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, over the last week the entire island experienced rains of 10 mm or less except for a few pockets about the ranges of Hawke’s Bay, Bay of Plenty, Gisborne, pockets of the Waikato, and about Whangārei. Many spots in the lower and western North Island recorded no rain over the past seven days. A moderate to large soil moisture decrease was experienced across the entire North Island. The driest soils, when compared to normal for the time of year, are found along the Northland coast west of Kaitaia and north to Cape Reinga, with the wettest soils, when compared to normal for the time of year, are found about Auckland, the western Waikato, Hawke’s Bay about Wairoa, and about the city of Wellington.
A hotspot has developed about the Manawatū-Whanganui coast near Foxton and in Northland about Tauroa Point to North Cape. As of 9 February, the New Zealand Drought Index (NZDI) map shows abnormally dry conditions in the Far North of Northland.
In the South Island, weekly rainfall totals of 50 to 100 mm were recorded in Fiordland, with pockets of 100 to 200 mm. Elsewhere, 10 to 30 mm of rain were recorded in Southland, about the Otago Peninsula, Queenstown, and Balclutha in Otago, and much of Westland. The remainder of the South Island recorded 10 mm or less of rain over the past week. A small to moderate soil moisture increase occurred about Bluff, Stewart Island and Fiordland in Southland. Soil moisture remained the same for Balclutha, most of inland Otago, and north Canterbury about Kaikōura. Elsewhere, a moderate to large decrease in soil moisture occurred over the past week. The driest soils in the South Island, when compared to normal for the time of year, are about Palmerston and in inland Otago, Southland about Bluff and Stewart Island, and Westland about Hokitika. The wettest soils, when compared to normal for the time of year, are in north Canterbury about Kaikōura.
The hotspots in Southland and Otago have dissipated. As of 9 February, the New Zealand Drought Index (NZDI) map shows no abnormally dry conditions across 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 (NZDI)
As of 9 February, the New Zealand Drought Index (NZDI) map shows abnormally dry conditions in the Far North of 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, a developing low moving south from the tropics will bring widespread showers and thunderstorms Friday and Saturday, with potential for heavy rain across the east of the North Island. Additionally, a front will move across the North Island from the southwest late Friday-Saturday (13-14 February), enhancing rain chances. The low pressure centre will be located east of the North Island on Sunday, then turn and move southward slowly, but will remain near New Zealand through Wednesday morning. This will produce broadly southwest to westerly wind flow and bring occasional showers and cool-than-average temperatures for much of the week. A ridge of high pressure will bring somewhat settled weather for the North Island late Wednesday and Thursday (18-19 February). Weekly rainfall totals of 50 to 100 mm are forecast for eastern Bay of Plenty, Gisborne, Hawke’s Bay, Wellington, and the Manawatū, with a few pockets exceeding 100 mm about Gisborne and the ranges and foothills of Hawke’s Bay and the Bay of Plenty. Expected rainfall over the next week will generally be 30 to 50 mm for the remainder of the North Island. There is uncertainty regarding rainfall totals over the next week due to the low confidence of the track of the tropical low.
Due to the expected rainfall in the next week, soil moisture levels will likely stay the same for much of the Auckland and Northland regions, while the remainder of the North Island will see a moderate to large increase in soil moisture levels. The hotspots in the Manawatū and Northland may weaken or dissipate over the next week.
In the South Island, a front moves across on Friday with rain and showers, along with a few thunderstorms along the east of the South Island, and a cool southwest wind flow. The cool weather pattern will remain into next week, with a few showers and isolated thunderstorms expected both Saturday and Sunday (14-15 February), primarily in the east of the South Island. A tropical low pressure system impacting the North Island over the weekend will move southward and will remain just east of the South Island from late Sunday through Wednesday, which will bring occasional showers and even a few thunderstorms for the east of the South Island. A front will move in on Thursday afternoon with some rain for the west and the south of the South Island. Weekly rainfall totals of 50 to 100 mm of rain are expected for the high country of north Canterbury including the Kaikōura region, the ranges of Marlborough, along the Southern Alps in Westland, and in Fiordland. Elsewhere, expect 30 to 50 mm of rain for the western and southern Tasman district, remainder of Canterbury from Timaru northward, and pockets of Otago mainly about Dunedin and Queenstown. For the remainder of the South Island expect 10 to 30 mm of rain over the next week. There is some uncertainty regarding rainfall totals over the next week due to the low confidence of the track of the tropical low.
Due to the expected rainfall in the next week, soil moisture levels will likely see a small to moderate increase across Canterbury. Expect soil moisture to remain the same or decrease slightly in Westland, Tasman, western Southland, and Otago. Elsewhere, the remainder of the South Island will see moderate decreases in soil moisture. Hotspots may form Otago and Southland over the next seven day period.
Long-term outlook (through mid-March):
- The drier (25th percentile) scenario, drier than normal conditions are signalled for the entire South Island and for the north, west, and south of the North Island, with very dry conditions for much of the South Island and wetter than normal conditions about Hawke’s bay in the North Island.
- The middle (50th percentile) scenario shows dry conditions for all of the South Island and Taranaki, wetter than normal conditions for the east of the North Island including very wet conditions for Hawke’s Bay, with near normal conditions elsewhere across New Zealand.
- In the wetter (75th percentile) scenario, wetter than normal conditions are signalled for the south, east, and north of the North Island, and the north and east of the South Island, with potential for very wet conditions in eastern Northland, Auckland, the Waikato, Bay of Plenty, Hawke’s Bay, Gisborne, eastern Wellington, and Marlborough, and dry conditions for the west and the south of the South Island.
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.
