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:
- Rainfall totals of 25-60 mm were observed in much of the upper half of the North Island in the past week.
- However, much of the lower North Island received lighter amounts of 20 mm or less.
- Particularly meagre rainfall was observed in Wairarapa.
- This resulted in small to moderate soil moisture increases in the upper North Island, while 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 western Wellington, while the wettest soils for this time of the year are found in the Far North and parts of Gisborne and Hawke’s Bay.
- While no hotspots currently exist in the North Island, a new hotspot is close to emerging along the Kapiti Coast.
- As of 5 December, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in parts of Wellington.
South Island:
- Rainfall amounts of 50-100 mm were generally observed in the West Coast, Tasman, and parts of Marlborough Sounds in the past week, with pockets up to 150 mm in higher terrain.
- Elsewhere in the South Island, light rainfall amounts of 15 mm or less were generally observed.
- This resulted in at least minor soil moisture increases in the West Coast, Tasman, and Marlborough Sounds, while minor decreases were generally observed elsewhere.
- The driest soils across the South Island, when compared to normal for this time of the year, are found in parts of Marlborough Sounds and western Southland, while the wettest soils for this time of the year are found in Tasman and Fiordland.
- In the past week, the hotspot located in Marlborough Sounds remained in place, while new, small hotspots emerged in Nelson and Banks Peninsula. Conditions are also approaching hotspot criteria in South Canterbury and parts of Otago and Southland.
- As of 5 December, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in parts of Nelson, Marlborough, North Canterbury, Banks Peninsula, South Canterbury, and interior Otago.
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 5 December, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in parts of Wellington, Nelson, Marlborough, North Canterbury, Banks Peninsula, South Canterbury, and interior Otago. 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:
- After a generally dry Saturday, a weakening front will bring showers to the island on Sunday afternoon and Sunday night (10 December), followed by isolated showers on Monday.
- Additional showers will be possible on Tuesday and Wednesday (12-13 December), but then dry weather returns late next week.
- Weekly rainfall totals of 15-30 mm are possible in the lower and eastern North Island, but meagre accumulations of 10 mm or less are expected elsewhere.
- Due to the expected rainfall in the next week, moderate soil moisture decreases may occur in the upper North Island, with minor decreases possible in the east and west.
- There is a small chance that a new hotspot may form in western Wellington in the next week.
South Island:
- A strong cold front will bring heavy rain to the West Coast from Saturday afternoon through Sunday morning (9-10 December), with moderate rain potentially affecting Canterbury.
- After a mostly dry Monday (11 December), scattered showers will be possible on Tuesday, with more widespread showers possible on Wednesday.
- Generally dry weather is likely on Thursday, but another front may bring more rain to the West Coast on Friday (15 December).
- Weekly rainfall totals of 100-250 mm are possible in the West Coast, with 30-50 mm possible in interior Canterbury and Southland.
- However, lighter amounts of 25 mm or less are forecast in the upper South Island, coastal Canterbury, and Otago.
- Due to the expected rainfall in the next week, soil moisture decreases may occur in Nelson, Marlborough Sounds, coastal Canterbury, and Otago, with minor to moderate increases possible elsewhere.
- The current hotspots in Marlborough Sounds and Nelson may strengthen in the next week, while the hotspot in Banks Peninsula may not change substantially.
Long-term outlook (through early January):
- The drier (25th percentile) and middle (50th percentile) rainfall scenarios both favour widespread drier or much drier than normal conditions across both islands through early January.
- Even in the wetter (75th percentile) scenario, below normal rainfall is still favoured in large parts of both islands.
- Very dry soil conditions could begin to affect large swaths of both islands in the drier rainfall scenario.
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.
