In each case a pre-determined set of rules and criteria were applied to the flow record to derive time series of D-day annual minima and to ensure that these were both stationary and independent. So that low flow events of different duration could be examined a range of values were used for D including 1, 7, 30, 60, 90, 180 and 365 days. For each time series, estimates of the non-exceedance probabilities corresponding to the annual minima were derived using the Gringorten Plotting Position Formula. These were then used to build the observed probability curve. It is important to note that the shape of this curve is influenced by the number and rank order of the annual minima derived for each series, as well as the range of annual minima observed.