Given a record of annual maximum flows or other measures of flood events, a number of methods can be used to estimate P and T (Gordon et al. 2004). One begins with a list of the single highest flow of each year, preferably based on the peak of the flood hydrograph rather than average daily discharge. This is especially important in small rivers where the peak flow passes in hours and will be underestimated by the daily average, although this may not be critical for large rivers. By fitting a probability distribution to the data set, it is possible to predict the average recurrence interval for floods of a given magnitude or, conversely, the magnitude of the flood that occurs with a given frequency. The recurrence interval (T) for an individual flood is calculated as:

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