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, weekly rainfall totals of 100 to 200 mm were recorded for eastern Northland, the Coromandel Peninsula, and the Bay of Plenty about the ranges and foothills. The remainder of eastern Northland except about Cape Reinga, northern and eastern Auckland, about Tongariro, most of the Waikato, and the remainder of the Bay of Plenty region, 50 to 100 mm of rain were recorded. Elsewhere, 25 mm or less were recorded for most of the Wellington region, most of the Taranaki region, most of Manawatū-Whanganui, Hawke’s Bay, and Gisborne. The remainder of the North Island received 25 to 50 mm of rain over the past seven days. Taranaki, southern Manawatū-Whanganui, Wellington, Hawke’s Bay, and Gisborne experienced a moderate to large decrease in soil moisture over the last week. Elsewhere, soil moisture experienced moderate to large increases for the Waikato and northern Manawatū-Whanganui. For the remainder of the North island, soil moisture remained the same or increased slightly over the past week. The driest soils, when compared to normal for the time of year, are found in Taranaki, southern Manawatu-Whanganui, northern Wellington, about Napier in Hawke’s Bay, and about Gisborne city in Gisborne. The wettest soils, when compared to normal for the time of year, are in Northland, Auckland, and northern Coromandel Peninsula.
The previous hotspots about the Manawatū-Whanganui coast near Foxton and in Hawke’s Bay about Napier and Hastings remain the same or have slightly strengthened, while a new hotspot has formed in the Wairarapa. As of 7 April, the New Zealand Drought Index (NZDI) map shows abnormally dry conditions along the coast of the Horowhenua District and about Napier in Hawke’s Bay, with abnormally dry to very dry conditions in Taranaki.
In the South Island, weekly rainfall totals of 100 to 200 mm of rain were recorded about northern Fiordland, and Westland south of Greymouth. 50 to 100 mm of rain were recorded in Tasman, most of Nelson, western Marlborough including the sounds, remaining Fiordland, and the Kakanui Mountains in Otago. The remainder of the South Island from eastern Marlborough to Southland generally recorded less than 30 mm of rain over the past week. Soil moisture generally remained the same or decreased slightly for Tasman, Westland, Southland, Otago, and Canterbury. Elsewhere in Marlborough, soil moisture increased slightly. The driest soils in the South Island, when compared to normal for the time of year, are in Westland about Reefton. The wettest soils, when compared to normal for the time of year, are in Tasman about Abel Tasman National Park.
There are currently no hotspots in the South Island. As of 7 April, the New Zealand Drought Index (NZDI) map shows abnormally dry conditions in Central Otago and in a small area of Westland about Reefton.
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 7 April, the New Zealand Drought Index (NZDI) map shows abnormally dry conditions along the coast of the Horowhenua District and about Napier in Hawke’s Bay, Central Otago, and in a small area of Westland about Reefton, with abnormally dry to very dry conditions in Taranaki. 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, former tropical cyclone Vaianu will move across this weekend, with a period of heavy rain for many parts of the North Island. Heaviest rain and wind will be on Sunday (12 April). The cyclone will clear by early Monday. A series of fronts will move across the North Island on Monday and Tuesday, with additional rain and showers, heaviest in the west. Showers clear Wednesday morning, with dry weather through to early Thursday (16 April), follows by another period of rain and showers on Thursday. Weekly rainfall totals of 100 to 150 mm are expected for eastern Northland, Auckland, coastal Waikato, the Coromandel, the Bay of Plenty, northern Gisborne, the Hawke’s Bay ranges, the Tararua Ranges, and about Tongariro and Taranaki Maunga, with isolated totals exceeding 200 mm in the Coromandel and the ranges of the Bay of Plenty and Gisborne. Weekly rainfall totals of 50 to 100 mm are forecast for the remainder of Northland, remainder Taranaki, the Manawatū-Whanganui, inland Hawke’s Bay, remainder of Gisborne, and the Wellington region. The remainder of the North Island will likely receive 25 to 50 mm of rain over the next seven days.
Due to the expected rainfall in the next week, the North Island except coastal Hawke’s Bay will see a moderate to large increase in soil moisture, with Hawke’s Bay likely to see soil moisture remain the same or increase slightly. The hotspots in the Manawatū-Whanganui and the Wairarapa are likely to weaken and possibly dissipate, while the hotspot in Hawke’s Bay will likely remain.
In the South Island, dry weather is expected late Friday through Saturday evening, except for rain with an approaching front about Fiordland late on Saturday (11 April). On Sunday, former tropical cyclone Vaianu will move north and east of the island, bringing a period of rain and wind to the Marlborough and far North Canterbury regions. Several fronts will cross the South Island on Monday through Wednesday morning, with additional rain and showers, heaviest along the west coast. A brief period of settled weather is expected Wednesday, with rain showers return to the south and west of the South Island Wednesday evening. Showers will continue across the island on Thursday (16 April). Weekly rainfall totals of 50 to 100 mm are expected about Fiordland, Westland and eastern Marlborough. For western and coastal Southland, Tasman, remaining Marlborough, and far north Canterbury about Kaikōura weekly rainfall totals of 30 to 50 mm are expected. The remainder of the South Island is likely to record 10 to 30 mm of rain over the next seven days.
Due to the expected rainfall in the next week, soil moisture levels are likely to remain the same or increase slightly across Southland, Westland, western Tasman, with potential for a large increase in Marlborough. For the remainder of the South Island, soil moisture will likely see a small to moderate decrease. Hotspots may return to Otago and Canterbury.
Long-term outlook (through mid-May)
- In the drier (25th percentile) scenario, drier than normal conditions are signalled for the entire South Island and Taranaki, with very dry conditions for the south of the South Island, wet conditions for small parts of the upper North Island, and near normal conditions elsewhere.
- The middle (50th percentile) scenario shows drier than normal conditions for the west and south of South Island, very dry conditions for much of Otago and far south Canterbury, wet conditions for Northland, Auckland, eastern Waikato, Bay of Plenty, wet to very wet conditions the Coromandel, Hawke’s Bay, and Gisborne, and near normal conditions for the remainder of the country.
- In the wetter (75th percentile) scenario, dry conditions are signalled for Southland, Otago, and far south Canterbury, wet to very wet conditions are expected for the east coast of Marlborough, and the north and east of the North Island, and near normal conditions 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.
