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 were observed in small areas of the Central North and about the ranges of Hawke’s Bay and Bay of Plenty over the last week. The remainder of the North Island recorded amounts of less than 10 mm over the past seven days, with some locations receiving no rainfall at all. 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.
A hotspot has formed in the Far North about Cape Reinga. As of 9 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, 30 to 50 mm of rain fell across a small portion of Otago, much of the West Coast, and Stewart Island, with some areas in the West Coast recording 50 to 100 mm of rain over the last week. The upper West Coast, Southland, and the remainder of Otago generally recorded 10 to 30 mm of rain, with remaining areas of the South Island recording only minimal rainfall over the last seven days. Soil moisture remained the same for western Southland, 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 and western Southland.
The existing hotspot in South Canterbury has strengthen and expanded in the past week. As of 9 December, the New Zealand Drought Index (NZDI) map shows abnormally dry conditions in parts of coastal 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 9 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 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 front will move through early Friday with little to no rain, followed by several days of high pressure. Generally dry weather will prevail from Friday to Monday (12-15 December), followed by isolated showers on Tuesday ahead the next cold front. The front moves in on Wednesday with a period of moderate to possibly heavy rain and some thunderstorms. Lingering showers are possible on Thursday (18 December). Weekly rainfall totals of 30 to 50 mm are expected 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 the entire North Island. The hotspot about Cape Reinga could possibly strengthen. Elsewhere, dry portions of the eastern North Island and about Whanganui could possibly approach hotspot status.
In the South Island, a front clears early on Friday followed by another quick moving front in the afternoon accompanied by few showers for the east and the south of the South Island. High pressure builds in Saturday and brings mainly dry weather for the weekend. A stronger cold front advances across the South Island with some heavy rain from early Monday through Tuesday night (15-16 December). Lingering showers and thunderstorms are expected Wednesday and Thursday (17-18 December). Weekly rainfall totals of 50 to 100 mm are expected for the West Coast, western Southland and western Tasman district, 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 Steward Island, remainder Otago, and Tasman-Nelson will generally record 30 to 50 mm, with 10 to 30 mm of rain over the Canterbury low country and Marlborough.
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, 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 North Otago.
Long-term outlook (through mid-January):
- The drier (25th percentile) and middle (50th percentile) rainfall scenario shows normal to below normal rainfall across both islands, with the drier scenario (25th percentile) showing areas of very dry conditions in the north and east of the North Island, and top of the South Island.
- In the wetter (75th percentile) scenario, wetter than normal condition are signalled for almost all of the North Island and the east of the South Island, with near normal rain elsewhere.
- Very dry soil conditions continue to be signalled in the east of both the South and North Island 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 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.
