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, the greatest rainfall amounts over the last seven days fell across parts of the Waikato, the Bay of Plenty, and the east coast of Northland, where 70 to 90 mm of rain fell. Elsewhere, 50 to 70 mm of rain was observed across the remaining areas of the Waikato, Northland, Auckland, Taranaki, and inland Bay of Plenty. A paltry 5 to 20 mm of rain fell in lower Wellington and most of the Wairarapa, Central Hawke’s Bay, and about Gisborne. The raining portions of the North Island generally recorded 20 to 50 mm of rain. Soil moisture levels increased slightly or remained the same for the North Island except for Hawke’s Bay around Napier and the Wellington region east of the Tararua Ranges, where soil moisture decreased slightly. The driest soils across the North Island, when compared to normal for this time of the year, are found around Napier, while the wettest soils for this time of the year are generally found in the upper North Island, the Bay of Plenty, and portions of the Manawatu.
No hotspots are currently located in the North Island. However, as of 30 September, the New Zealand Drought Index (NZDI) map shows that drier than normal conditions are currently found in the Napier area.
In the South Island, 100 to 150 mm of rain fell in the past week across most of the West Coast and Fiordland, with approximately 50 to 100 mm in portions of the Tasman District and high country of Otago. Conversely, a mere 5 to 15 mm of rain fell in the low country of Canterbury, Otago, and Marlborough over the last seven days. Elsewhere, rainfall amounts were generally between 15 to 30 mm, with a few high-country locations in Canterbury, Otago, Marlborough, Nelson, and remaining areas of the Tasman District seeing 30 to 50 mm of rain. Soil moisture levels generally remained steady in the South Island in the past week, except for most of Marlborough, coastal Canterbury, and coastal Otago regions, where soil moisture levels generally decreased slightly, while interior South Canterbury and interior Otago saw slight increases. The driest soils across the South Island, when compared to normal for this time of the year, are found in a small portion of coastal North Otago and in Marlborough around Blenheim and the Wairau Valley, while the wettest soils for this time of the year are generally found in interior South Canterbury and interior Otago.
No hotspots are currently located 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.
New Zealand Drought Index (NZDI)
As of 30 September, the New Zealand Drought Index (NZDI) map shows that drier than normal conditions are currently found in the Napier area.
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:
Rain and some thunderstorms will move across the North Island on Friday and the weekend, 3 to 5 October, with some areas seeing heavy rainfall. Another period of rain will likely arrive on 7 October due to a passing front. For the North Island, weekly rainfall totals will generally range from 10 to 30 mm for the coastal areas of Hawke’s Bay, Gisborne, and the Wairarapa, with a forecast of 30 to 60 mm across the rest of the North Island. A few locations could see upwards of 90 mm of rain over the next seven days, most likely in central and western regions.
Due to the expected rainfall in the next week, only small changes to soil moisture levels are likely for most of the North Island, except for the coastal portions of Hawke’s Bay and the east coast of the Manawatu region, where slight decreases in soil moisture may occur. Slight increases may occur in the western North Island.
The same weather system affecting the North Island will bring heavy rainfall for the central and upper South Island on Friday and the weekend, 3 to 5 October. A cold front follows this system on Monday, 6 October, with heavy rainfall primarily in the West Coast. Light rainfall may affect the West Coast in the middle of next week, with heavier rainfall again possible by Thursday, 9 October. For weekly rainfall totals, the West Coast will generally see 100-150 mm of rain, and 30-60 mm across Southland, Marlborough, Tasman, Nelson, the high country of Canterbury, about Christchurch, and interior Otago. However, coastal Otago and Southland may see just 10 to 30 mm of rain.
Due to the expected rainfall in the next week, slight soil moisture increases may occur in much of the upper and eastern South Island. For coastal Otago and Southland, a slight decrease in soil moisture may occur.
Long-term outlook (through early November):
- The drier (25th percentile) rainfall scenario shows below normal rainfall across the east and the top of the South Island and most of the east coast of the North Island and the east coast of Northland, including very dry conditions in interior Otago, and coastal Hawke’s Bay and Gisborne.
- The middle (50th percentile) and upper (75th percentile) rainfall scenarios show a wet signal for the west of both the North and South Island, along with a wet signal around Christchurch, with dry conditions for inland Otago and coastal Hawke’s Bay.
- Very dry soil conditions may begin to affect the east coasts of both islands in the drier 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)
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.
