Runoff From Two Slopes
Two identical trays hold the same kind of soil packed the same way, tilted at the same angle. Tray A is bare soil. Tray B is covered with a layer of grass and mulch. Students pour 500 mL of water onto the top of each tray at the same steady rate, catching the runoff in jars below. From Tray A they collect 350 mL of muddy water and a thick layer of soil at the bottom. From Tray B they collect 420 mL of mostly clear water with a thin layer of soil. A student writes in a conclusion, 'Grass stops erosion completely.'
1. Use the class's own numbers to judge that conclusion.
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The conclusion is too strong. Tray B still lost soil - there was a thin layer in the jar - so erosion was reduced, not stopped. Compare the two layers: A's thick layer is far more soil than B's thin one, which supports the claim that plant cover slows erosion a lot. But the class should say 'grass greatly reduces erosion,' not 'stops erosion.' A conclusion has to match the size of the effect actually measured, and here the effect is large but not zero.
2. Look at the water volumes: 350 mL collected from A and 420 mL from B. What does that difference tell you, and how does it weaken the comparison?
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Only part of the 500 mL came back from either tray, and the two trays returned different amounts. The missing water soaked into the soil, sat in the mulch, or evaporated. That means the two trays were not in the same condition when the water hit them - for example, the grass and mulch may have held water and slowed it down, letting more soak in, or the bare soil may have crusted over. Because the two runs did not deliver water the same way, the difference in soil loss is not perfectly clean evidence. The class should repeat the test and measure the soil by drying and weighing it, not by eyeballing layer thickness.
3. Write a better prediction for a third tray of bare soil tilted more steeply, and explain your reasoning.
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A steeper bare tray should lose more soil and possibly more water in the same time. Steeper slope means water runs downhill faster, so it has more energy when it hits the soil and can carry grains away instead of soaking in. That is exactly why the class had to keep the tilt the same in the first test: slope is another variable that changes erosion, so if it changes, the comparison between grass and bare soil no longer isolates the effect of the plant cover.