Question 15·Hard·Command of Evidence
Simulated Change in Annual Aquifer Input and Irrigation Output if Precipitation Concentration Increases as Climate Models Predict
| Baseline concentration of annual precipitation | % change in water entering aquifers | % change in surface water used for irrigation | % change in groundwater used for irrigation |
|---|---|---|---|
| Precipitation is currently somewhat concentrated | 4.9 | 0.4 | 0.9 |
| Precipitation is currently evenly distributed | 11.0 | 9.0 | 7.9 |
Some climate models for the western United States predict that while total annual precipitation may remain unchanged from the present level, precipitation will become concentrated into fewer but more intense rain and snow events. University of Texas climate scientist Geeta Persad and her colleagues simulated how the amount of water entering aquifers and the amount being used for irrigation purposes would change if this were to occur. Persad and her colleagues concluded that concentration of precipitation into fewer events would result in a higher number of dry days, triggering more irrigation, but that this change in irrigation output is highly sensitive to the baseline concentration of precipitation that currently exists in an area.
Which choice best describes data from the table that support Persad and her colleagues’ conclusion?
For data-support questions like this, first underline the key idea in the conclusion (here, that irrigation output increases and that the size of the increase strongly depends on the baseline precipitation pattern). Then, go to the table and locate exactly those variables (surface and groundwater used for irrigation) under each condition you need to compare. Note the pattern (small vs large changes, increases vs decreases), and finally choose the answer that (1) talks about the right variables, (2) compares the correct rows, and (3) matches the numbers and direction shown in the table. Ignore extra details that do not directly support the stated conclusion.
Hints
Focus on what the question is really asking
You are not being asked to restate the whole table. You are being asked which data support the conclusion about how irrigation output changes when precipitation becomes more concentrated.
Identify the key variables
Look back at the conclusion. Is it mainly about aquifer input or irrigation output? Make sure you know which columns in the table are most relevant.
Compare the two baseline conditions
Carefully compare the irrigation percentages in the row where precipitation is currently somewhat concentrated to the row where it is evenly distributed. How big is the change in each case?
Check both content and numbers in the answer choices
Eliminate any choice that (1) focuses on the wrong type of data (for example, aquifer input instead of irrigation output), (2) gets the direction wrong (increase vs decline), or (3) mismatches the percentages from the table.
Step-by-step Explanation
Understand the conclusion you must support
Reread the key conclusion in the passage: when precipitation becomes more concentrated into fewer events, (1) irrigation output increases, and (2) the amount of that increase depends strongly on the baseline concentration of precipitation (whether it was already somewhat concentrated or evenly distributed). So the right answer must:
- Talk about irrigation output (surface water and/or groundwater), and
- Compare what happens under the two different baseline conditions.
Locate the relevant data in the table
From the table:
- For baseline: somewhat concentrated:
- Surface water used for irrigation: +0.4%
- Groundwater used for irrigation: +0.9%
- For baseline: evenly distributed:
- Surface water used for irrigation: +9.0%
- Groundwater used for irrigation: +7.9% These are all increases in irrigation, but the increases are tiny in the first row and much larger in the second row.
Connect the data to the idea of “highly sensitive”
“Highly sensitive to the baseline concentration” means that the size of the irrigation increase changes a lot depending on whether the baseline was somewhat concentrated or evenly distributed.
- When baseline is somewhat concentrated: irrigation barely changes (less than 1%).
- When baseline is evenly distributed: irrigation jumps by about 8–9%. So the best answer must describe small increases in irrigation for the somewhat concentrated baseline and much larger increases for the evenly distributed baseline.
Match the correct description to the data
Check each choice:
- The correct choice must (1) focus on irrigation, (2) compare the two baseline situations, and (3) get the percentages and direction (increase vs decline) right. Choice B does exactly this: it says that when baseline precipitation is somewhat concentrated, water use for irrigation increases only slightly, but when baseline precipitation is evenly distributed, it increases 9.0% for surface water and 7.9% for groundwater—perfectly matching the table and supporting the idea that irrigation changes are highly sensitive to baseline precipitation concentration.