Rational Method is suitable for small catchments only because the time of concentration of small catchments is small.
In Rational Method the peak runoff is calculated based on the assumption that the time of concentration is equal to the rainfall duration.
For small catchments, this assumption may hold true in most circumstances.
One of the assumptions of Rational Method is that rainfall intensity over the entire catchment remains constant during the storm duration.
However, in case of a large catchment it stands a high probability that rainfall intensity varies in various part of the large catchment.
In addition, for long duration of rainfall, it is rare that the rainfall intensity remains constant over the entire rainstorm and a shorter duration but a more intense rainfall could produce a higher peak runoff.
Moreover, a reduction of peak runoff is also brought about by the temporary storage of stormwater like channels within the catchment.
In actual condition, the runoff rate within the catchment varies from place to place because of different soil properties and past conditions.
As suggested by Bureau of Public Roads (1965), sometimes the peak discharge occurs before all of the drainage area is contributing.
For instance, when a significant portion of drainage area within the catchment has very small time of concentration so that a higher rainfall intensity can be used for this portion, the runoff coming solely from this portion is higher than that of the whole catchment in which a lower rainfall intensity is adopted because the remaining part of the catchment has comparatively large time of concentration.
Therefore, this results in incorrect estimation of peak runoff of large catchments if Rational Method is adopted.