What size watershed is the rational method used?

• A. Large, over 1000 acres
• B. Small, under 200 acres
• C. Medium, 200-500 acres
• D. Very large, over 5000 acres

Answer: (B)

The rational method is designed for estimating peak discharge in small watersheds. It relies on the idea that rainfall intensity is effectively uniform across the entire basin and that runoff from all parts of the watershed arrives at the outlet with little delay. Those conditions hold best when the watershed is small, typically under about 200 acres, so the time of concentration is short and the peak flow occurs during the rainfall's peak intensity. For larger basins, rainfall varies across the area and storage and routing effects spread the response, making the simple rational method less accurate. So, it’s best suited for small watersheds under about 200 acres.