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 30 to 50 mm were observed in small areas of the Central North, about Taranaki, and about the ranges of Hawke’s Bay and Bay of Plenty over the last week. Conversely, large portions of Northland, Auckland, the Coromandel, about East Cape in Gisborne, remainder of Hawke’s Bay, and the Wairarapa recorded amounts of less than 10 mm over the past seven days. The remainder of the North Island generally observed 10 to 30 mm of rain in the past week. Moderate to large soil moisture decreases occurred for the entire North Island. The driest soils across the North Island, when compared to normal for this time of year, are found in the Far North, while the wettest soils for this time of year are found in Auckland and northern Waikato.
The hotspot in the Far North about Cape Reinga has strengthened and expanded in the past week, with a new hotspot across southern Hawke’s Bay. As of 16 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.
In the South Island, 30 to 50 mm of rain fell across a small portion of Otago, much of the West Coast, western Tasman district, portions of coastal Canterbury, and around the divide between Nelson and Marlborough, with some areas in the West Coast along the Southern Alps recording 50 to 100 mm of rain over the last week. A portion of Marlborough around Blenheim recorded under 10 mm of rain over the last seven days, while the remainder of the South Island received 10 to 30 mm of rain. Soil moisture saw a small to moderate increase across the upper West Coast, coastal Otago, and coastal Canterbury. Elsewhere, soil moisture remained the same for the lower West Coast and interior portions of Canterbury, with a small to moderate decrease in soil moisture 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 central Canterbury, while the wettest soils for this time of the year are found in western Tasman district, coastal Otago, and western Southland. The existing hotspot in South Canterbury remains with little change over the last seven days. As of 16 December, the New Zealand Drought Index (NZDI) map shows abnormally dry conditions in parts of coastal Canterbury.
Soil Moisture Anomaly Maps, relative to this time of year. The maps show soil moisture anomalies over the past two weeks.
As of 16 December, the New Zealand Drought Index (NZDI) map shows that abnormally dry conditions are currently found in coastal Hawke’s Bay, Gisborne, the Wairarapa, parts of coastal Canterbury, and very dry conditions are currently found in coastal Southern Hawke’s Bay. 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 front will clear Friday overnight, with lingering shower Saturday into Sunday morning. A weak ridge of high pressure quickly moves through on Monday, followed by a fast moving front with light rain or showers Monday night and Tuesday. Tropical moisture will occasionally bring rain showers to the North Island Wednesday through Friday (24-26 December), with a front moving across New Zealand late Friday. Weekly rainfall totals of 30 to 50 mm are expected in Taranaki, Waikato, western Northland, about the ranges and foothills of the Bay of Plenty, Hawke’s Bay, and about the central North Island, with localised rain totals of 50 to 75 mm. For the remainder of the North Island, expect totals of 10 to 30 mm over the next seven days.
Due to the expected rainfall in the next week, a slight to moderate decrease of soil moisture is possible for much of Northland, the lower and east of the North Island, with soil moisture expected to remain the same or see a slight increase across the rest of the North Island. The hotspot about Cape Reinga could possibly strengthen, while the hotspot across Hawke’s Bay is likely to strengthen and expand.
In the South Island, a front clears Friday evening, leaving a cool southwesterly wind and rain showers across the South through the weekend. On Monday, an area of low pressure to the south of New Zealand will bring a surge of moisture, with rain and wind, through the South Island, before clearing to westerly flow Tuesday through Friday morning, bringing occasional light rain or showers, mainly in the west and south of the South Island. A cold front will bring a period of rain to the South Island next Friday (26 December). Weekly rainfall totals of 50 to 100 mm are expected for the West Coast and far western Southland, with high elevations locations seeing upward of 150 mm. Some spillover may lead to 50 to 75 mm of rain for interior Otago and the high country of Canterbury. The remainder of Southland and Stewart Island, remainder Otago, and western Tasman will generally record 30 to 50 mm, with 10 to 30 mm of rain over the Canterbury low country, Marlborough, and Nelson.
Due to the expected rainfall in the next week, soil moisture levels will likely remain the same or increase slightly in all of the South Island except for the east and the top of the South, where soil moisture will likely see small to moderate decreases. The current hotspot about central Canterbury is likely to grow, with hotspots potentially developing across coastal Canterbury and eastern Marlborough.
Long-term outlook (through mid-January)
- The drier (25th percentile) scenario shows normal to below normal rainfall across most of the South Island except in Canterbury, and the majority of the North Island, with very dry conditions in the Far North, east of the North, and the top of the South.
- The middle (50th percentile) rainfall scenario shows dry conditions for the Far North, the east of the North Island, and the southwest and the top of the South Island, with wet conditions signalled for coastal Canterbury.
- In the wetter (75th percentile) scenario, wetter than normal conditions are signalled for almost all of the North Island, and the east of the South Island, with very wet conditions about Gisborne, parts of the Waikato, and for coastal Canterbury, and near normal rain elsewhere.
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.