A weekly update describing soil moisture patterns across the country to show where dry to extremely dry conditions are occurring or imminent. Regions experiencing significant soil moisture deficits are deemed “hotspots”. Persistent hotspot regions have the potential to develop into drought.
Recent rainfall and current soil moisture conditions:
In the North Island, the greatest rainfall amounts over the last seven days fell in the western North Island, mainly in Taranaki, Waikato, the Manawatū, and about the Tararua Range, where 30 to 75mm of rain fell, with a few locations in the Tararua Range seeing over 100 mm of rain. Less than 10mm of rain fell across Northland, the low country of the Bay of Plenty, eastern Wellington, Gisborne, and Hawke’s Bay. Elsewhere, generally 10 to 30 mm of rain fell across the North Island. Soil moisture levels decreased across most of the North Island, with the exception of most of the Manawatū, western Wellington, southern Taranaki, and western Auckland where soil moisture remained the same. The driest soils across the North island, when compared to normal for this time of year, are found around Hastings and eastern Northland. The wettest soils for this time of the year are generally found in the Auckland region, around Wanganui, and the Māhia Peninsula.
No hotspots are currently located in the North Island. However, as of 20 October, the New Zealand Drought Index (NZDI) map shows that drier than normal conditions are currently found in the Hastings area.
In the South Island, 100 to 200 mm of rain fell in the past week across most of the West Coast and western Tasman districts, with pockets greater than 200 mm. Eastern Tasman, western Southland, and a small portion of the Nelson district generally saw 50 to 100 mm of rain. Over the last seven days, generally 30 to 50 mm of rain fell in the High Country of Canterbury, interior Otago, and for small sections of Southland. The remaining areas of the South Island generally received 10 to 30 mm of rain, with many portions of coastal and Central Otago, the Canterbury Plains, and the low country of Marlborough recording less than 10 mm of rain. Soil moisture levels generally decreased in Canterbury, Marlborough, coastal and Central Otago, while soil moisture levels generally increased or remained the same across the West Coast and southern Tasman, and Southland districts. The driest soils across the South Island, when compared to normal for this time of the year, are found in South Canterbury and in Marlborough around Blenheim, while the wettest soils for this time of the year are generally found in Otago and the Nelson-Tasman area.
No hotspots are currently located in the South Island.
Pictured above: Soil Moisture Anomaly Maps, relative to this time of year. The maps show soil moisture anomalies over the past two weeks.
New Zealand Drought Index
As of 20 October, the New Zealand Drought Index (NZDI) map shows that drier than normal conditions are currently found in the Hastings area.
Please note: some hotspots in the text above may not correspond with the NZDI map. This difference exists because the NZDI uses additional dryness indices, including one which integrates the rainfall deficit over the past 60 days. Changes are therefore slower to appear in the NZDI compared to soil moisture anomaly maps that are instantaneously updated.
The week ahead:
For the North Island, weak cold fronts will bring occasional light rain to western areas from 24 to 26 October, with dry conditions in the east prevailing. A stronger area of low pressure spreads moderate to heavy rain across the North Island 27 to 28 October, followed by settled weather on 29 October. Next week Thursday (30 October), a system north of New Zealand may bring heavy rain to Northland. Weekly rainfall total will be 30 to 50 mm for most regions in the North Island, including much of Northland, the Waikato, Taranaki, the Manawatū, around northern Wellington, with a few pockets seeing up to 75 mm. Expect 10 to 30 mm of rain elsewhere, with parts of coastal Hawke’s Bay and Gisborne possibly receiving less than 10 mm.
Due to the expected rainfall in the next week, only small changes to soil moisture levels are likely for most of the North Island. A slight increase may occur in the central North Island and the far North, with the best chance for decrease in soil moisture along the east coast. The area of dryness around Hastings may start to approach hotspot status. For the South Island, cold fronts will bring occasional rain to western areas from 24 to 26 October, with dry conditions in the east prevailing. A stronger area of low pressure spreads moderate to heavy rain across the South Island 27 to 28 October, with some spillover rain possible in the east. Snow is a possibility in the high country of Otago, Southland, and Canterbury on 28 October.
Unsettled weather continues for the South Island 29 to 30 October, with continued light to moderate rains in the west and the south. Weekly rain for the West Coast and western Tasman district will likely reach 100 to 200 mm, with high terrain locations potentially recording 200 mm or
more. Interior Otago, western Southland, and the eastern Tasman district will generally expect 50 to 100 mm. Central Otago, Southland, the Canterbury High Country and Nelson will see weekly rain of 25 to 50 mm, with remaining portions of the South Island recording 10 to 25 mm.
Due to the expected rainfall in the next week, soil moisture increases will be possible in much of the western South Island. For coastal Otago, Marlborough, and the low country of Canterbury, a slight decrease in soil moisture may occur.
Long-term outlook (through mid-November):
- The drier (25th percentile) rainfall scenario shows below normal rainfall across the east coast of the South Island and most of the east coast in the North Island, including very dry conditions in southern Hawke’s Bay.
- The middle (50th percentile) and upper (75th percentile) rainfall scenarios show a very wet signal for the west of South Island, with potential for wet weather in the north, west, and lower North Island and most of the South Island in the wetter scenario.
- Very dry soil conditions may begin to affect the east coasts of both islands in the drier scenario.
Pictured above: 35-day forecast rainfall anomaly scenarios (Top), and 35-day forecast dryness and drought scenarios (Bottom). These maps are updated daily at https://niwa.co.nz/climate/seasonal-climate-outlook.
Background:
Hotspot Watch: a weekly advisory service for New Zealand media. It provides soil moisture and precipitation measurements around the country to help assess whether extremely dry conditions are imminent.
Soil moisture deficit: the amount of water needed to bring the soil moisture content back to field capacity, which is the maximum amount of water the soil can hold.
Soil moisture anomaly: the difference between the historical normal soil moisture deficit (or surplus) for a given time of year and actual soil moisture deficits.
Definitions: “Extremely” and “severely” dry soils are based on a combination of the current soil moisture status and the difference from normal soil moisture (see soil moisture maps))
Hotspot: A hotspot is declared if soils are "severely drier than normal" which occurs when Soil Moisture Deficit (SMD) is less than -110 mm AND the Soil Moisture Anomaly is less than -20 mm.
