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 along interior parts of southern Northland and Auckland, the Waikato, portions of the western Bay of Plenty, Gisborne, northern Hawke’s Bay, the central plateau, with pockets of 50 to 100 mm of rain in the Waikato, Gisborne, and Hawke’s Bay about Wairoa. Elsewhere, 10 to 30 mm of rain fell in Taranaki, southern Hawke’s Bay, and portions of the Manawatū. The remainder of the North Island generally observed less than 10 mm of rainfall over the past week, with some locations recording no rain. Soil moisture decreased across the Wellington-Wairarapa region, most of the Manawatu, Taranaki, the west and south of Hawke’s Bay, and in the Coromandel Peninsula. Soil moisture increased moderately across Gisborne and Hawke’s Bay north of Napier. Elsewhere, soil moisture either remained the same or saw a slight increase across the North Island. The wettest soils for this time to year across the North Island, when compared to normal, are located in Gisborne and northern Hawke’s Bay. The driest soils, when compared to normal for this time of year, are found in the Far North.
There are currently hotspots in the Far North about Cape Reinga, about Whanganui, in interior southern Hawke’s Bay, and a small hotspot in the Wairarapa in the foothills of the Tararua Range. As of 5 January, the New Zealand Drought Index (NZDI) map shows that abnormally dry conditions are currently found in southern Hawke’s Bay and the Far North.
In the South Island, 25 to 50 mm of rain fell across the lower West Coast and Fiordland, while the remainder of the South Island recorded 25 mm or less of rain over the last seven days, with many locations recording no rainfall. Soil moisture saw moderate decreases across the entire South Island over the past week. The driest soils in the South Island, when compared to normal for this time of the year, are located about Invercargill, while the wettest soils for this time of the year are found in Fiordland and the lower West Coast.
There are currently hotspots in coastal North Otago, about Invercargill and Bluff in Southland, and about Nelson city and Richmond in the Tasman District. As of 5 January, the New Zealand Drought Index (NZDI) map shows that abnormally dry conditions are currently found in small 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.
As of 5 January, the New Zealand Drought Index (NZDI) map shows that abnormally dry conditions are currently found in the Far North, southern Hawke’s Bay, and parts of coastal Canterbury. 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, high pressure will be in place from Friday, 9 January, through to the following Monday. A weak cold front will cross the North Island with a few showers and isolated thunderstorms, heaviest in the lower North on Monday. High pressure follows again on Tuesday, with tropical moisture moving south into the North Island for the remainder of the week, bringing increasing chances for showers and thunderstorms each day through next week Friday, 16 January. There is large uncertainty in the weekly rainfall, due to potential tropical activity north of New Zealand. Weekly rainfall totals of 10 to 30 mm are expected for lower Wellington, Northland, Auckland, Hawke’s Bay and Gisborne regions, while 30 to 50 mm are expected across the remainder of the North Island. However, there is a low chance that some portions of the North Island may see heavier totals of 50 to 100 mm over the next seven days should a tropical low pressure system move into the North Island late next week.
Due to the expected rainfall in the next week, a slight to moderate decrease of soil moisture is possible for much of Northland, and the lower and eastern North Island, with soil moisture expected to remain the same across the rest of the North Island. However, should tropical moisture move in from the north, soil moisture could increase substantially for a large portion of the North Island. The current hotspots in the Far North, the Wairarapa, and southern Hawke’s Bay could possibly strengthen and expand in the current drier forecast scenario, although a wetter scenario would likely lead to the hotspots weakening.
In the South Island, a few fronts will bring rain in the west and south through Sunday, 11 January, heaviest in the West Coast, followed by high pressure building in from the west on Monday and Tuesday. Several weak fronts will push across the lower South Island late Tuesday and Wednesday, followed by another period of high pressure centered over or just to the southwest of the island through Friday. Weekly rainfall totals of 50 to 100 mm are expected across the West Coast over the next seven days, although isolated areas may see 150 mm or more. 30 to 50 mm is expected for western Tasman, Southland, the highest spots in the Canterbury high country, and much of Otago. Elsewhere, the rain forecast is for 10 to 30 mm over the next week.
Due to the expected rainfall in the next week, soil moisture levels will likely remain the same or increase slightly in most regions except for the east and the top of the South Island, where soil moisture will likely see small to moderate decreases. The current hotspot about Nelson is likely to grow and expand, with hotspots in Southland and Otago likely remaining the same or weakening.
Long-term outlook (through early-February)
- The drier (25th percentile) scenario shows below normal rainfall across the majority of New Zealand, with very dry conditions for Otago and South Canterbury.
- The middle (50th percentile) rainfall scenario shows dry conditions for Otago and small portions of the top of the South Island, with near normal to above normal rainfall in the upper North Island.
- In the wetter (75th percentile) scenario, wetter than normal conditions are signalled for almost all of the North Island, and the West Coast, north and east of the South Island, with very wet conditions about Kaikoura and Blenheim, and near normal rain elsewhere.
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.
