How do runoff coefficients affect pollutant loads in a watershed?

• A. They measure water temperature.
• B. They determine the chemical composition of runoff.
• C. They are used to convert rainfall to runoff volume.
• D. They indicate soil permeability.

Answer: (C)

Runoff coefficients are the factor that tells us how much of the rain that falls on a watershed actually becomes surface runoff. They vary with land cover, soil type, slope, and drainage, and they let us convert rainfall depth into runoff depth or volume for the whole watershed. Since pollutant loads are typically estimated as the product of pollutant concentration and runoff volume, the runoff coefficient directly affects the potential amount of pollutants carried to streams. It doesn’t measure water temperature, determine chemical makeup, or directly indicate soil permeability; its role is to scale rainfall into runoff so we can estimate loads.