Where's the Point - Student Worksheet

How to Construct a Three-Dimensional Watershed Model

Three-dimensional watershed models are constructed by cutting out selected contours shown on a topographic map of the area being modeled, and then stacking the cutouts together. If you are not familiar with topographic maps, you may want to visit http://topomaps.usgs.gov/ for some background information. Demonstrations using this type of model typically involve placing various “pollutants” on the the model, and then causing these to run off into one or more simulated water bodies by “raining” onto the model surface with small watering cans, spray bottles, or large sponges. You may want to visit http://www.pyr.ec.gc.ca/EN/IPM/index.shtml and http://clubs.ca4h.org/sanluisobispo/r2rwe/pdf/jr_high.pdf for some examples of watershed models and how they have been used in information programs about contaminated runoff.


Step 1

Determine the desired overall size of the finished model. Larger models can accommodate more detail, but are heavier and more awkward to transport than smaller models. Typical models range in size from a 2 ft square to a full sheet of plywood (4 ft x 8 ft).

Step 2

Make one or more copies of the topographic map for your watershed, enlarging or reducing if necessary to the desired size.

Step 3

Decide whether the horizontal and vertical scales of the model will be the same or different. Many models exaggerate the vertical scale by a factor of 3 or 4 to make topographic features more visible. On the map in Figure 1, a distance of 6000 ft is represented by one inch, so a hill 1000 ft high would be 1000 ÷ 6000 = 0.166 inch high without any vertical exaggeration; with a 4-fold exaggeration the hill would be 0.66 inches high on the model.

Step 4

Decide which contour lines will be used to construct the model. This will usually depend upon the overall range of elevations represented on the map, and how much time is available for cutting out the individual contours. Most models will need at least five contours; the more contours there are, the smoother the slopes will be on the finished model. So, if the highest point on the topographic map is 2300 ft and the lowest point is 200 ft, the overall range of elevations is (2300 ft – 200 ft) = 2100 ft, and the minimum interval would be (2100 ft ÷ 5) = 420 ft. Use a colored marker to trace the selected contour lines on the map copy to make the lines easier to follow when cutting.

If the contour interval on the map does not match the desired interval, use a smaller interval that will match the map interval. For example, if the desired interval is 420 ft and the interval between contour lines on the map is 100 ft, use 400 ft instead of 420 ft. So, using the map of Figure 1, we would use the 400 ft, 800 ft, 1200 ft, 1600 ft, and 2000 ft contours.

Step 5

Determine the proper thickness of material from which the contours will be constructed. The scale of the map in Figure 1 is one inch = 6000 ft, so a 400 ft contour interval would be represented by a thickness of 400 ft ÷ 6000 ft/inch = 0.0667 inch. If we use a 4-fold vertical exaggeration, the required thickness would be 4 x 0.0667 inch = 0.267. So one-fourth inch thick foamcore display board would be a suitable material for this model. In some cases, more than one layer of the material may be needed for each contour.

Step 6

Cut the map copy along the lines representing the next-to-lowest elevation, since the lowest elevation is the base on which the model will be built (plywood, etc). In the example of Figure 1, the lowest elevation is 200 ft, so the first cutout would be done along the 400 ft contour line (see Figure 2).

Step 7

Trace the outline of the cutout onto the material that will be used to construct the contours, and cut along the traced line (see Figure 3).

Step 8

Place the cutout on the base, but don’t glue it down yet. You may want to glue another copy of the topographic map onto the base to help position the cutouts.

Step 9

Repeat steps (6), (7), and (8) for the next contour interval (which would be along the 700 ft contour line in the example). Continue until all contours have been cut.

Step 10

Starting with the lowest (largest) contour, carefully glue successive contours together to build the three-dimensional model. Some contours may be in several pieces (See Figure 4).

Step 11

Waterproof the model. There are several ways to do this, depending upon the material used to construct the contours. Exterior latex paint or polyester resin are the simplest options, but it may be difficult to cover the model thoroughly if porous material (such as foam) has been used to make the cutouts. A more elaborate technique, that gives smoother results, is to drape the model with pieces of cheesecloth soaked in a plaster- of-paris mixture. The plaster is added to approximately 2 liters of water in a bucket until the mixture is smooth and thick, but still pourable. The cheesecloth is pressed into the bucket until it is completely coated with plaster, then draped over the model. The process is repeated until the entire model is covered with at least two layers. The “stair-steps” of the contours can be smoothed out with additional plaster. After the plaster has dried, the model can be painted with exterior latex paint. Sand can be sprinkled over the wet paint to provide texture.

Step 12

Add details to the model, according to the features and processes to be demonstrated. Students may be willing to contribute toy animals, people, buildings, etc. from model kits. Sponges, floral foam, or dried moss may be cut into appropriate shapes to simulate structures and vegetation. Colored fleece fabric can represent vegetation as well as bare earth. Asphalt shingles can simulate paved areas. If students plan to use the model to simulate runoff conditions as described above, they will need to decide on materials that will simulate pollutants. Some commonly used options are cocoa powder (motor vehicle exhaust), chocolate syrup (motor oil), colored drink powder (chemical runoff), and chocolate cake sprinkles or chips (animal waste).

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