If the average distance from a micro-sprinkler emitter to the edge of the wetted area is 15 inches, how many acres in a 15-acre site are being treated if there are 250 emitters per acre?

• A. 0.21 acres
• B. 0.42 acres
• C. 0.83 acres
• D. 1.25 acres

Answer: (B)

Think of each emitter as spraying a circular area with radius equal to the distance to the edge, so the wetted area per emitter is a circle with r = 15 inches = 1.25 feet. The area per emitter is πr^2 = π(1.25)^2 ≈ 4.91 square feet. With 250 emitters per acre, the total wetted area per acre is 250 × 4.91 ≈ 1,227 square feet. Since one acre is 43,560 square feet, the fraction of each acre that's wetted is 1,227 / 43,560 ≈ 0.0282 acres of coverage per acre. On a 15-acre site, the total treated area is 0.0282 × 15 ≈ 0.423 acres, about 0.42 acres.