Abstract
Flash drought is a relatively new term describing drought events that develop rapidly, differentiating from typical droughts that slowly develop over a period extending from months to years. Conventional droughts are typically driven by periods of sustained below-average precipitation, whereas flash droughts are also commonly linked to a high evaporative demand and above-normal temperatures. Their rapid development means they can be highly significant in terms of impact on agriculture, water storage and electricity production, yet they have not been studied before in a New Zealand context. There is no universal method to identify flash drought events, however, soil moisture, evapotranspiration, precipitation and temperature have all been used. In this study flash droughts were identified using soil moisture, the following three criteria were used: a decrease from the 40th to the 20th soil moisture percentile, a decrease within 20 days and a decrease of at least 5% for each pentad. 13 locations were selected across New Zealand using soil moisture data from July 2003 to January 2023.
Research showed that Lake Tekapo has experienced the highest number of flash droughts (12), and Blenheim the second highest (10). These locations had lower average yearly rainfall and a lower percentage occurring during the summer months in comparison to locations that had fewer flash droughts. It was also found that flash droughts can transition into seasonal droughts with two in Blenheim and one in Paraparaumu and Gisborne transitioning to seasonal drought. Analysis of Kidson types showed a domination of trough Kidson types the month before flash drought event initiation, transitioning to a domination of zonal Kidson types in Gisborne and Paraparaumu and blocking Kidson types at all the other locations. The H, HW and HSE Kidson types increased the most during event initiation, all associated with above-normal temperatures and below-normal precipitation. Analysis of climatological patterns showed an increase in the La Niña phase during the month before and initiation of flash drought events. There were also increases in the positive phase of SAM. In summer, the La Niña phase of ENSO reduces rainfall across the entire country except for eastern areas of the North Island and has a connection to blocking types. In some instances, La Niña and a positive SAM co-occur, with the effects of the climate patterns amplified when they are in phase. The positive phase of SAM is associated with less rainfall over the country except for northeast coasts of the South and North Island and is associated with blocking types during summer. IOD was also examined, but has a weaker connection to flash droughts, with most occurring during neutral IOD phases. This research has formed the basis for flash drought research in New Zealand, identifying where flash droughts occur and Kidson types and climate patterns that have increased in occurrence from the month before to the initiation of flash drought events. The research has also highlighted some key areas of research that are yet to be investigated that will help advance knowledge of flash droughts, especially in New Zealand.