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 across Northland, Auckland, the Waikato, most of the Bay of Plenty, a majority of Gisborne, southern and central Hawke’s Bay, the Tararua District, the Wairarapa, coastal parts of the Manawatu-Whanganui, and coastal parts of Taranaki. Elsewhere, 30 to 50 mm of rain was recorded about remainder of Taranaki, Manawatū-Whanganui, eastern Hawke’s Bay, eastern Bay of Plenty Ranges, and about the western Wellington region, with a few pockets of 50 to 75 mm about the Tararua Ranges and the Stratford District. In the past week, soil moisture increased about Taranaki, parts of the Waikato, the Manawatū-Whanganui, Hawke’s Bay, and most of the Wellington region, while in the remainder of the North Island soil moisture decreased slightly or remained the same. 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.
There are currently no hotspots in place across the North Island. However, as of 19 October, the New Zealand Drought Index (NZDI) map shows that abnormally dry conditions are currently found in western Northland.
In the South Island, rainfall amounts of 50 to 100 mm were recorded about the Canterbury High Country west of Christchurch, the Marlborough Sounds, and Buller and Grey districts over the past week, with a few pockets of 100 to 150 mm. Elsewhere, less than 10 mm of rain was recorded across much of Fiordland, the lower portions of Westland, and about Nelson and Richmond. The remainder of the South Island generally recorded 20 to 40 mm of rain over the past week. Moderate soil moisture increases were observed in eastern areas of the South Island in the past week, with soil moisture decreasing 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 located in Nelson and the far northwest of Marlborough, while the wettest soils for this time of the year are found in coastal South Canterbury, the Waitaki District and the Kaikōura District.
There are currently no hotspots in place across the South Island. As of 19 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.
New Zealand Drought Index (NZDI)
As of 19 October, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in western 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, high pressure will remain overhead through Sunday, leading to mainly dry weather. A quick moving cold front will bring a period of wet weather overnight Monday into Tuesday (26-27 October), with localised downpours possible. For the remainder of the week westerly winds will settle in, leading to another period of rainfall late Wednesday and Thursday. Expect 30 to 60 mm of rainfall about Taranaki, King Country, and about the Tararua Ranges over the next seven days, with higher amounts about the ranges. Weekly rainfall totals of 10 to 30 mm is expected for the remainder of the North Island, with localised heavier amounts.
Due to the expected rainfall in the next week, a majority of the North Island will see small to moderate soil moisture decreases, with the exception of Taranaki and the central North Island. No hotspots are expected to form in the next week across the North Island.
In the South Island, aside from drizzle or light showers about western areas, dry weather will prevail through Sunday morning. Showers will turn to rain about Fiordland on Sunday, spreading across the remainder of the South Island overnight Sunday through Tuesday (26-27 October). Heavy rain is likely for the Southern Alps, Westland, western Tasman, and potential spillover rain for western Southland, inland Otago, and Canterbury High Country. Showery westerly wind flow will continue the rest of the week, with a period of more widespread rain on Thursday (29 October). Weekly rainfall totals of 150 to 250 mm will be widespread in Fiordland and the ranges of Westland, with some areas receiving 300 mm of rainfall or more.
Coastal Westland, western Southland, and Otago and Canterbury about the divide will likely record 50 to 100 mm of rainfall. Eastern Tasman, Nelson, Marlborough, coastal Canterbury, 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) and middle (50th percentile) scenarios both show significant dryness across the North Island and eastern South Island, with near normal to above normal rainfall for the western South Island.
The wetter (75th percentile) scenario shows pockets of drier than normal conditions in the northern and eastern North Island and eastern South Island, and wetter than normal conditions for the western and southern South Island
Notably, drier than normal conditions are indicated for the northern and eastern North Island in all three 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.
