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 30-60 mm occurred in portions of the central North Island in the past week, along with the Coromandel Peninsula and East Cape. The rest of the central North Island generally received 15-25 mm.
- However, the upper and lower North Island generally received meagre rainfall totals of 5 mm or less.
- This resulted in small soil moisture decreases across much of the North Island, although small increases were observed in western and southern Waikato.
- The driest soils across the North Island, when compared to normal for this time of the year, are found in southern Taranaki, while the wettest soils for this time of the year are found in southern Waikato.
- Hotspots are currently located in a small portion of eastern Northland, the south coast of Taranaki, and parts of Wellington-Wairarapa.
- As of 24 January, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in parts of eastern Northland, central Waikato, and much of the lower North Island. Very dry to extremely dry conditions are located in southern Manawatū-Whanganui and Wellington.
South Island:
- Widespread significant rainfall amounts of 200-400 mm (with isolated higher amounts) occurred in the central and lower West Coast in the past week, resulting in areas of flooding. The upper West Coast and western Tasman generally received 50-100 mm.
- Southland, interior Otago, and far interior Canterbury received 25-50 mm, while the rest of the eastern South Island generally saw amounts of 10-25 mm. However, meagre amounts of less than 5 mm occurred in Marlborough and northern Canterbury.
- This resulted in large soil moisture increases for much of the West Coast and western Southland, while small to moderate increases were observed in most other areas.
- The driest soils across the South Island, when compared to normal for this time of the year, are found in interior Marlborough and far northern Canterbury, while the wettest soils for this time of the year are found along the West Coast and western Southland.
- Most South Island hotspots dissipated in the past week, although a hotspot remains in place in southern Marlborough.
- As of 24 January, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in Tasman, Nelson, Marlborough, Canterbury, northern and interior Otago, coastal Southland, and Stewart Island. Very dry to extremely dry conditions are now located in Marlborough, parts of Canterbury, and interior Otago, while small areas of meteorological drought are indicated in far southern Marlborough and far northern Canterbury.
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 24 January, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in parts of eastern Northland, central Waikato, much of the lower North Island, Tasman, Nelson, Marlborough, Canterbury, northern and interior Otago, coastal Southland, and Stewart Island. Very dry to extremely dry conditions are now located in southern Manawatū-Whanganui, Wellington, Marlborough, parts of Canterbury, and interior Otago, while small areas of meteorological drought are indicated in far southern Marlborough and far northern Canterbury. 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:
- An approaching front will bring showers to the lower and central North Island on Saturday (27 January). However, as this front stalls, more substantial rainfall is likely on Sunday afternoon and Sunday night.
- An area of low pressure forming north of the North Island will direct periods of at least moderate rainfall into the eastern North Island from Monday to Wednesday (29-31 January), although other regions will likely remain much drier.
- Late next week a brief period of drier weather may occur.
- Weekly rainfall totals could reach 75-100 mm along the east coast and Central Plateau, with isolated higher amounts.
- However, more modest amounts of 25-50 mm are likely in most other areas, although Northland and Auckland may only receive 15-25 mm.
- Due to the expected rainfall in the next week, substantial soil moisture increases may occur in the east coast, with small increases possible in most other areas. However, the upper North Island may see small soil decreases.
- The current hotspot in eastern Northland may strengthen at least slightly in the next week, while those in Taranaki and Wellington-Wairarapa could ease somewhat.
South Island:
- A potent front moving across the island on Friday night and Saturday morning (26-27 January) will bring heavy rain to the West Coast and light to moderate rain to the eastern South Island.
- A round of showers and isolated thunderstorms will affect the West Coast and lower South Island on Sunday (28 January).
- However, much drier weather is expected from Monday across the island, at least through late next week.
- Weekly rainfall totals of 100-250 mm are expected in the central and lower West Coast, with 50-75 mm possible in the upper West Coast along with western portions of Canterbury, Otago, and Southland.
- However, Marlborough and coastal Canterbury may see rainfall totals of less than 20 mm.
- Due to the expected rainfall in the next week, additional soil moisture increases may occur in many regions, but at least small decreases are possible in Marlborough and coastal Canterbury.
- The current hotspot in Marlborough could strengthen in the next week, with a chance for additional hotspots to emerge in Marlborough Sounds and northern Canterbury.
Long-term outlook (through late February):
- The drier (25th percentile) rainfall scenario shows drier or much drier than normal conditions across a majority of New Zealand, although in the middle (50th percentile) scenario, many areas see closer to normal rainfall, or even above normal rainfall in the eastern North Island. Based on the agreement of the drier and middle scenarios, the upper portions of both islands and the lower South Island have the highest chances of experiencing below normal rainfall.
- However, in the wetter (75th percentile) scenario, near normal to above normal rainfall could occur across the entire country.
- Very dry soil conditions could affect the lower North Island and upper South Island in all three rainfall 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.
