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, weekly rainfall totals of 10 mm or less were recorded for nearly all locations except for a few spots near Taranaki Maunga, where 10 to 20 mm of rain were recorded over the last seven days. In fact, some locations received no rainfall at all in the past week. A moderate to large decrease in soil moisture occurred across the entire North Island. The driest soils, when compared to normal for the time of year, are found about Cape Reinga in Northland, with the wettest soils, when compared to normal for the time of year, in Wairarapa and the Tararua District.
The hotspot about the Manawatū-Whanganui coast near Foxton remains while the hotspot in Northland about Tauroa Point to North Cape has grown slightly. As of 24 February, the New Zealand Drought Index (NZDI) map shows abnormally dry to very dry conditions in the Far North of Northland and abnormally dry conditions in the Manawatū-Whanganui coast near Foxton.
In the South Island, weekly rainfall totals of 100 to 200 mm of rain were recorded about Fiordland and portions of lower Westland. Across Westland south of Hokitika 50 to 100 mm of rain were recorded over the past week. Elsewhere, 30 to 50 mm of rain were recorded above Greymouth and Karamea in the foothills of the Southern Alps in Westland and in a few pockets of coastal Southland and Stewart Island. The remainder of the South Island recorded less than 30 mm of rain over the past seven days, with some locations receiving no rainfall at all. Soil moisture decreased or remained the same in all areas except Fiordland and lower Westland, where soil moisture experienced a slight to moderate increase. The driest soils in the South Island, when compared to normal for the time of year, are in Southland about Lumsden, Westland about Hokitika, Otago south of Oamaru, and east of Richmond in the Marlborough region. The wettest soils, when compared to normal for the time of year, are in North Canterbury about Kaikōura and Banks Peninsula.
Hotspots have formed in Southland about Lumsden, in Otago south of Oamaru, and along the Nelson-Tasman border with Marlborough east of Richmond. The previous hotspot in Southland just west of Invercargill has dissipated. As of 24 February, the New Zealand Drought Index (NZDI) map shows a very small area of abnormally dry conditions in Central Otago.
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 24 February, the New Zealand Drought Index (NZDI) map shows abnormally dry to very dry conditions in the Far North of Northland, and abnormally dry conditions in the Manawatū-Whanganui coast near Foxton and a very small area in Central Otago.
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 weakening front and southerly wind flow will bring scattered showers and a few thunderstorms on Friday, followed by a turn to easterly winds for the upper North Island on the weekend. On Saturday and Sunday (28 February-1 March) the easterly winds will bring scattered showers to the north, central, and east of the North Island. Another front will bring more southerly winds to the North Island starting Sunday evening through Tuesday (3 March), bringing more rain and showers to the east of the North Island. High pressure moves in with settled weather on Wednesday and Thursday. Weekly rainfall totals of 30 to 50 mm are expected for northern and central Hawke’s Bay and Gisborne. Elsewhere, the remainder of the North Island is expecting 10 to 30 mm of rain, with some areas seeing less than 10 mm of rain over the next week.
Due to the expected rainfall in the next week, soil moisture will remain the same or increase slightly across Hawke’s Bay and Gisborne, while soil moisture levels will likely see moderate to large decreases across the rest of the North Island. The hotspots in the Manawatū-Whanganui and Northland will likely strengthen over the next week.
In the South Island, south-southwesterly wind flow will bring cool temperatures and a few showers for Friday and Saturday, followed by an increase in showers and rain for the south and east of the South Island as a cold front moves in Saturday night. Scattered showers and rain will continue Sunday and into Monday (2 March), followed by high pressure. Settled weather is expected most of Tuesday and Wednesday, with rain moving into the west and south of the South Island ahead of another front on Thursday (5 March). Weekly rainfall totals of 30 to 50 mm of rain are forecast for the west and south of Southland and Westland from Westport south to Haast, with 50 to 100 mm of rain expected for lower Westland, the Southern Alps, and Fiordland. Parts of Southland and North Canterbury are expecting 10 to 30 mm of rain over the next week, with 10 mm or less for the remainder of the South Island.
Due to the expected rainfall in the next week, soil moisture levels will likely see a small to moderate decrease across the entire South Island except for North Canterbury, where soil moisture is likely to remain the same. The hotspots in Southland, Otago, and Nelson-Tasman will likely remain the same or strengthen some over the next seven day period. Additional hotspots may form in Otago and inland South Canterbury.
Long-term outlook (through late-March):
- In the drier (25th percentile) scenario, drier than normal conditions are signalled for the entire country, with very dry conditions for all regions of the South Island except North Otago and South Canterbury, and for all of the North Island except Gisborne, Hawke’s Bay, and eastern Northland.
- The middle (50th percentile) scenario shows dry conditions for all of the South Island, including a very dry signal along the Southern Alps, northern Westland, Nelson-Tasman, and Marlborough, and dry conditions for the north, west, and south of the North Island, including a very dry signal from Taranaki southward to western Wellington, with near normal conditions elsewhere across New Zealand.
- In the wetter (75th percentile) scenario, dry conditions are signalled for the south, west, and north of the South Island and for the west of the North Island, while wetter than normal conditions are indicated for the north and east of the North Island, with potential for very wet conditions in eastern Northland, Hawke’s Bay, and Gisborne, and near normal conditions elsewhere.
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.
