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, a heavy rainfall event led to 100 to 250 mm of rain across eastern Northland, a large portion of the Auckland region, much of the Waikato from Hamilton and Taumarunui eastward excluding Taupo, the Bay of Plenty, Gisborne, large portions of Hawke’s Bay, the Central Plateau, most of Taranaki, and the north of the Manawatū, with many spots recording 250 to 350 mm of rain along the eastern Northland coast, in the Coromandel Peninsula, the Bay of Plenty, and about the ranges and foothills of Gisborne, and Hawke’s Bay. This heavy rainfall resulted in areas of flooding and slips in the upper North Island. Elsewhere, 50 to 100 mm of rain was recorded in western Northland, the remainder of the Waikato, Auckland, and Hawke’s Bay regions, and the Manawatū between Wanganui and Mount Ruapehu. Over the last seven days, generally 10 to 30 mm of rain was recorded for the Manawatū about Palmerston North and in the south and east coasts of the Wellington region. The remainder of the North Island received 30 to 50 mm of rain over the last week. A moderate to large increase in soil moisture was observed across Gisborne, Hawke’s Bay, the Bay of Plenty, the Waikato, Auckland, Northland and most of the Manawatū. Meanwhile, a slight decrease in soil moisture was observed in the Manawatū about Palmerston North and lower Wellington-Wairarapa. The driest soils, when compared to normal for the time of year, are found the Manawatū about and west of Palmerston North, with the wettest soils, when compared to normal for the time of year, are found about eastern Northland, Auckland, the Waikato, Bay of Plenty, Gisborne, Hawke’s Bay, and Taranaki.
The previous hotspot in Northland has now dissipated due to the recent heavy rainfall. As of 20 January, the New Zealand Drought Index (NZDI) map shows that abnormally dry conditions are currently found in southern Hawke’s Bay, southern Manawatū, and the north of Northland, with very dry conditions about the Bay of Islands in Northland. However, this dryness indicated in Northland will likely dissipate in the coming days due to the past week’s rainfall.
In the South Island, weekly rainfall totals of 100 to 150 mm were recorded in North Canterbury about Kaikoura. Elsewhere, 50 to 100 mm of rain fell in the foothills of the western Tasman district, and in parts of the Canterbury high country, with 30 to 50 mm of rain about the remainder of the Tasman District, Nelson, the Marlborough Sounds, most of Canterbury, pockets of the West Coast, including all of Westland north of Karamea, and the ranges and hills of North Otago. The remainder of the South Island generally recorded 10 to 30 mm of rain, although much of Southland recorded less than 5 mm over the past week. Soil moisture saw slight to moderate increases across Tasman, Nelson, Marlborough, northern Westland, and Canterbury. Soil moisture saw a moderate decrease across Fiordland and the lower West Coast, Southland, and Central Otago. For the remainder of the South Island, soil moisture saw a slight decrease or remained the same over the last week. The driest soils in the South Island, when compared to normal for the time of year, are located about Southland around Lumsden, while the wettest soils for the time of the year are found in the western Tasman District and Canterbury about Kaikoura.
A hotspot has developed about Lumsden and Winton in Southland. As of 20 January, the New Zealand Drought Index (NZDI) map shows that abnormally dry conditions are currently found in small parts of coastal South Canterbury and North Otago, about Bluff in Southland, about Nelson City, and the Wairau Valley in Marlborough.
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 (NZDI)
As of 20 January, the New Zealand Drought Index (NZDI) map shows that abnormally dry conditions are currently found in small parts of coastal South Canterbury and North Otago, about Bluff in Southland, about Nelson City, the Wairau Valley in Marlborough, southern Hawke’s Bay, southern Manawatū, and the north of Northland, with very dry conditions about the Bay of Islands in Northland.
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:
In the North Island, westerly wind flow will dominate Friday-Monday behind a cold front, as low pressure sits to the south and east of New Zealand. This will bring gusty winds, occasional showers, and even isolated downpours through Monday (26 January). A building high pressure system across the Tasman Sea will bring more south or southeasterly wind flow starting Tuesday, allowing for showers and a few thunderstorms to impact the east of the North Island. On Wednesday, high pressure moves over the North Island, allowing for a light northly wind to return, bringing showers to the upper North Island Wednesday and Thursday (28-29 January). Overall, no widespread heavy rain is forecast over the next week or more, which means weekly rain totals will generally be 10 to 30 mm, with a few pockets of 30 to 50 mm in the west of the Wellington region, about Taranaki Maunga, in pockets of the Manawatū, the Waikato, and the western coasts of Auckland and Northland.
Due to the expected rainfall in the next week, soil moisture levels are likely to decrease slightly or remain the same across a majority of the North Island, with a moderate decrease in soil moisture expected for the east of the North Island. The former hotspot about North Cape in Northland may re-form over the next week, with possible hotspots developing in the Manawatū about Palmerston North and the southern Wellington coast.
In the South Island, low pressure will sit southeast of the island through Sunday evening, with a cold front bringing southwesterly winds and unsettled weather through the weekend. Showers and thunderstorms will bring potential downpours Friday through Monday. High pressure will move in from the Tasman Sea on Tuesday (27 January), allowing for more settled weather Tuesday and Wednesday. A front will move in with more showers Wednesday night and Thursday next week. Weekly rainfall totals of 30 to 50 mm of rain are expected for the Canterbury high country, about Christchurch, and Kaikoura, as well as most of Otago, Southland, and Westland, with pockets of 50 to 100 mm of rain about Westland, southern Fiordland, and the highest elevations of Otago and Canterbury. The remainder of the South Island is expected to see 10 to 30 mm of rain, with potentially less than 10mm of rain about the Marlborough Sounds.
Due to the expected rainfall in the next week, soil moisture levels will increase for much of Southland and Otago. Elsewhere, expect soil moisture to decrease or remain the same across the remainder of the South Island, with the largest decreases likely about the Marlborough Sounds. The current hotspot in Southland will likely weaken over the next week.
Long-term outlook (through late-February):
- The drier (25th percentile) and middle (50th percentile) scenario shows normal to above normal rainfall for much of the North Island, and normal to below normal rainfall across the majority of the South Island, with the middle (50th percentile) scenario depicting wet conditions about Christchurch in the South Island.
- In the wetter (75th percentile) scenario, wetter than normal conditions are signalled for all of the North Island and the east of the South Island, with potential for very wet conditions about the Canterbury low country.
- Unusual dryness is only indicated in parts of Southland 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 at https://www.niwa.co.nz/climate/nz-drought-monitor/droughtindicatormaps)
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.
