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 meagre rainfall totals of 5 mm or less in the past week, with a few areas seeing no rainfall at all.
- A few localised areas in the Coromandel Peninsula and Central Plateau saw amounts of 10-30 mm.
- This resulted in additional moderate to large soil moisture decreases across the entire North Island.
- The driest soils across the North Island, when compared to normal for this time of the year, are found in eastern Northland and the Coromandel Peninsula, while the wettest soils for this time of the year are found in southern Waikato.
- Hotspots are now located across much of the North Island, including Northland, Auckland, northern Waikato, Bay of Plenty, East Cape and coastal Gisborne, coastal Taranaki, Manawatū-Whanganui, and Wellington-Wairarapa.
- As of 21 February, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in parts of Northland, Auckland, eastern Bay of Plenty, East Cape, and much of the lower North Island. Very dry to extremely dry conditions are located in eastern Northland, far southern Manawatū-Whanganui, and Wellington.
South Island:
- Rainfall amounts of 20-40 mm affected the West Coast in the past week, although Fiordland received 75-150 mm.
- However, the rest of the South Island saw meagre rainfall amounts of 10 mm or less, with some locations receiving no rainfall at all.
- This resulted in moderate soil moisture decreases across nearly the entire South Island.
- The driest soils across the South Island, when compared to normal for this time of the year, are found in Nelson, Marlborough and northern Canterbury, while the wettest soils for this time of the year are found in western Southland.
- Hotspots are currently located across much of the upper and eastern South Island, including eastern Tasman, Nelson, Marlborough, northern and southern Canterbury, and eastern Otago. In addition, central Canterbury is close to hotspot status.
- As of 21 February, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in eastern Tasman, Nelson, Marlborough, northern and far southern Canterbury, and northern Otago. Very dry to extremely dry conditions are located in Nelson, Marlborough, northern Canterbury, and northern 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 21 February, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in parts of Northland, Auckland, eastern Bay of Plenty, East Cape, much of the lower North Island, eastern Tasman, Nelson, Marlborough, northern and far southern Canterbury, and northern Otago. Very dry to extremely dry conditions are located in eastern Northland, far southern Manawatū-Whanganui, Wellington, Nelson, Marlborough, northern Canterbury, and northern 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 isolated showers on Friday (23 February), mostly dry weather is expected on Saturday.
- A front will bring moderate rainfall on Saturday night and Sunday morning to many regions.
- While Northland may continue to see showers on Monday, the rest of the island will be generally dry on Monday and Tuesday (26-27 February).
- After a few showers on Wednesday, late next week will be mostly dry.
- Weekly rainfall totals could reach 20-30 mm in parts of the northern and central North Island, with generally less than 20 mm elsewhere.
- Due to the expected rainfall in the next week, small soil moisture decreases may occur in drier parts of the island, although little change is expected in areas that see higher rainfall amounts.
- The current hotspots in the North Island will likely not change much in the next week, although those in areas that see less rainfall may strengthen slightly.
South Island:
- A front moving up the South Island on Saturday (24 February) will bring moderate to heavy rain to the West Coast, Southland, and Otago.
- After dry weather on Sunday and Monday, an additional round of showers or rain may arrive on Tuesday (27 February).
- Thereafter, generally dry weather is expected through late next week.
- Weekly rainfall totals of 70-100 mm are possible in the lower West Coast, including Fiordland, with 30-40 mm in the upper West Coast and lower South Island.
- However, regions such as Nelson, Marlborough, and Canterbury will likely see lighter rainfall totals of less than 15 mm.
- Due to the expected rainfall in the next week, additional soil moisture decreases are likely in the upper and eastern South Island, but the lower West Coast and lower South Island may see small increases.
- Current hotspots may strengthen in the next week, while additional hotspots may form in central Canterbury.
Long-term outlook (through late March):
- The drier (25th percentile) and middle (50th percentile) rainfall scenarios show drier or much drier than normal conditions across much of the country.
- In the wetter (75th percentile) scenario, below normal rainfall is still forecast in isolated areas, but near normal rainfall is most favoured.
- 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. The drier scenario also shows the possibility for a small area of meteorological drought to form in the lower South Island.
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.
