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 variability of droughts. Three geographical regions in New Zealand with different climate and physical properties were used in the study. The annual maximum droughts in terms of severity (total deficit) were identified for 44 sites with at least 20 years of daily flow data using two truncation levels: the mean and 75% of the mean. For both approaches the mean of the standardised annual maximum severity was found to vary with physical and climatic catchment characteristics except for one region, where the severity was almost constant because of very high annual rainfall. The best regional estimates of mean severity were found for the lower truncation level. The severities and the durations of the annual maximum droughts at each site were almost linearly related. L moment statistics showed that two of the three regions were homogeneous, and that the three-parameter log-normal distribution gave the best fit to both severities and durations. A baseflow index integrating the influence of various physical and climatic catchment characteristics on 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.