GW communications adviser Donna Adlan said the winter-spring period in Wairarapa this year was unusual with "long periods of relatively low rainfall punctuated by a few heavy falls".
"For example, the slow moving southeasterly weather system that brought as much as 200mm to parts of the Wairarapa over a week in late September has been followed by a period of almost zero rainfall - meaning soil moisture levels have fallen again rapidly."
The Niwa material indicates soils are drier than normal throughout Wairarapa, especially so in the south, and rainfall over the past three months has been below average in the south and eastern Wairarapa.
A climate map of the region shows soils becoming progressively drier - compared to the norm - the further south they are.
The El Nino weather pattern has produced warmer than normal ocean temperatures in the north and mid Pacific Ocean, but at the same time the waters around New Zealand have been cooler than normal.
This has produced more westerly and southwesterly winds, meaning less rainfall for easterly regions.
"Calculations by Niwa indicate that most of Wairarapa has had lower than normal winter rainfall, and rainfall on the eastern coast has been near the lowest on record (eg, Castlepoint had the fourth lowest winter rainfall on record)," according to the report.
Two soil moisture sites in the eastern hill country at Whareama and Longbush show soil moisture well below normal through winter.
Niwa's weekly "hotspot" report for Friday said the driest soil in the North Island exists below a line from Levin to Castlepoint.
Northern Wairarapa has near normal soil moisture for this time of year.
-DairyNZ's "farmwatch" advisory is predicting summer pasture growth 25 per cent below the long-term average for "parts of the east coast of the lower North Island". Dairy farmers are advised to graze pastures low to make the most of the spring growth and improve the quality of regrowth - and to conserve any excess pasture as silage or summer crops.
