REGIONAL FREQUENCY-ANALYSIS OF ANNUAL MAXIMUM STREAMFLOW DROUGHT

A regional frequency analysis of annual maximum streaflow drought was presented as a method for investigating the spatial and temporal varia bility of droughts. Three geographical regions in New Zealand with dif ferent climate and physical properties were used in the study. The ann ual maximum droughts in terms of severity (total deficit) were identif ied for 44 sites with at least 20 years of daily flow data using two t runcation levels: the mean and 75% of the mean. For both approaches th e mean of the standardised annual maximum severity was found to vary w ith physical and climatic catchment characteristics except for one reg ion, where the severity was almost constant because of very high annua l rainfall. The best regional estimates of mean severity were found fo r the lower truncation level. The severities and the durations of the annual maximum droughts at each site were almost linearly related. L m oment statistics showed that two of the three regions were homogeneous , and that the three-parameter log-normal distribution gave the best f it to both severities and durations. A baseflow index integrating the influence of various physical and climatic catchment characteristics o n hydrographs was found to be useful in regionalising the mean annual maximum severity and the log-normal distribution, allowing estimation of extreme droughts and droughts in ungauged catchments.