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 30-70 mm occurred in much of Gisborne and parts of the Central Plateau in the past week, with up to 25 mm in the rest of the upper North Island.
- However, the lower North Island once again received meagre rainfall amounts of less than 5 mm.
- This resulted in soil moisture increases across portions of the upper North Island (most notably in Gisborne), while the lower North Island saw small decreases.
- The driest soils across the North Island, when compared to normal for this time of the year, are found in much of Wellington and southern Taranaki, while the wettest soils for this time of the year are found near Cape Reinga and in southern Waikato.
- Hotspots are currently located in a small portion of eastern Northland, the south coast of Taranaki, and around Wellington. In addition, Wairarapa is nearing hotspot status.
- As of 16 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 now located in Wellington.
South Island:
- Rainfall amounts of 50-100 mm were generally observed in the West Coast in the past week (through early Friday morning), with some higher terrain seeing more than 150 mm.
- Southland received 25-40 mm, while the rest of the eastern South Island generally saw meagre amounts of 10 mm or less.
- This resulted in moderate soil moisture decreases from Nelson to Canterbury, while the West Coast and Southland saw soil moisture increases.
- 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.
- South Island hotspots are now located in eastern Tasman, Nelson, much of Marlborough, and far northern Canterbury. The rest of Canterbury and northern Otago are nearing hotspot status.
- As of 16 January, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in Tasman, Nelson, Marlborough, Canterbury, 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 a small area of meteorological drought is 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 16 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, interior Otago, coastal Southland, and Stewart Island. Very dry to extremely dry conditions are now located in Wellington, Marlborough, parts of Canterbury, and interior Otago, while a small area of meteorological drought is 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:
- Scattered showers and thunderstorms will occur daily through Tuesday (23 January), especially in the central North Island.
- In addition, an area of low pressure could bring some heavy rain to Gisborne and eastern Bay of Plenty on Monday (22 January).
- Generally drier weather is expected from about Wednesday next week.
- Weekly rainfall totals could reach or exceed 100 mm in parts of Gisborne, with localised amounts above 50 mm in the central North Island. However, lesser amounts are expected in Northland and Auckland.
- Due to the expected rainfall in the next week, small soil moisture increases will be possible in the central and northeastern North Island, but Northland and Auckland may see at least small decreases.
- The current hotspot in eastern Northland may strengthen at least slightly in the next week, while those in the lower North Island may not change significantly.
South Island:
- Very heavy rain will continue to affect the West Coast through Saturday morning (20 January), with moderate rain in the lower South Island.
- More unsettled weather will arrive from late Sunday through Tuesday (21-23 January), affecting not only the West Coast but parts of the eastern South Island also.
- Generally drier weather is expected to arrive from Wednesday next week.
- Weekly rainfall totals (including Friday 19 January) of 200-500 mm or more are expected in the central West Coast, with 30-60 mm in the lower South Island, interior Otago, and interior Canterbury. However, Marlborough and coastal Canterbury may receive less than 25 mm.
- Due to the expected rainfall in the next week, additional soil moisture decreases may occur in Marlborough and coastal Canterbury, with some increases expected in most other areas.
- The current hotspots in the upper South Island may strengthen at least slightly in the next week, while there is a possibility for new hotspots to form in coastal Canterbury.
Long-term outlook (through mid-February):
- The drier (25th percentile) rainfall scenario shows drier or much drier than normal conditions across the western North Island and much of the South Island, although in the middle (50th percentile) scenario, many areas see closer to normal rainfall, with a chance for above normal rainfall in the north-eastern North Island. Based on the agreement of the drier and middle scenarios, Northland, Otago, and Southland have the highest chances of experiencing below normal rainfall.
- However, in the wetter (75th percentile) scenario, above normal rainfall could occur in large parts of the North Island and portions of the South Island.
- Very dry soil conditions could affect parts of the upper and eastern 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.
