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
In the North Island, recent heavy rainfall brought widespread amounts of 60 to 120 mm to the upper half of the island, with isolated pockets of 150 to 190 mm in parts of Bay of Plenty and the Central Plateau. Meanwhile, the east coast and lower North Island generally received 40 mm of rainfall or less in the past week. Significant soil moisture increases occurred across nearly all of the North Island in the past week, except in Wairarapa where slight soil moisture decreases were observed. The driest soils across the North Island, when compared to normal for this time of the year, are found in eastern Northland, coastal Gisborne, and southern Manawatū-Whanganui, while the wettest soils for this time of the year are found in West Auckland and western Bay of Plenty.
The previous hotspot in East Cape dissipated in the past week, but a new small hotspot has formed near the city of Gisborne. As of 8 November, the New Zealand Drought Index (NZDI) map below shows abnormally dry conditions are currently found in Northland, far northern Auckland, interior Waikato, coastal Bay of Plenty, coastal Gisborne, and southern Hawke’s Bay, with very dry to extremely dry conditions in southern Northland, coastal Bay of Plenty, and East Cape. However, due to recent heavy rainfall in the upper North Island, these areas of dryness will likely improve significantly or even dissipate in future updates.
In the South Island, rainfall amounts of 50 to 120 mm were observed in most of the West Coast in the past week, with higher amounts up to 170 mm in parts of Tasman and Nelson. However, significantly lower amounts of 30 mm or less were observed in the eastern and lower South Island. Significant soil moisture increases occurred in the upper South Island in the past week, while other parts of the island generally saw minor soil moisture decreases. The driest soils in the South Island, when compared to normal for this time of the year, are found near Christchurch, while the wettest soils for this time of the year are found in eastern Tasman and coastal South Canterbury.
The previous hotspot near Christchurch has dissipated in the past week, and no hotspots are currently located in the South Island. As of 8 November, the New Zealand Drought Index (NZDI) map shows no unusually dry conditions are currently present in the South 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 November, the New Zealand Drought Index (NZDI) map below shows abnormally dry conditions are currently found in Northland, far northern Auckland, interior Waikato, coastal Bay of Plenty, coastal Gisborne, and southern Hawke’s Bay, with very dry to extremely dry conditions in southern Northland, coastal Bay of Plenty, and East Cape. However, due to recent heavy rainfall in the upper North Island, these areas of dryness will likely improve significantly or even dissipate in future updates. 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
In the North Island, scattered showers will affect northern and western regions on Thursday (12 November), followed by several days of generally dry weather due to high pressure overhead. However, by late Monday and Tuesday (16-17 November), a weakening front will bring another round of showers. Weekly rainfall totals will generally be 25 mm or less for much of the North Island, although locally higher amounts of 50 mm or more may be observed in East Cape, the Central Plateau, Taranaki, and Kapiti Coast.
Due to the expected rainfall in the next week, much of the North Island may see slight soil moisture decreases. There is a small chance that new hotspots may form in eastern Northland and coastal Hawke’s Bay, while the current hotspot near Gisborne may strengthen.
In the South Island, western and southern regions may continue to see showers on Thursday (12 November), with generally dry weather over the following couple of days. An arriving front will bring heavy rain to the West Coast on Sunday and Monday (15-16 November), with lighter rain in the eastern South Island. Thereafter, showers may continue to affect the West Coast with dry weather elsewhere. Weekly rainfall totals of 100 to 200 mm will affect much of the West Coast, while amounts will generally be 30 mm or less across the rest of the South Island.
Due to the expected rainfall in the next week, soil moisture levels may increase along the West Coast, but small decreases may occur elsewhere. The previous hotspot in the Christchurch area may re-form in the next week.
Long-term outlook (through mid-December)
- All scenarios show wetter than normal conditions in the upper North Island, particularly in the 50th and 75th percentile scenarios.
- Near normal to below normal rainfall is most strongly signaled in the eastern and lower South Island in the 25th and 50th percentile scenarios.
- The risk for very dry conditions is generally not signaled in these scenarios, with the exception of parts of the eastern North Island in the driest 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.
