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, generally 10 to 30 mm of rain fell in the past week across Taranaki, King Country, Ruapehu District, Tararua District, and Wellington region from Wellington city northward to Masterton. The remainder of the North Island only received 10 mm or less. In the past week, soil moisture remained mostly unchanged for much of Wellington and small portions of Taranaki and northwest Manawatū-Whanganui, while in the remainder of the North Island soil moisture decreased. The driest soils across the North Island, when compared to normal for this time of the year, are found about Whangārei and in Gisborne about East Cape, while the wettest soils for this time of the year are found in the south coast of the Wellington region.
A small hotspot has formed in the past week about East Cape in Gisborne. As of 25 October, the New Zealand Drought Index (NZDI) map shows that abnormally dry conditions are currently found in far eastern Bay of Plenty, Gisborne about East Cape, and most of southern Northland, with very dry conditions in southwest Northland.
In the South Island, rainfall amounts of 50 to 100 mm were recorded across western Southland and about the Southern Alps from Arthur’s Pass southward, with pockets of 150 to 250 mm about Fiordland and parts of the Southern Alps. Elsewhere, 20 to 50 mm of rain fell across coastal Westland, the Southern Alps north of Arthur’s Pass, Mackenzie Country, Queenstown-Lakes, and remainder of Southland. Elsewhere, less than 20 mm of rain was recorded across the remainder of the South Island over the past week. Moderate soil moisture increases were observed in the entire South Island except about eastern Tasman, Nelson, and northwest Marlborough, where soil moisture decreased. The driest soils in the South Island, when compared to normal for this time of the year, are located in Nelson and the far northwest of Marlborough, while the wettest soils for this time of the year are found in inland Otago, coastal South Canterbury, the Waitaki District and the Kaikōura District.
There are currently no hotspots in place across the South Island. As of 25 October, the New Zealand Drought Index (NZDI) map shows no areas of abnormally dry conditions are currently found 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.
As of 25 October, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in far eastern Bay of Plenty, Gisborne about East Cape, and most of southern Northland, with very dry conditions in southwest 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, southwesterly flow will bring fast-moving fronts across the island Thursday and Friday (29-30 October), followed by a period of high pressure through the weekend. From Monday onward, expect moist northerly flows, with showers likely each day, especially the north of the North Island. Weekly rainfall totals of 10 to 30 mm are expected across much of the North Island, with localised heavier amounts up to 40 mm, especially in central and western regions.
Due to the expected rainfall in the next week, a majority of the North Island will see a small to moderate soil moisture decrease, although little change is expected in central and western areas. Additional hotspots may form about eastern Northland and the eastern Bay of Plenty over the next week, while the hotspot about Gisborne is likely to grow stronger.
In the South Island, southwesterly flow will bring fast moving fronts across the island Thursday and Friday (29-30 October), followed by generally settled weather under high pressure on Saturday. On Sunday, moist northerly flow ahead of a front will bring rain to the west of the island, turning heavy for a time overnight Sunday into Monday (2 November). Rain and showers will continue to impact the west and the north of the South Island for the remainder of the week due to a moist northerly wind flow. Expect weekly rainfall totals of 150 to 250 mm to be widespread in Fiordland and the ranges of Westland, with some areas receiving 300 mm of rainfall or more. Coastal Westland, most of Southland, and Otago and Canterbury about the divide will likely record 50 to 100 mm of rainfall. Eastern Tasman, Nelson, Marlborough, Canterbury low country, and coastal Otago are expected to see 20 mm or less of rain. Meanwhile, the remainder of the South Island will record 25 to 50 mm of rainfall over the next seven days.
Due to the expected rainfall in the next week, soil moisture levels will likely decrease about eastern Tasman, Nelson, Marlborough, and much of the eastern South Island, with a small to moderate increase in soil moisture for the remainder of the South Island. No hotspots are expected to form in the next week across the South Island.
Long-term outlook (through late November)
- The drier (25th percentile) scenario shows significant dryness across the North Island and the north and east of the South Island, with wetter than normal conditions about Fiordland, and near normal rainfall elsewhere.
- The middle (50th percentile) scenario shows dryness across the north and east of the North Island and Otago in the South Island, with above normal rainfall for the southwestern South Island, and near normal rainfall elsewhere.
- The wetter (75th percentile) scenario shows pockets of drier than normal conditions in the eastern North Island, wet to much wetter than normal conditions for the western and southern South Island, and near normal rainfall elsewhere.
- Drier than normal soils are expected in the upper North Island in each of these scenarios.
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.
