Given a watershed area of 2 square kilometers, rainfall of 25 mm, and runoff coefficient of 0.35, calculate the runoff volume in cubic meters?

• A. Runoff volume = area * rainfall * runoff coefficient = 2,000,000 m^2 * 0.025 m * 0.35 = 17,500 m^3
• B. 1,750 m^3
• C. 175,000 m^3
• D. 875 m^3

Answer: (A)

The main idea is to use the runoff formula: runoff volume equals rainfall depth times watershed area times the runoff coefficient. First convert to consistent units: area is 2 km^2, which is 2,000,000 m^2, and rainfall depth is 25 mm, which is 0.025 m. Multiply area by rainfall depth to get the total rainfall volume over the watershed: 2,000,000 m^2 × 0.025 m = 50,000 m^3. Then apply the runoff coefficient (a dimensionless fraction of rainfall that becomes runoff): 50,000 m^3 × 0.35 = 17,500 m^3. The units work out to cubic meters because area times depth gives volume, and the coefficient is just a fraction. So the runoff volume is 17,500 cubic meters.