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:
- Substantial rainfall totals of 40-100 mm (with isolated totals up to 150 mm) occurred in the central North Island in the past week, along with Gisborne and Hawke’s Bay.
- However, the upper and lower North Island both received much less rainfall in the past week—generally less than 20 mm.
- This resulted in significant soil moisture increases in the central North Island, with generally little change observed elsewhere.
- The driest soils across the North Island, when compared to normal for this time of the year, are found in parts of eastern Northland and Wairarapa, while the wettest soils for this time of the year are found in southern Waikato and coastal Hawke’s Bay.
- Hotspots are currently located in a portion of eastern Northland, the south coast of Taranaki, and parts of Wellington-Wairarapa.
- As of 29 January, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in parts of Northland 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 50-100 mm (with isolated higher amounts) occurred in the central and lower West Coast in the past week. The upper West Coast and western Tasman generally received 25-50 mm.
- While Southland and parts of Otago received 10-25 mm, meagre rainfall amounts of less than 5 mm again occurred in much of Nelson, Marlborough, and Canterbury.
- This resulted in small soil moisture decreases across a majority of the South Island.
- The driest soils across the South Island, when compared to normal for this time of the year, are found in interior Marlborough and northern Canterbury, while the wettest soils for this time of the year are found in the upper West Coast and western Tasman.
- A hotspot remains in place in portions of southern Marlborough and northern Canterbury, while the remainder of these areas are close to hotspot status.
- As of 29 January, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in Tasman, Nelson, Marlborough, Canterbury, Otago, and far eastern Southland. Very dry to extremely dry conditions are located in Marlborough, parts of Canterbury, and interior 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 29 January, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in parts of Northland, much of the lower North Island, Tasman, Nelson, Marlborough, Canterbury, Otago, and far eastern Southland. Very dry to extremely dry conditions are located in southern Manawatū-Whanganui, Wellington, Marlborough, parts of Canterbury, and interior 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:
- Unsettled weather will affect much of the North Island from Friday afternoon through Saturday (2-3 February), producing areas of moderate to heavy rain in western and central regions.
- However, a large area of high pressure will move overhead thereafter, bringing several days of dry weather from Sunday (4 February).
- Weekly rainfall totals could reach 30-50 mm in the western and central North Island.
- However, meagre rainfall totals of 15 mm or less are likely in all other regions.
- Due to the expected rainfall in the next week, substantial soil moisture decreases may occur in much of the North Island, although little change is expected in western and central regions.
- The current hotspots in eastern Northland and Wairarapa will likely strengthen and expand in the next week, while the hotspot located in Taranaki could ease at least slightly.
South Island:
- A front will bring heavy rain to the West Coast on Friday (2 February), with areas of showers and moderate rain east of the Alps. Showers and lighter rain will continue on Saturday (3 February).
- Thereafter, several days of dry weather are expected in the upper and eastern South Island.
- However, the lower West Coast and lower South Island will continue to see periods of moderate to heavy rainfall through the middle of next week as a moisture-rich front moves back and forth across those regions.
- Weekly rainfall totals of 150-300 mm are possible in the central and lower West Coast, including Fiordland, while the upper West Coast and western Tasman will receive up to 100 mm.
- Substantial rainfall is also possible in the lower South Island, where Southland and Otago could receive 50-75 mm.
- However, more modest amounts of 15-40 mm are likely in much of the eastern South Island.
- Due to the expected rainfall in the next week, additional soil moisture increases are likely in the West Coast and lower South Island, with some decreases possible elsewhere.
- The current hotspot in Marlborough and northern Canterbury could strengthen at least slightly 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 the entire North Island and eastern South Island, although the West Coast and lower South Island would see near normal rainfall. Based on the agreement of the drier and middle scenarios, the North Island and eastern South Island have the highest chances of experiencing below normal rainfall.
- However, in the wetter (75th percentile) scenario, near normal rainfall could occur across large portions of the country.
- Very dry soil conditions could affect the lower North Island and upper South Island in all three rainfall scenarios, along with Northland in the drier scenario. A small area of meteorological drought is indicated in the drier scenario in Wairarapa.
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.
