The official declaration of a drought in parts of New Zealand should not come as a surprise at all. The West Coast of the South Island, known for its high rainfall, has not had any heavy rainfall warnings since mid-January. Northland is currently experiencing drought conditions with significant blazes proving a challenge to tackle. And Canterbury experienced fast moving grass fires in the month of January, which claimed a few properties and caused evacuations on multiple occasions. As yet there have not been significant economic losses on farms, caused by the lack of water, but the potential is certainly there.
There have been significant droughts in New Zealand in the past. The eastern seaboard is typically drier than the western seaboard, because the topography of both islands ensures most of the moisture that is carried by the prevailing westerly airflow, is dumped on the west coast’s of both islands in the Southern Alps, on Mount Taranaki and the western flanks of the Tararua Ranges.
The La Nina phenomenon involves anti-cyclonic weather systems that become slow moving over New Zealand. They tend to strengthen the prevailing easterly in locations such as Christchurch and Dunedin, tends to block the southerly fronts coming up from Antarctica, which bring the heaviest rain to the eastern South Island. However, it is often a different story over the North Island where the same phenomenon can induce more sub-tropical low pressure systems, sometimes of cyclonic origin, to pass over the eastern North Island often bringing heavy easterly quarter rain. Northland, Auckland and Waikato are also more likely to experience significant rainfall events during these events. Normally a La Nina phase will reduce the amount of rain that the West Coast of the North and South Islands are likely to receive, and often these areas can find themselves in drought after just a matter of several weeks with little rain.
The El Nino phase is different. The anti-cyclonic weather pattern is weakened and more frequently depressions that south of New Zealand or Tasmania tend to induce more frequent and prolonged westerly quarter airstreams. During an El Nino phase, typically the west coast of both islands have heavier and more frequent rainfall events. Although there is still drought in the Hawkes Bay, Marlborough, Canterbury and Otago regions, their larger alpine catchment rivers can receive significant spillover rain from precipitation events on the West Coast. The low pressure systems
In 1988-89, whilst constant heavy rain caused economic losses from the ruining of crops by excess precipitation on the West Coast, and in Taranaki, the east coast of both islands were in significant droughts along with Northland. The impact of the drought in these areas was substantial. Some farmers were driven from their land by the fact that their stock had no food or water and had to be either sold to farmers in other parts of New Zealand or sent to the slaughter house. The cost was estimated to be in the hundreds of millions of dollars. Over 5500 farms were affected and job losses put at risk or lost totalled 11,000.
Droughts brought on by the 1997-98 El Nino and the 1998-99 La Nina phases by contrast, only caused about $3-4 million in losses. The El Nino phase drought occurred in Northland and Auckland, along with Marlborough, Canterbury and Otago. Irrigation ponds completely dried up, many rivers and significant streams acting as irrigation water sources were put on minimum flow, which meant that irrigators could not use the water lest their ecosystem and other users be affected. Canterbury rivers such as the Ashley, Selwyn, Orari, Opihi, Temuka, Tengawai all virtually stopped flowing.
Climate change is forecast to intensify the El Nino and La Nina phases over New Zealand. The neutral phases, where no one airstream is dominant, may either weaken and become less of a feature, or be shorter lasting but be more intense. Eastern sides of both islands may experience more droughts, and be forced to adopt new crops in the place of water intensive farming practises such as those that are used now.
How much longer Northland and Auckland’s current drought lasts remains to be seen, as does whether or not it will spread to other parts of New Zealand. However the less than minimum flow readings of rivers printed in The Press newspaper daily suggest that irrigators must be starting to feel the effects in Canterbury.