Three Stream Tables, One Storm
Students test erosion using three stream tables filled with the same soil and fed the same 1 L/min of water for 3 minutes. Trial A: bare soil, slope 10 degrees, 40 g of sediment collected. Trial B: bare soil, slope 20 degrees, 95 g collected. Trial C: soil covered with grass, slope 20 degrees, 22 g collected. Each trial is run only once. A classmate concludes, 'Grass stops erosion completely, and slope hardly matters.' The class also has photos of a nearby hill: the bare section loses far more soil after storms than the grassy section beside it.
1. Which two trials isolate the effect of slope, and what does that comparison show?
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Trials A and B differ in only one variable - slope (10 degrees vs 20 degrees). Soil, water flow, and time are held constant, so any difference in sediment must be caused by the slope.
The result: 40 g at 10 degrees versus 95 g at 20 degrees. Doubling the slope more than doubled the sediment loss (about 2.4 times). So steeper ground erodes faster, which directly contradicts the classmate's 'slope hardly matters.'
Trials A and C cannot be used to answer this question because they differ in two things at once (slope and grass), so their difference cannot be pinned on either one.
2. Evaluate the claim that grass stops erosion completely.
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The claim is too strong. The fair comparison is B versus C, which differ only in the grass cover: 95 g fell to 22 g. Grass reduced erosion by about 77%, which is a large effect - but 22 g is not zero, so soil still moved even with grass.
Notice also that comparing A (40 g) and C (22 g) would understate the grass effect, because C also has the steeper, more erosive slope. Choosing B vs C is what makes the conclusion valid.
So the defensible statement is: 'Grass greatly reduces erosion but does not eliminate it.'
3. State one weakness in the design and predict what the real hillside would do if a fire removed the grass cover.
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Weakness: each trial ran only once, so there is no way to tell whether the differences are real or just run-to-run variation (soil packing, a clump of soil, a splash). There are also no repeats and no bare-soil trial at intermediate slopes. Scale is another limit: 3 minutes on a table is not a season of storms.
Prediction: with grass gone, the hillside would behave like the vegetated-versus-bare comparison at the same slope - far more soil would be lost, at minimum several times more - because roots no longer hold soil particles and raindrops strike bare ground directly. The photos already support this: same area, same weather, much greater loss on the bare section.