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 20 to 40 mm were recorded for eastern Northland and spots in the Far North, eastern Coromandel, and about Wairoa in Hawke’s Bay. Elsewhere, the remainder of the North Island recorded less than 10 mm of rain over the past seven days, while many locations in the central and western North Island received no rainfall at all. A moderate to large decrease in soil moisture occurred across the entire North Island in the past week. The driest soils, when compared to normal for the time of year, are found about the Far North in Northland, with the wettest soils, when compared to normal for the time of year, are in coastal areas of Wairarapa, the Tararua District, and in Northland about Whangārei.
The hotspot about the Manawatū-Whanganui coast near Foxton has strengthened and grown while the hotspot in Northland about the Far North has strengthened and expanded southward. An additional hotpot in the Bay of Plenty just east of Whakatāne remains, while new hotspots have formed in parts of Auckland, including Waiheke Island, in the Hauraki District southwest of Thames, and along the South Taranaki coast west of Hāwera. As of 9 March, the New Zealand Drought Index (NZDI) map shows abnormally dry conditions along the coast of the Horowhenua District and abnormally dry to very dry conditions in the Far North of Northland.
In the South Island, weekly rainfall totals of 30 to 50 mm were recorded in the west and south of Southland, small portions of Stewart Island, and in lower Westland. Elsewhere, Fiordland recorded 75 to 150 mm of rain, with a few locations seeing 150 to 200 mm over the past seven days. The remainder of the South Island generally recorded 5 to 25 mm of rain over the past week, with the lowest totals located in the upper South Island. A small to moderate soil moisture increase occurred in much of Southland and Fiordland. Soil moisture increased slightly or remained the same for most of Canterbury, remainder Otago, Stewart Island and Southland. In the upper South Island moderate to large soil moisture decreases occurred. The driest soils in the South Island, when compared to normal for the time of year, are in Westland about Reefton and Karamea, and Nelson and western Marlborough region. The wettest soils, when compared to normal for the time of year, are in North Canterbury about Kaikōura and in Banks Peninsula.
The previous hotspot along the Nelson-Tasman border with Marlborough has expanded west and southward toward Richmond, Tapawera and the inland Wairau Valley, while a new hotspot has developed about the Marlborough Sounds. As of 9 March, the New Zealand Drought Index (NZDI) map shows a 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 9 March, the New Zealand Drought Index (NZDI) map shows abnormally dry conditions along the coast of the Horowhenua District, abnormally dry to very dry conditions in the Far North of Northland, and abnormally dry conditions in a 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 tropical low pressure will bring moist northeasterly flow on Friday, with isolated heavy rain through the evening on Friday (13 March). The remaining trough and front will clear on Saturday, with lingering showers possible for the entire North Island, followed by a period of mostly settled weather from Sunday through Wednesday as high pressure moves across the country. A weak front might bring a few showers across the North Island on Thursday (19 March). Weekly rainfall totals of 30 to 50 mm are expected for eastern Bay of Plenty and northern Gisborne, with the possibility of larger totals of up to 75 mm in the foothills and the ranges. Elsewhere, the remainder of the North Island is expecting 10 to 30 mm of rain, with the potential for some areas to see 10 mm or less.
Due to the expected rainfall in the next week, soil moisture will likely increase in parts of the Bay of Plenty and Gisborne, while soil moisture levels will likely see moderate to large decreases across the rest of the North Island. The hotspots in Manawatū-Whanganui, Auckland, the Far North, Hauraki District, and South Taranaki will remain the same or slightly strengthen and grow over the next week, while the hotspot in Bay of Plenty may weaken based on current forecast rain projections.
In the South Island, a front will move across on Friday (13 March) with showers or rain, clearing early Saturday, followed by southwesterly wind flow. The weather pattern will produce occasional showers for the south and west of the South Island from Saturday through Tuesday, with mainly dry weather elsewhere as a ridge builds over the upper South Island. Another front will push across the South Island on Wednesday (18 March), with potentially heavier showers and rain for Fiordland and the Southern Alps and a few showers elsewhere. Weekly rainfall totals of 30 to 50 mm of rain are forecast for the west and south of Southland and Westland south of Westport, with 100 to 250 mm of rain for Fiordland and the foothills of Westland. The remainder of the South Island is forecast to receive 30 mm or less, with potentially 10 mm or less across northern Canterbury, Marlborough, and Nelson-Tasman over the next week.
Due to the expected rainfall in the next week, soil moisture levels will likely increase across Fiordland. Soil moisture is likely to remain the same or decrease slightly for southern Westland and Southland. For the remainder of the South Island, soil moisture levels are likely to see a small to moderate decrease. The hotspots in Nelson-Tasman and Marlborough will likely strengthen and grow over the next seven day period. Additional hotspots may form in Otago and South Canterbury.
Long-term outlook (through mid-April):
- In the drier (25th percentile) scenario, drier than normal conditions are signalled for the entire country, with very dry conditions for Canterbury, coastal Otago, and the north of the South Island, and very dry conditions for the west, central, and parts of the north of the North Island.
- The middle (50th percentile) scenario shows drier than normal conditions for the South Island except Fiordland, and dry conditions for all of the North Island except Northland and the eastern North Island, with a wet signal for coastal Hawke’s Bay, and near normal conditions for the remainder of the country.
- In the wetter (75th percentile) scenario, dry conditions are signalled for Nelson-Tasman in the South Island and for Taranaki in the North Island, while wet to very wet conditions are indicated for Northland, the east of the North Island, Fiordland, and Canterbury about Kaikōura, 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.
