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:
- Moderate to substantial rainfall of 30-60 mm was widespread across the upper half of the North Island in the past week.
- In addition, amounts of 75-150 mm were observed in parts of eastern Northland, the Coromandel, and East Cape.
- In the lower North Island, weekly rainfall totals were generally 10-25 mm.
- This resulted in moderate soil moisture increases in the upper North Island (particularly in Northland), while some small decreases were observed in the lower North Island.
- The driest soils across the North Island, when compared to normal for this time of the year, are found in interior Hawke’s Bay, while the wettest soils for this time of the year are found in parts of Northland along with southern Hawke’s Bay.
- No hotspots currently exist in the North Island.
- As of 29 October, 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 50-100 mm occurred across much of the West Coast in the past week, including Tasman.
- Parts of Southland and interior Otago received 20-30 mm.
- However, the rest of the South Island received meagre rainfall of less than 10 mm.
- This resulted in small to moderate soil moisture decreases across the upper and eastern South Island, with some increases observed in the west and south.
- The driest soils across the South Island, when compared to normal for this time of the year, are found in Nelson and Marlborough Sounds, while the wettest soils for this time of the year are found in north-central Canterbury.
- The previous hotspot located along the border of eastern Marlborough and the Kaikōura District dissipated in the past week, and no hotspots currently exist in the South Island.
- As of 29 October, 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 29 October, 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:
- Numerous convective showers and isolated thunderstorms are affecting the east-central North Island today (1 November), resulting in locally substantial rainfall amounts.
- After a fairly dry Thursday, moderate to heavy rain will overspread the North Island on Friday (3 November).
- Scattered showers will occur during the weekend, followed by a better chance for showers to gradually move north across the island during early-to-mid next week (6-8 November).
- Weekly rainfall totals of 20-40 mm will be widespread across the North Island, but locally higher totals of 40-60 mm will be possible in central and eastern areas.
- Due to the expected rainfall in the next week, soil moisture levels may increase at least slightly in many areas, especially in east-central regions.
- No hotspots are expected to form in the North Island in the next week.
South Island:
- A few scattered showers will be likely on Thursday, followed by more widespread light to moderate rain on Friday (3 November).
- The lower South Island will see more showers or rain on Saturday, which will then move into the central and upper South Island on Sunday.
- Early-to-mid next week (6-8 November) will feature generally drier weather with only isolated, light showers.
- Weekly rainfall totals of 15-30 mm are expected across most of the South Island, with isolated higher totals around 40 mm.
- Due to the expected rainfall in the next week, soil moisture levels are unlikely to change significantly across the South Island. Minor increases or decreases may occur due to localised rainfall totals.
- No hotspots are expected to form in the South Island in the next week, although conditions may begin to approach hotspot criteria in parts of eastern Marlborough and coastal central Canterbury.
Long-term outlook (through early December):
- The wetter than normal anomalies (green) shown in the maps below are partially the result of the moisture from ex-Tropical Cyclone Lola that affected the North Island earlier this week.
- The drier (25th percentile) and middle (50th percentile) scenarios both favour drier than normal conditions across the South Island and western North Island, with very dry conditions potentially signalled in parts of the South Island.
- Even in the wetter (75th percentile) scenario, near normal to below normal rainfall is still favoured for these same areas.
- In the drier rainfall scenario, there is a signal for dry conditions to emerge in parts of the eastern and lower South Island and lower North Island over the next 35 days.
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.
