In the calculation example, what is the assumed weight of one acre-foot of water in pounds?

• A. 2,700,000 pounds
• B. 27,000 pounds
• C. 2,700 pounds
• D. 27,000,000 pounds

Answer: (A)

The idea being tested is converting a volume of water into its weight using the density of water. One acre-foot is the volume of water that covers one acre to a depth of one foot, which equals 43,560 cubic feet. Water weighs about 62.4 pounds per cubic foot, so the weight is 43,560 × 62.4 ≈ 2,718,144 pounds. In practice problems this is rounded to about 2,700,000 pounds for simplicity. So the assumed weight of one acre-foot of water is roughly 2.7 million pounds.

The other totals are not consistent with the standard conversion: 27,000 pounds would only correspond to a much smaller volume of water, around 434 cubic feet; 2,700 pounds is even smaller, about 43.4 cubic feet; 27,000,000 pounds would be nearly ten times the actual weight of an acre-foot.