Estimation of the peak flow on a small to medium sized rural catchment is probably one of the most common applications of flood estimation as well as having a significant economic impact. While the terms ¿small¿ and ¿medium¿ are difficult to define, upper limits of 25 km² and 500 km² can be used as guides. The Rational Method, which can be traced back to the mid-eighteenth century, is probably the most commonly used method for estimating the peak flow of a flood. Most urban drainage systems and culverts for rural roads, particularly those for small subdivisions, are designed using the Rational Method.

However, there are a number of problems associated with the use of the Rational Method. Most of these problems are associated with the estimation of parameter values such as the time of concentration and the runoff coefficient. As a result, the rational method may be easy to implement, but it is difficult to ensure that the predictions adequately represents processes occurring in the catchment.

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