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 the Central Plateau, the Ruapehu District, and the Tararua and Ruahine Ranges, with a few pockets of 30 to 50 mm about the ranges. The remainder of the North Island only received 10 mm or less. In the past week, a small to moderate decrease in soil moisture occurred across the entire North Island, except in western Waitomo where soil moisture increased. The driest soils across the North Island, when compared to normal for this time of the year, are found in eastern Northland, about Whakatane in the Bay of Plenty, and in the north of Gisborne, while the wettest soils for this time of the year are found on the south coast of the Wellington region and western Waitomo.
The small hotspot about East Cape in Gisborne remains. As of 1 November, the New Zealand Drought Index (NZDI) map shows that abnormally dry conditions are currently found in coastal Bay of Plenty, Gisborne about East Cape, central Waikato, and the south and west of Northland, with very dry conditions in the north of Gisborne, and very dry to extremely dry conditions in southern Northland.
In the South Island, rainfall amounts of 75 to 150 mm were recorded across Fiordland, Westland, and about the Southern Alps, 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 Buller District, the Southern Alps, western Tasman, Banks Peninsula, Coastal Otago, and remainder of Southland. Elsewhere, less than 20 mm of rain was recorded across the remainder of the South Island over the past week. A small decrease in soil moisture occurred in northwest Marlborough, parts of North Canterbury, and Central Otago. Elsewhere, soil moisture increased or remained the same across the remainder of the South Island. The driest soils in the South Island, when compared to normal for this time of the year, are in the far northwest of Marlborough, while the wettest soils for this time of the year are found in North Otago, coastal South Canterbury, and the Kaikōura District.
There is a very small hotspot in eastern Christchurch in the South Island. As of 1 November, the New Zealand Drought Index (NZDI) map shows no areas of abnormally dry conditions are currently present 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 1 November, the New Zealand Drought Index (NZDI) map below shows abnormally dry conditions are currently found in coastal Bay of Plenty, Gisborne about East Cape, central Waikato, and the south and west of Northland, with very dry conditions in the north of Gisborne, and very dry to extremely dry conditions in southern 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, northerly flow will bring periods of showers and rain to the North Island Thursday through Saturday (5-7 November), with potential downpours and thunderstorms Friday and Saturday. Sunday will be somewhat settled, with only isolated showers. Low pressure will approach on Monday (9 November) and bring another unsettled northeasterly to northerly wind flow, with another period of showers and potential downpours through to Wednesday night (11 November). Weekly rainfall totals of 50 to 100 mm are expected across Northland, Auckland, Taranaki, the Waikato, western Bay of Plenty, Manawatū-Whanganui, the ranges of Hawke’s Bay, and the Tararua Ranges, with localised heavier amounts up to 150 mm about the ranges of Northland, the Coromandel, Bay of Plenty, Tongariro, Taranaki Maunga, and Gisborne/Tairāwhiti. Coastal areas of the Wairarapa, Hawke’s Bay, and Gisborne/Tairāwhiti are expected to see 10 to 30 mm of rain, with the remainder of the North Island likely to record 30 to 50 mm of rain over the next seven days.
Due to the expected rainfall in the next week, a majority of the North Island will see an increase in soil moisture, with large increases possible in areas that receive widespread downpours. The hotspot in Gisborne is likely to weaken or remain the same.
In the South Island, northerly flow will bring moisture across the South Island ahead of low pressure that arrives on Friday (6 November). Expect heaviest rainfall in the west of the South Island. Showers and possible downpours move across the South Island on Saturday, with showers becoming confined to inland areas on Sunday (8 November) with a return to southerly wind flow. Monday is somewhat settled, with showery northeasterlies returning for Tuesday and Wednesday, as low pressure again approaches from the Tasman Sea. Expect weekly rainfall totals of 100 to 200 mm in Fiordland, and southern Westland. 50 to 100 mm of rain is expected for most of Tasman, Nelson, and the ranges in the west of Marlborough. Coastal areas of Otago, Canterbury, and eastern Southland are likely to record 10 to 30 mm of rain, with the remainder of the South Island likely to record 30 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 for coastal Canterbury, coastal Otago, and much of Southland, with moderate to large increases in soil moisture for the west and north of the South Island. Elsewhere, soil moisture is likely to remain the same. The hotspot about Christchurch is likely to remain the same or grow slightly.
Long-term outlook (through early December)
- The drier (25th percentile) scenario shows dryness in the north and the east North Island, about Whanganui, and the east and south of the South Island, with near normal rainfall elsewhere.
- The middle (50th percentile) scenario shows dryness across parts of Southland, Canterbury, and the east of the North Island, above normal rainfall for the lower West Coast and North Taranaki, and near normal rainfall elsewhere.
- The wetter (75th percentile) scenario shows wet to much wetter than normal conditions for the west and north of the South Island, the western Wellington region, and the majority of the upper North Island, and near normal rainfall 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.
