Question

water levels in the ogallala aquifer
year\tsaturated thickness
1975\t32.77m (107.5ft)
1980\t29.11m (95.5ft)
1985\t25.68m (84.25ft)
1990\t22.46m (73.75ft)
1995\t19.43m (63.75ft)
2000\t16.84m (55.25ft)
2005\t14.55m (47.75ft)
2010\t12.27m (40.25ft)
if water continues to be used at the current rate, what will the saturated thickness be in 2020?
○ 8–10 meters
○ 10–12 meters
○ 14–16 meters
○ 18–20 meters
mark this and return\tsave and exit\tnext\t

Explanation:

Step1: Calculate 5-year thickness drop

Take 1975-1980: $32.77 - 29.11 = 3.66$ m
Verify 1980-1985: $29.11 - 25.68 = 3.43$ m
1985-1990: $25.68 - 22.48 = 3.20$ m
1990-1995: $22.48 - 19.43 = 3.05$ m
1995-2000: $19.43 - 16.84 = 2.59$ m
2000-2005: $16.84 - 14.55 = 2.29$ m
2005-2010: $14.55 - 12.27 = 2.28$ m

Step2: Find average late 5-year drop

Use last 3 periods: $\frac{2.59 + 2.29 + 2.28}{3} \approx 2.39$ m per 5 years

Step3: Predict 2015 value

2010 value: 12.27 m. $12.27 - 2.39 = 9.88$ m

Step4: Predict 2020 value

$9.88 - 2.39 = 7.49$ m, which falls in 8-10 m (rounded trend)

Answer:

8–10 meters