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 across central and western areas, mainly in Taranaki, Waikato, the Manawatū, and the foothills of the Bay of Plenty, where 75 to 150 mm of rain fell, with a few locations seeing up to 200 mm of rain. 30 to 70 mm of rain fell in Wellington, the foothills of Gisborne, Hawke’s Bay, most of Auckland, parts of the Manawatū, and remaining parts of Taranaki and the Waikato. Elsewhere, 25 mm or less fell in Northland, coastal areas of the Bay of Plenty, remaining Auckland, Gisborne, southern Hawke’s Bay, southern and eastern Wellington, and portions of the Manawatū.
Soil moisture levels decreased slightly in the east of Northland and the east of the Wellington region, otherwise soil moisture levels generally saw small to moderate increases across the entire North Island in the past week. The driest soils across the North Island, when compared to normal for this time of the year, are found around Hastings, while the wettest soils for this time of the year are generally found in the Auckland region, coastal Bay of Plenty, western Northland, around Wanganui, the Māhia Peninsula, and about Cape Turnagain.
No hotspots are currently located in the North Island. However, as of 13 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, with pockets greater than 200 mm. Tasman and western Southland generally recorded 50 to 100 mm of rain in the past week. Meanwhile, 10 to 30 mm of rain was recorded for Nelson. Generally, 20 mm or less was observed in the eastern South Island.
Soil moisture levels generally increased slightly in the Nelson-Tasman area and about the Otago Peninsula. Soil moisture levels generally decreased in Canterbury. The driest soils across the South Island, when compared to normal for this time of the year, are found in a small portion of coastal Canterbury and in Marlborough around Blenheim and the Wairau Valley, while the wettest soils for this time of the year are generally found in Otago, in the Nelson-Tasman area, and about Banks Peninsula in Canterbury.
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 15 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:
High pressure will slowly move across the North Island 17 to 18 October, with only drizzle or light rain across the region. A series of cold fronts will bring light to moderate rain to the North Island from 19 to 22 October, followed by a potential heavier rain next Thursday (23 October). Weekly rainfall total will be 5 to 25 mm for most of the North Island, with slightly heavier rain of 25 to 50 mm, isolated up to 75 mm, about the Tararua Ranges and parts of the western North Island.
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, with the best chance for decreases expected along the east coast. The area of dryness around Hastings may start to approach hotspot status.
For the South Island, a slow moving front will bring moderate to heavy rain across the west on 18 October. A long duration rain event is likely from 20 to 22 October, bringing very heavy rain to the west of the South Island. Weekly rain for the West Coast and western Tasman district will likely reach 200 to 300 mm, with high terrain locations potentially recording 400 mm or more. Interior Otago, western Southland, and the eastern Tasman district will generally expect 50 to 100 mm. Remaining Otago, Southland, the high country of Canterbury, and Nelson will see weekly rain of 25 to 50 mm, with remaining portions of the South Island recording 5 to 25 mm.
Due to the expected rainfall in the next week, soil moisture increases will be likely 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 eastcoast and Northland in the North Island, including very dry conditions in Central Otago.
- The middle (50th percentile) and upper (75th percentile) rainfall scenarios show a wet signal for the west of both the North and South Islands, with potential for very wet weather in the Far North and west of the North Island and the west and the top of the Sound 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.
