The rational method for calculating peak flow is typically used for which size watershed?

The rational method for calculating peak flow is particularly suitable for small watersheds, generally defined as those that are less than 200 acres in size. This method simplifies the process of estimating peak discharge from a rainfall event by using a straightforward formula that takes into account the area of the watershed, the rainfall intensity, and the runoff coefficient. Since the rational method assumes uniform conditions across the watershed, it works best when applied to smaller areas where those conditions can be reasonably assumed to be consistent.

In contrast, larger watersheds often exhibit more variability in land surface conditions, drainage patterns, and hydrological responses, which makes the rational method less effective and accurate for estimating peak flows in those scenarios. Larger watersheds may require more complex hydrological models that consider various factors influencing runoff, such as infiltration rates, storage, and time of concentration due to the increased complexity associated with larger environments.

Thus, the method's design for efficiency and straightforward calculations aligns closely with the specific characteristics of smaller watersheds, reinforcing why it is typically applied to areas less than 200 acres.