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:
- Much of the North Island saw rainfall totals of 15-25 mm in the past week, along with some smaller pockets of 25-50 mm.
- However, Northland, much of Auckland, and parts of the lower east coast received 5 mm or less.
- This resulted in small to moderate soil moisture decreases in the central North Island, although decreases were more substantial in Northland.
- The driest soils across the North Island, when compared to normal for this time of the year, are found in eastern Northland, while the wettest soils for this time of the year are found in southern Waikato.
- Hotspots are currently located in much of eastern Northland and parts of Wellington-Wairarapa.
- As of 5 February, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in parts of Northland, Auckland, and much of the lower North Island. Very dry to extremely dry conditions are located in southern Manawatū-Whanganui and Wellington.
South Island:
- Widespread rainfall amounts of more than 200 mm affected Fiordland in the past week, while the West Coast generally received 50-100 mm.
- Lower Southland received 30-60 mm, while the rest of Southland, Otago, and the upper South Island generally received up to 25 mm.
- However, much of Canterbury received less than 10 mm.
- While many areas didn’t see much change in soil moisture in the past week, some increases were observed in the upper South Island and Southland, while central and southern Canterbury saw soil moisture decreases.
- The driest soils across the South Island, when compared to normal for this time of the year, are found in Nelson, Marlborough and parts of Canterbury, while the wettest soils for this time of the year are found in the upper West Coast and western Tasman.
- The previous hotspot in portions of southern Marlborough and northern Canterbury dissipated in the past week, and no other hotspots are currently present. However, parts of the upper and eastern South Island remain close to hotspot status.
- As of 5 February, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in Tasman, Nelson, Marlborough, Canterbury, Otago, and far northern Southland. Very dry to extremely dry conditions are located in eastern Tasman, Nelson, Marlborough, parts of Canterbury, and northern Otago, while small areas of meteorological drought are indicated in 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 5 February, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in parts of Northland, Auckland, much of the lower North Island, Tasman, Nelson, Marlborough, Canterbury, Otago, and far northern Southland. Very dry to extremely dry conditions are located in southern Manawatū-Whanganui, Wellington, eastern Tasman, Nelson, Marlborough, parts of Canterbury, and northern Otago, while small areas of meteorological drought are indicated in 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:
- Dry weather will continue on Thursday (8 February), followed by a few showers in the eastern and central North Island on Friday.
- Afternoon showers and isolated thunderstorms will impact much of the upper and central North Island on Saturday, with isolated showers possible into Sunday (11 February).
- However, from early next week another large area of high pressure will arrive, bringing multiple days of dry weather.
- Weekly rainfall totals could reach 15-25 mm in parts of the central North Island that see the heaviest showers and thunderstorms.
- However, meagre rainfall totals of 15 mm or less are likely in the rest of the North Island.
- Due to the expected rainfall in the next week, moderate soil moisture decreases may occur in much of the North Island, although generally minor change is expected in central regions.
- The current hotspots in eastern Northland and Wairarapa could both strengthen further in the next week.
South Island:
- A weakening front will continue moving up the West Coast on Thursday (8 February), with light showers farther east.
- After dry weather on Friday, another front will bring moderate rainfall totals to the lower and central South Island from Saturday afternoon to Sunday morning (10-11 February).
- From Sunday afternoon through the middle of next week, high pressure will bring mostly dry weather.
- Weekly rainfall totals of 40-80 mm are possible in the central and lower West Coast, including Fiordland, while much of Canterbury, Otago, and Southland will receive 15-30 mm.
- However, meagre rainfall amounts are expected in the upper South Island, where weekly totals may be 5 mm or less.
- Due to the expected rainfall in the next week, additional soil moisture decreases are likely in the upper South Island, but areas from central Canterbury to Southland may see slight increases.
- Hotspots could re-emerge in Nelson, Marlborough, and northern Canterbury in the next week.
Long-term outlook (through early March):
- The drier (25th percentile) and middle (50th percentile) rainfall scenarios show drier or much drier than normal conditions across nearly the entire country.
- Even in the wetter (75th percentile) scenario, below normal rainfall is still forecast for several regions across both islands.
- Such agreement across all three rainfall scenarios gives an increased confidence in drier than normal conditions in the coming weeks.
- Very dry soil conditions could affect the lower North Island and eastern South Island in all three rainfall scenarios, along with the upper North Island in the drier scenario. Areas of meteorological drought are indicated in parts of Wairarapa and the eastern South Island in the drier 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.
