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Figure 5.1 Plots of extreme value distributions for EV1 distribution (red line) and GEV distribution (blue line) for Waitangi, Chatham Island annual maxima for 1957-1992, for 10 standard durations

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The annual maxima have been plotted on the diagrams.

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The figure consists of 10 different graphs of extreme rainfall versus return period, each for a different duration: 10, 20, and 30 minutes, 1, 2, 6, 12, 24, 48, and 72 hours. All graphs have Rainfall Depth (in mm) on the vertical axis plotted against "Reduced Variate", which is a measure of return period or annual recurrence interval (ARI). The horizontal axis is the same for each graph and ranges from -2 to 5, but the vertical axis changes and ranges from 0mm to 20mm for the 10 minute storm, to 0mm to 200mm for the 72h storm. The annual maxima in rainfall, for each duration, are sorted into ascending order and superimposed on two lines representing theoretical extreme value distributions, known as EV1 and GEV (Jenkinson, 1955). These two theoretical lines are almost the same except at extreme high rainfall where they diverge. In general, the observed data fit equally well to either of the theoretical distributions, and show increasing rainfall depth for larger ARI.