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, rainfall amounts of 10 to 30 mm generally fell in Northland, most of the Coromandel, coastal Bay of Plenty, Gisborne north of Gisborne city, and interior parts of Hawke’s Bay. Conversely, parts of the Manawatū, Taranaki, the Waikato (excluding the Coromandel Peninsula) recorded 50 to 75 mm, with localised amounts up to 100 mm. The remainder of the North Island recorded 30 to 50 mm of rain over the past seven days. Soil moisture levels decreased in eastern Northland. Elsewhere, soil moisture increased across a majority of the North Island, with a large increase across Hawke’s Bay, Gisborne, the Manawatu, and the Wellington region. The driest soils across the North Island, when compared to normal for this time of the year, are found in the east of the Far North, while the wettest soils for this time of the year are found in Auckland, coastal Taranaki, and coastal Bay of Plenty.
All hotspots across the North Island have dissipated. As of 2 December, the New Zealand Drought Index (NZDI) map shows that abnormally dry conditions are currently found in coastal Hawke’s Bay, Gisborne, and the Wairarapa, and very dry conditions are currently found in coastal Southern Hawke’s Bay and the Mahia Peninsula.
In the South Island, portions of interior Otago, the Canterbury High Country, North Canterbury about Kaikōura, western Tasman, the Marlborough Sounds, and the West Coast recorded 30 to 50 mm of rain, with localised amounts of 50 to 100 mm in the West Coast. Remaining areas of the South Island generally recorded 10 to 30 mm, with little to no rain recorded about Christchurch and Banks Peninsula. Soil moisture increased in the Marlborough region, Canterbury, and parts of Otago. Soil moisture slightly decreased or remained the same for the rest of the South Island. The driest soils in the South Island, when compared to normal for this time of the year, are in Canterbury, while the wettest soils for this time of the year are found in western Tasman district.
The small hotspot in South Canterbury remains, while a hotspot in North Canterbury has dissipated. As of 2 December, the New Zealand Drought Index (NZDI) map shows abnormally dry conditions in coastal North Canterbury.
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 2 December, the New Zealand Drought Index (NZDI) map shows that abnormally dry conditions are currently found in coastal Hawke’s Bay, Gisborne, the Wairarapa, coastal North Canterbury, and very dry conditions are currently found in coastal Southern Hawke’s Bay and the Mahia Peninsula.
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, a very dry and hot week is expected as high pressure sits over or near the North Island from Friday 5 December through to Wednesday 10 December. Despite high pressure, isolated showers are possible through the weekend and next week, mainly about the ranges and foothills. An area of low pressure will move from the north, possibly bringing some rain to the top of the North Island on Thursday (11th). Weekly rainfall totals will be less than 10 mm for most of the North Island, with localised totals of up to 30 mm about the Tararua Ranges, Mount Ruapehu, Taranaki Maunga, and the foothills in Gisborne.
Due to the expected rainfall in the next week, a moderate to large decrease of soil moisture is likely for the entire North Island. Dry portions of the east of the North Island could possibly approach hotspot status.
In the South island, a front passes along the east coast, bringing showers Friday and Friday night (5th), followed by a moist north-westerly flow on Saturday ahead of the next cold front. On Sunday and Monday (7-8 December) rain will become heavy at times in the west of the South Island as the aforementioned cold front moves through the South Island and weakens by Monday night. One day of high pressure on Tuesday (9th) is followed by another front, bringing rain to the south and west of the South Island. Additional cold fronts will push across the lower South Island with rain on Thursday and Friday (11-12 December). Weekly rainfall totals of 50-100 mm are expected for the West Coast and western Southland, with high elevations locations seeing upward of 150 mm. Southland and Steward Island will generally record 30 to 50 mm, with 10 to 30 mm of rain expected across Otago. The remainder of the South Island is set to receive 10 to 30 mm of rain, with portions of North Canterbury and Marlborough recording little to no rain.
Due to the expected rainfall in the next week, soil moisture levels will likely remain the same or decrease slightly in the west of the South Island, Southland and Otago. Soil moisture will likely see small to moderate decreases for the remainder of the South Island. The current hotspot about Canterbury is likely to grow, with hotspots potentially developing across Canterbury and North Otago.
Long-term outlook (through early-January):
- The drier (25th percentile) rainfall scenario shows widespread below normal rainfall, most of the South Island and the top of the North Island, with areas of very dry conditions.
- In the middle (50th percentile) scenario, dry conditions are expected for the interior of the South Island, Tasman and upper West Coast, and small portions of Northland, wet to very wet conditions for the east of the North Island and upper South Island, and near normal rain elsewhere.
- In the wetter (75th percentile) scenario, wet to very wet condition for most of the North Island, the east of the upper South Island, and western Southland and Stewart Island, with near normal rain elsewhere.
- Very dry soil conditions continues to emerge in the east of the 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.
