Tech to track the drought
Scientists trying to rate the severity of the current drought are able to compare conditions across a grid of virtual climate stations with data going back four decades.
Grid points covering the whole country are about 5km apart. Estimates made at the virtual stations include daily rainfall, potential evapotranspiration, air and vapour pressure, maximum and minimum air temperature, soil temperature, relative humidity, solar radiation, wind speed and soil moisture.
They are based on actual data observations made at climate stations around the country.
"If we're going to make a comparison we have to have some quantitative measure of how severe droughts are," Niwa climate scientist Dr Brett Mullan said.
A measure often used at Niwa was known as the potential evapotranspiration deficit (PET).
"You can think of this as the amount of water that you need to add to a crop to prevent loss of production due to water shortage," Mullan said.
In some of the worst affected areas as much as 500mm of water would need to have been added from irrigation through the growing season to keep plants growing at the optimum level.
PED data across the 5km grid going back 41 years to 1972-73 showed this summer's drought was the worst in that time, "if you like to put it that way" in large parts of Northland, Auckland, Waikato, Bay of Plenty, Gisborne, Hawke's Bay, and quite a large part of the West Coast.
As at the end of February, 17 per cent of grid points were the worst they had been since the system was introduced, Mullan said.
That was second only to 1973 which had 20 per cent coverage, although that was primarily in the South Island, in Otago, Southland, and Tasman as well as Wairarapa.
The third biggest coverage was for 1997/98, when the PED, or intensity, had been worse in the Hawke's Bay than it was this year but that drought had been confined to east coast areas.
In some of the worst affected areas this summer, as much as 500mm of water would need to have been added from irrigation through the growing season to keep plants growing at the optimum level, Mullan said.
Looking back further, weather station records through to the 1940s indicated that in the most affected areas of the North Island the current drought was comparable to one in 1946.
"Those two stand out as the worst in that 70 year record. Some places the '46 one is worse, some places the 2013 is worse, but they're fairly comparable," Mullan said,
Outside the worst affected regions, the 2013 drought was more like a one-in-10-year event.