Downpours with enough grunt to cause flooding are needed to reverse dry soil conditions that are already killing off century-old native trees, a climate scientist says.
Jim Salinger says 70-100mm of rain is needed over a week to redress the driest soil conditions since records began 70 years ago in Northland, Auckland, Waikato, Bay of Plenty, Wairarapa and Westland.
Already, native trees including rimu - some over 100 years old - were starting to die out in Auckland.
Masterton's soil moisture was near Auckland's, meaning natives there were also at risk.
With no serious rain on the horizon, there was little that could be done for native trees and shrubs in the face of these conditions.
''It's going to rain over the next couple of weeks but that will be it for the next while.''
The 70-100mm of rain needed could cause flooding if it arrived over a short period but would not if it fell steadily over a week.
He has used a measure of soil dryness called potential evapotranspiration deficits (PED), which measures water going into the soil from rain, against that leaving through run-off, evaporation, and use by plants.
''Until there is an extended spell of rainfall during April in these drought effected areas it is probable farmers will continue to struggle to maintain adequate home grown feed supplies for stock. Producers will face extended difficulties in maintaining production on their land," Dr Salinger said.
Anything with a PED rating over 500 was extreme.
Between July last year and March this year, Masterton recorded 554, Palmerston North 432, Taupo 467, Auckland 560, and Tauranga 601.
Wellington Airport had a PED rating of 509, the third driest since records began there in 1960. At Kelburn, the rating was 346, which was above average and not unusual, Dr Salinger said.
- The Dominion Post