What is the formula for calculating effective rainfall?

• A. Drainage area / Rainfall amount
• B. Drainage area / Volume of runoff (area under the curve)
• C. Volume / Surface Area
• D. Runoff coefficient x Total rainfall

Answer: (B)

The formula for calculating effective rainfall focuses on determining the portion of total rainfall that contributes to direct runoff. This is critical in hydrology and drainage design, as it helps engineers assess how much rainfall will actually lead to runoff that needs to be managed.

The most appropriate formula for effective rainfall is the product of the runoff coefficient and the total rainfall. The runoff coefficient represents the fraction of rainfall that becomes runoff, which takes into account losses due to evaporation, infiltration, and other factors. Therefore, effective rainfall is calculated by multiplying the total amount of rainfall by this runoff coefficient.

Using runoff coefficient multiplied by total rainfall captures the essence of effective rainfall, as it reflects the actual contribution of rainfall to runoff based on the characteristics of the watershed, such as soil type, land use, and surface conditions. This provides a practical and actionable figure for designing drainage systems.

The other options presented either misconstrue the relationships involved in hydrology or do not directly relate to effective rainfall. For example, one option focuses on the drainage area and volume of runoff, which do not directly correspond to the simplified calculation needed for effective rainfall.