rain gardens to 3. Adults 2. Fill a cake pan to the rim with soil. Gently clean water before it is eventually tap on the floor a few times to settle discharged into surface waters. Recommended group size? the soil and add more if necessary. Use a 4. For computations, the runoff 20-50 board to strike off any excess soil and to coefficient (runoff volume/rainfall create a uniform surface even with the volume) can be calculated. Where could you offer this? top of the pan. 1. Local school 3. Place the narrow dimension of the filled 2. Any place with an audience that is pan on two pieces of 2 by 4 and the other interested in runoff or storm water end of the pan on a single piece of 2 by 4. management. The runoff will travel the long dimension of the pan. What type of room do 4. Place the empty pan at the downslope you need? end of the filled pan to collect runoff. Floor that can get wet and dirty, or 5. Measure 1000 milliliters (mL) of water Celebrating the outdoors on grass or cement. into your watering can (or spray bottle or sprayer) and distribute uniformly over the surface of the soil in the filled pan. Try not to place water anywhere but on

continued... soils.org/IYS IYS Activities: Soil Science Society America–2 Where Does All the Rainfall Go?

the soil surface and do not apply water Discussion Questions in one place for very long. The water What would happen if: should be added over several minutes. The volume of water should be sufficient 1. we placed three pieces of 2 by 4 under to generate at least several hundred mil- the pan at the upslope end (increasing liliters of runoff. If it does not, use more the slope)? than 1000 mL but keep track of the total 2. we placed a piece of sod on top of amount of water added. After runoff has the soil? stopped, use the measuring cup to deter- 3. we increased the area of the pan covered mine the volume of runoff. by an impervious surface? 6. To simulate the effects of impervious 4. we increased the size of the rain garden? surfaces on runoff volume, fill another pan of soil as described above. Place the 5. we compressed the soil in the pan before 3 by 12.5 inch piece of gridded tem- we added the “rainfall”? C) We obtained 350 mL of runoff from 1000 mL of plate plastic lengthwise in the center of 6. we used sand instead of soil? the pan and secure at the top using the “rain” on bare soil. Plenty of soil moved too! 7. we started with soil that was already wet? binder clip. This would be equivalent to converting an agricultural field to a subdi- vision. Repeat the application of 1000 mL of water (or the same volume as the first run) and measure the runoff volume. 7. To simulate the effects of a rain garden on runoff volume, fill another pan of soil as described above and place the other 3 by 12.5 inch piece of gridded template plastic (with the 2 by 6 inch center piece removed) in the center of the pan and secure at the top with the binder clip. D) Simulated impervious surface in watershed Using the plastic spoon, as best as you increases runoff volume by 85%. Note the drastic can, scoop out all of the soil in the 2 by soil erosion! 6 inch area of the template and replace A) Soil and potting mix are readily available in with compost or potting soil. Repeat the garden stores, or you can collect your own soil application of 1000 mL of water (or the and use locally prepared compost. same volume as the first run) and mea- sure the runoff volume.

E) “Rain garden” construction. Potting mix was B) Bare soil run ready to go. used to replace soil removed from the center of the pan. The rain garden helps to absorb water and slow runoff. Runoff volume was reduced by 23% compared to the impervious surface without the rain garden. Real rain gardens could function as wetlands, contain plants to use collected wa- ter, or simply retain water for slower discharge.

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