Drought worst in nearly 70 years
The 2012-13 drought has been confirmed as the worst in nearly 70 years for large parts of the country.
An assessment published today said the dry conditions had been unusually widespread, with a severity similar to that of 1945-46.
The paper was prepared by Niwa for the Ministry for Primary Industries.
The ministry had been told the drought was the worst some farmers had experienced but it was important to have the analysis to understand how dry the regions had been compared to previous droughts, said Stuart Anderson, ministry resource policy manager for North Island regions.
Comparisons were principally based on estimates of soil water content. The duration and intensity of the latest drought could be compared with historically similar events by a calculation called the potential evapotranspiration deficit (PED).
PED could be thought of as the amount of water needed to be added as irrigation, in order to keep pastures growing at optimum levels.
The PED calculations for the period between July 2012 and May 2013 were applied to selected station observations going as far back as the 1940s. They were also applied to a 5km gridded data set covering the country, known as Niwa's virtual climate station network (VCSN), which started in 1972.
"Both analyses show that the 2012-13 drought was one of the most extreme on record for New Zealand," the paper said.
"For much of southern Northland, Auckland, Waikato, Bay of Plenty, Gisborne, Hawke's Bay, and West Coast, the July 2012-May 2013 PED accumulation was the largest in the 41-year VCSN record," the paper said.
"The longer-record station calculations indicate an event of similar severity occurred in the 1945-46 season."
An estimate of the area covered by the drought could be made using the PED calculations on the VCSN record, then comparing that to previous events over the 41 years of VCSN data.
At 34 per cent of North Island VCSN grid points, the PED accumulation to the end of May was the highest in the 41 years of data. That was much larger than any other drought in the period of VCSN coverage.
"For the Auckland, Waikato and Bay of Plenty regional council regions, the 2012-13 PED was the highest since 1972 over about 70 per cent of each region, and one of the three highest over about 95 per cent or more of each region," the paper said.
Niwa principal scientist Brett Mullan said previous severe droughts, in 1972-73 and 1997-98, were both El Nino years.
"This latest drought was different, being related to persistent high pressure centres over New Zealand during summer - a trend that is increasing according to century-long pressure records."
Although drought conditions had dissipated with the onset of winter, and many farms had started to recover following good autumn conditions, the economic and social impact of the drought continued to be felt around the country, Anderson said.
Recent snow made it harder for farmers to see themselves through the winter and manage feed supply and pastures.
The adverse event declaration of the drought would remain in place until September 30, which will allow rural communities to get through winter and into early spring.