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, rainfall amounts of less than 30 mm generally fell across most of the island in the past week, with the exception of small portions of the Coromandel, parts of the Bay of Plenty ranges, and the foothills of Hawke’s Bay, where 30 to 50 mm was recorded. Localised amounts of 50 to 75 mm were recorded in the eastern Bay of Plenty. Soil moisture decreased across nearly all of the North Island except the western Bay of Plenty and the Waikato about Taupo. The driest soils across the North Island, when compared to normal for this time of the year, are found in coastal Hawke’s Bay and Wairarapa, while the wettest soils for this time of the year are found in Northland, Auckland, and the Bay of Plenty.
A hotspot in lower coastal Wairarapa has formed over the last week. In addition, the hotspot in coastal Hawke’s Bay weakened. As of 25 November, the New Zealand Drought Index (NZDI) map shows that abnormally dry conditions are currently found in coastal Hawke’s Bay, Gisborne, and the Wairarapa, and very dry conditions are currently found in coastal Southern Hawke’s Bay and the Mahia Peninsula.
In the South Island, much of the West Coast, Fiordland, and western Tasman district received 75-150 mm of rain in the past week, with larger amounts in high terrain. Elsewhere, generally less than 30 mm of rain fell across Southland, Otago, Canterbury, Marlborough, Nelson and eastern Tasman district, with some portions of Canterbury, Otago, and Marlborough recording under 15 mm of rain. Remaining areas of the South Island generally recorded 30 to 50 mm of rain. While little change in soil moisture levels was observed in the top of the South Island, West Coast, and Fiordland, the rest of the South Island generally saw small to moderate soil moisture decreases. The driest soils in the South Island, when compared to normal for this time of the year, are in Canterbury, while the wettest soils for this time of the year are found in interior Otago and western Tasman district.
Small hotspots in both North Otago and South Canterbury have strengthened and grown over the last week, while a new hotspot has formed in North Canterbury about Waipara. As of 25 November, the New Zealand Drought Index (NZDI) map shows abnormally dry conditions in coastal North Canterbury.
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
As of 25 November, the New Zealand Drought Index (NZDI) map shows that abnormally dry conditions are currently found in coastal Hawke’s Bay, Gisborne, the Wairarapa, and coastal North Canterbury, and very dry conditions are currently found in coastal Southern Hawke’s Bay and the Mahia Peninsula.
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 wetter week is expected, with a front and rain clearing the North on Friday, 28 November. After a one day break, another front will move through the North Island on Sunday, 30 November with additional rain, clearing on Monday morning. Brief high pressure will follow Monday’s front, before a complex low pressure system brings moderate to heavy rain to the North Island on Tuesday evening through Thursday of next week (2-4 December). High pressure is set to bring dry weather next Friday. Weekly rainfall totals could reach 50 to 75 mm for Northland, Auckland, much of Taranaki, the eastern Bay of Plenty, and about the Wellington region. The Waikato, remainder Bay of Plenty, the Manawatu and the Gisborne foothills will generally see 30 to 50 mm. The remainder of the North Island will receive lower amounts of 30 mm or less, with less than 10 mm for Southern Hawke’s Bay.
Due to the expected rainfall in the next week, a slight increase of soil moisture is expected across most of the North Island, however, small decreases may occur in the eastern and lower North Island. A hotspot in lower coastal Wairarapa has formed over the last week and is likely to grow stronger in the week ahead. The hotspot in coastal Hawke’s Bay continues to weaken but could grow stronger in the days ahead, with the area around Wanganui and Gisborne possibly approaching hotspot status.
In the South island, rain returns to the southwest of the island on Saturday ahead of a front that moves in for Sunday, 30 November, bringing heavy rain to the west. Weaker fronts will bring light to moderate rain Monday-Tuesday of next week, before a complex low pressure system brings moderate to heavy rain to the South Island on Wednesday and Thursday of next week (3-4 December). The Wednesday-Thursday system may bring beneficial rains to parts of the east coast. Weekly rainfall totals of 50 to 100 mm are expected for western Southland, the West Coast, Tasman-Nelson, parts of Marlborough, and Canterbury country. Elsewhere, expect under 30 mm of rain for parts of Central and North Otago, with the remainder of the South Island set to receive 30 to 50 mm of rain.
Due to the expected rainfall in the next week, soil moisture levels will likely remain the same or decrease slightly in the west of the South Island, Southland, and the western Tasman district. Soil moisture will continue to decrease in interior Otago, with a likely increase of soil moisture for North Canterbury and Marlborough. The current hotspots about Canterbury may weaken with expected rain over the next week, while the hotspot in North Otago remain the same or could continue to grow over the next seven days.
Long-term outlook (through late-December):
- The drier (25th percentile) rainfall scenario shows widespread below normal rainfall, especially in lower South Island and the top of the North Island, with areas of very dry conditions.
- In the middle (50th percentile) scenario, dry conditions are expected for the east of the South Island and Northland and parts of the east coast of the North Island, with near normal rain elsewhere.
- In the wetter (75th percentile) scenario, dry conditions are forecast about Otago in the South Island, with normal to wet conditions elsewhere across the North and South Islands.
- Very dry soil conditions will continue to affect the east of the North Island and likely continues to emerge in the east of the South Island.
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.
