Drill 24 · Reading & Writing · Hard Command of Evidence
SAT R&W Command of Evidence (Hard) — Drill 24 is a Reading & Writing practice drill covering Hard Command of Evidence. It contains 5 original questions created by Brian Stewart, a Barron's test prep author with over 20 years of tutoring experience.
How much water a river carries past a point each second -- its discharge -- responds to rain, snowmelt, and what the land upstream does with water before it reaches the channel. The finding items here ask which gauge reading or watershed fact decides between rival causes of a flow change; the two data items, a table and a bar comparison, reward reading for the difference between two responses rather than for either one alone.
This year a river's spring flood crested three weeks earlier than its long-term average. One explanation is that an unusually warm spring melted the mountain snowpack early, sending meltwater down sooner. Another is that the early crest came from a run of heavy spring rainstorms rather than from snowmelt timing at all. Hydrologists at a downstream gauge want a measurement that points to one source over the other.
Question 1. Which finding, if true, would most strongly support the early-snowmelt explanation over the heavy-rain explanation?
Explanation: Correct: The two explanations differ in the source of the water, so timing the peak to an early snowpack melt-out -- and away from any storm -- directly favors snowmelt. It links the flood to meltwater rather than rain. A: Higher total discharge could come from either heavy rain or a big snowmelt, so it does not separate the two. C: Overtopping shows the peak was large, not what supplied it; both explanations allow a large peak. D: Warm spring air is consistent with early melt but also with the general season; on its own it does not pin the peak to snowmelt over rain.
Table: storm response at three subbasins
After one storm that dropped nearly the same rainfall across a watershed, hydrologists compare how three subbasins sent that water to the river. For each they note the land cover and two response numbers: the peak flow rate reached and the lag time between the rain and that peak. The mostly paved and mixed subbasins are filled in; the heavily forested subbasin's row is left blank. Forest and soil soak up and slow water, so across the rows the peak should fall and the lag should lengthen as cover shifts from paved to mixed to forested, and the forested row should continue that pattern.
Question 2. Which values, entered for the heavily forested subbasin, best continue the pattern that forest cover lowers the peak and lengthens the lag?
Explanation: Correct: From paved (95 cfs, 2 h) to mixed (55 cfs, 5 h) the peak falls and the lag lengthens. The forested row should extend that trend in both columns: 18 cfs with a 9 h lag is the only option lower in peak and longer in lag than the mixed subbasin. A: 110 cfs and a 1 h lag are higher and faster than even the paved subbasin, the reverse of what forest cover does. B: A 70 cfs peak is higher than the mixed subbasin's 55 cfs and the 3 h lag is shorter than its 5 h, so neither column continues the trend. C: The 8 h lag continues the trend, but a 60 cfs peak is higher than the mixed subbasin's 55 cfs, so the peak fails to keep falling -- only one column fits.
Through a rainless late summer, one tributary keeps flowing steadily while a neighboring tributary dwindles to a trickle. A hydrologist proposes that the steady tributary is fed between rains by groundwater seeping in from a large aquifer -- its baseflow -- whereas the dwindling one drains a watershed with little groundwater storage and so relies on whatever surface runoff is left over from earlier rains. She sets up a test: if groundwater baseflow sustains the steady stream, then its dry-season water should carry the temperature and chemical signature of deep groundwater, while the dwindling stream's should not. The two sources differ enough that a sample from each should settle which is feeding the steady flow.
Question 3. Which result would best fit the prediction that follows from the groundwater-baseflow explanation?
Explanation: Correct: Groundwater emerges cool and carries a distinct dissolved-mineral signature, so dry-season water that stays cool and groundwater-like in the steady stream -- and runoff-like in the other -- is the direct test of the baseflow idea. It confirms the predicted contrast. B: A larger drainage area could raise flow generally but does not show the dry-season water comes from groundwater. C: Both running high in spring reflects shared snowmelt or rain, not what sustains one stream once the rains stop. D: Channel steepness affects how fast water moves, not whether groundwater is feeding the flow.
A watershed was partly paved over two decades. Hydrologists compare river flow before and after the paving under two conditions: the peak flow during storms and the baseflow during dry spells. They want to know which condition the paving changed more, since paving sheds storm water fast off hard surfaces but can starve the dry-season flow by keeping rain from soaking in to recharge the ground. A change concentrated in the storm peak would point to fast runoff; a change concentrated in the baseflow would point to lost recharge. The grouped bars show before-and-after values for storm peak and for baseflow.
Question 4. Which statement best compares how much paving changed the two flow conditions, and is supported by the graph?
Explanation: Correct: The question is which condition paving moved more, so it asks for the bigger before-to-after change. The storm-peak bars rise by more than the baseflow bars shift, making the peak the larger response. A: Comparing peak to baseflow after paving is true from the bars but compares two conditions in one period, not the size of the change paving caused. B: A drop in baseflow is one true before-after change, yet on its own it does not show the peak changed more, which is the comparison asked. D: A higher post-paving peak is true but describes one bar pair; the question is whether that change exceeds the baseflow change.
A water manager claims that removing a small irrigation diversion raised the river's late-summer flow at a downstream gauge, pointing out that flow there rose the year after the diversion was removed. A reviewer is unconvinced. He agrees the flow rose, but argues that the diversion's removal was not the only thing that changed on the river that summer, and that some other change could account for the increase just as well, with the river otherwise behaving as before. Before crediting the removal, he says, one would need to know whether anything else upstream altered how much water reached the gauge that same season.
Question 5. Which finding, if true, would most weaken the claim that removing the diversion caused the higher late-summer flow?
Explanation: Correct: If an upstream reservoir started releasing extra water the same summer, that release -- not the diversion's removal -- could have raised the downstream flow, with everything else about the river held the same. That rival cause undercuts the claim. A: That the diversion long supplied irrigation is background; it does not offer a competing cause for the flow increase. C: Year-to-year variation invites caution but names no specific alternative cause, so it weakens the claim only weakly. D: A reliable gauge makes the measured rise trustworthy, which does not argue against the diversion having caused it.