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:
- Meagre rainfall totals of 5 mm or less were observed in much of the western half of the North Island in the past week.
- Gisborne, Hawke’s Bay, and eastern Bay of Plenty generally received 10-25 mm in the past week, with small pockets up to 40 mm.
- This resulted in significant soil moisture decreases across a majority of the North Island.
- The driest soils across the North Island, when compared to normal for this time of the year, are found in eastern Northland, coastal Manawatū-Whanganui, and Wellington, while the wettest soils for this time of the year are found near Cape Reinga, southern Auckland, and interior Waikato.
- Hotspots are currently located in eastern Northland, coastal Manawatū-Whanganui, and western Wellington.
- As of 8 January, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in parts of Waikato and much of the lower North Island. Very dry to extremely dry conditions are also now located in Wellington.
South Island:
- Rainfall amounts of 25-75 mm were generally observed in the lower West Coast and Fiordland in the past week.
- However, elsewhere in the South Island rainfall amounts were minimal—generally less than 15 mm.
- This resulted in moderate to large soil moisture decreases in nearly all of the South Island.
- The driest soils across the South Island, when compared to normal for this time of the year, are found in the central West Coast, coastal Southland, and Stewart Island, while the wettest soils for this time of the year are found in coastal Tasman and far northern West Coast.
- South Island hotspots are now located in far northern Canterbury, interior South Canterbury, and coastal eastern Southland.
- As of 8 January, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in parts of Tasman, Nelson, Marlborough, Canterbury, Otago, coastal Southland, and Stewart Island. Very dry to extremely dry conditions are now located in Marlborough, parts of Canterbury, interior Otago, and Stewart Island.
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 8 January, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in parts of Waikato, much of the lower North Island, Tasman, Nelson, Marlborough, Canterbury, Otago, coastal Southland, and Stewart Island. Very dry to extremely dry conditions are now located in Wellington, Marlborough, parts of Canterbury, interior Otago, and Stewart Island. 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:
- High pressure will bring mostly dry weather through Sunday (14 January), with only isolated, light showers possible.
- A front will bring moderate to heavy rain to most locations on Monday (15 January), followed by scattered showers and isolated thunderstorms from Tuesday through Thursday.
- Weekly rainfall totals could reach 40-60 mm across much of the North Island, although totals could be somewhat lower along the east coast.
- Due to the expected rainfall in the next week, small soil moisture increases will be possible in much of the North Island.
- The current hotspots in the North Island may weaken at least slightly in the next week.
South Island:
- After a mostly dry Friday (12 January), a front will bring moderate to heavy rain to the West Coast and lower South Island from late Saturday through Sunday.
- After some additional showers on Monday (15 January), mostly dry weather will occur through Thursday.
- Weekly rainfall totals of 75-150 mm are possible in the West Coast, with 30-50 mm in the lower South Island. However, amounts less than 20 mm are likely elsewhere.
- Due to the expected rainfall in the next week, additional soil moisture decreases will be likely in Marlborough and Canterbury, with some increases possible in Otago and Southland.
- The current hotspot in Southland may weaken in the next week, while the hotspots in Canterbury will likely strengthen and expand.
Long-term outlook (through early February):
- The drier (25th percentile) rainfall scenario shows drier or much drier than normal conditions across nearly all of New Zealand, although in the middle (50th percentile) scenario, many areas see closer to normal rainfall.
- However, in the wetter (75th percentile) scenario, above normal rainfall could occur in large parts of the North Island and upper South Island.
- Very dry soil conditions could affect parts of both islands in the drier rainfall scenario, along with a risk for meteorological drought to develop in the interior 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.
