Source: NIWA – National Institute of Water and Atmospheric Research
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:
North Island:
- In the past week, substantial rainfall amounts of 40-100 mm were observed in parts of Bay of Plenty, Gisborne, northern Hawke’s Bay, the Central Plateau, and Taranaki.
- Across the rest of the North Island, rainfall amounts in the past week were generally 25 mm or less.
- This resulted in large soil moisture increases across much of the eastern and central North Island where the heaviest rainfall occurred. Elsewhere, soil moisture changes were generally minimal.
- The driest soils across the North Island, when compared to normal for this time of the year, are found in northern Waikato, while the wettest soils for this time of the year are found in southern Gisborne and northern Hawke’s Bay.
- No hotspots currently exist in the North Island.
- As of 7 November, the New Zealand Drought Index (NZDI) map below shows that no unusually dry conditions are currently found across the North Island.
South Island:
- Rainfall of 30-60 mm occurred across much of the upper South Island in the past week, along with pockets of central Canterbury and Otago.
- Most other locations received 15-25 mm.
- However, western Southland and much of Fiordland received only 5 mm or less in the past week.
- This resulted in moderate soil moisture increases across much of the upper and eastern South Island, with small decreases observed in parts of Southland and Fiordland.
- The driest soils across the South Island, when compared to normal for this time of the year, are found in south western Southland, while the wettest soils for this time of the year are found in northern Canterbury.
- No hotspots currently exist in the South Island.
- As of 7 November, the New Zealand Drought Index (NZDI) map below shows that no unusually dry conditions are currently found across 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 7 November, the New Zealand Drought Index (NZDI) map below shows that no unusually dry conditions are currently found across the country. 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:
North Island:
- A large area of high pressure over the North Island will result in generally dry weather through Monday (13 November). During this time only isolated, light showers may occur.
- On Tuesday and Wednesday (14-15 November), a more organised front may bring moderate rain to the western North Island.
- Towards the end of next week, another front could deliver more moderate to heavy rainfall.
- Weekly rainfall totals of 30-60 mm will be favoured across the western half of the North Island, although the east coast may see much less rainfall, generally less than 20 mm.
- Due to the expected rainfall in the next week, soil moisture levels may increase in the western North Island, while minor decreases may occur in the east.
- No hotspots are expected to form in the North Island in the next week.
South Island:
- Generally dry weather is expected through this weekend, although Fiordland and the lower West Coast will see rain developing on Sunday (12 November).
- Heavy rain will affect the West Coast on Monday (13 November), with a period of moderate rain on Tuesday night.
- By late Thursday and Friday, low pressure in the Tasman Sea could deliver another round of heavy rain to the western South Island, and a potential for some moderate rain in the east.
- Weekly rainfall totals could exceed 150 mm in the central and lower West Coast, with a bit less farther north. Totals of 20-50 mm will be possible from southern Canterbury to Southland.
- However, eastern Marlborough and northern Canterbury may receive less than 20 mm.
- Due to the expected rainfall in the next week, soil moisture levels could increase moderately in the western and lower South Island, but minor decreases may occur in eastern Marlborough and northern Canterbury.
- No hotspots are expected to form in the South Island in the next week.
Long-term outlook (through mid-December):
- High pressure and less rainfall than normal will become strongly favoured by late November.
- The drier (25th percentile) and middle (50th percentile) rainfall scenarios both favour drier or much drier than normal conditions across much of the country, especially in the upper North Island and eastern South Island.
- Even in the wetter (75th percentile) scenario, below normal rainfall is still favoured in parts of these same areas.
- In the drier rainfall scenario, there is a signal for dry conditions to emerge in pockets of both islands.
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.
