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 portions of eastern Northland, the Waikato including the Coromandel Peninsula, about Tongariro, Taranaki, and eastern Bay of Plenty about the ranges and foothills, with greater than 200 mm in the ranges of the Coromandel. In Northland south of the Bay of Islands, Auckland, northern portions of the Manawatū-Whanganui, the remainder of the Bay of Plenty, parts of Central Hawke’s Bay, and inland Gisborne, 50 to 100 mm of rain were recorded. Elsewhere, 25 mm or less were recorded for most of the Wellington region and about the Far North in Northland. The remainder of the North Island received 25 to 50 mm of rain over the past seven days. Eastern Northland experienced a small to moderate decrease in soil moisture over the past week. In the Waikato, Taranaki, the Manawatū-Whanganui, Gisborne, Hawke’s Bay, and most of Wellington, a moderate to large increase in soil moisture occurred. Elsewhere, soil moisture remained the same across the North Island. The driest soils, when compared to normal for the time of year, are found in the Tararua Ranges. The wettest soils, when compared to normal for the time of year, are located in Northland, Auckland, and portions of the Waikato.
The previous hotspots about the Manawatū-Whanganui coast near Foxton and in Hawke’s Bay about Napier and Hastings have dissipated, while the hotspot in the Wairarapa has weakened but remains. As of 14 April, the New Zealand Drought Index (NZDI) map shows abnormally dry conditions about Napier in Hawke’s Bay and Taranaki, with abnormally dry to very dry conditions along the coast of the Horowhenua District.
In the South Island, weekly rainfall totals of 100 to 200 mm were recorded in most of Fiordland and southern Westland. Over the past week, 50 to 100 mm of rain were recorded for Westland about the foothills and ranges, western Tasman, most of Nelson, and western Marlborough. In coastal Westland and the remainder of Marlborough, 30 to 50 mm of rain were recorded. For the remainder of the South Island, less than 25 mm of rain were recorded over the past week, with little to no rainfall recorded in portions of Otago and Canterbury. A moderate to large increase in soil moisture occurred in Marlborough, Nelson, Fiordland, coastal Southland, and Stewart Island over the past week. Elsewhere, Otago and Canterbury experienced a small to moderate soil moisture decrease. For the remainder of the South Island, soil moisture remained the same. 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 about eastern Marlborough.
A hotspot has formed in Canterbury just to the west of Christchurch. As of 14 April, the New Zealand Drought Index (NZDI) map shows abnormally dry conditions in Central Otago and about Reefton in Westland.
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 14 April, the New Zealand Drought Index (NZDI) map shows abnormally dry conditions about Napier in Hawke’s Bay, Taranaki, Central Otago, and about Reefton in Westland, with abnormally dry to very dry conditions along the coast of the Horowhenua District. 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 slow-moving area of low pressure will move through over the weekend, bringing periods of showers and thunderstorms with downpours, starting late Friday night through the weekend and into Monday (20 April). Forecast confidence is lower beyond Monday, with the low pressure system forecast to be over or near the top of the North Island on Tuesday, bringing scattered showers. Showers are likely to become less numerous but will remain in the North Island for Wednesday and Thursday (22-23 April) with low pressure nearby. Weekly rainfall totals of 100 to 150 mm are expected about and north of Taranaki Maunga, portions of King Country, the eastern Bay of Plenty ranges, and about the Taraua Ranges, with weekly rainfall totals of 50 to 100 mm for the remainder of the Wellington region, remainder of Taranaki, remainder of the Manawatu-Whanganui, and about the Bay of Plenty. There is a low probability of rainfall totals much higher in the Wellington region, between 100 and 150 mm, depending on the track of the slow-moving low pressure system over the next week. The remainder of the North Island will likely receive 25 to 50 mm of rain over the next seven days, with heavier isolated totals due to potential downpours.
Due to the expected rainfall in the next week, Northland, Auckland, Gisborne, and Hawke’s Bay are likely to see soil moisture remain the same or increase slightly. The remainder of the North Island will likely see a moderate to large increase in soil moisture. The current hotspot in the Wairarapa is likely to dissipate in the next week.
In the South Island, low pressure moves across the country, along with a front, on Friday (17 April) through the weekend, which will produce showers and thunderstorms, along with downpours, mainly Friday and Saturday. Southeasterly flow will bring rain and potential downpours to the east coast of the South Island starting Sunday, with heaviest rain potential Monday through Wednesday (20-22 April). Forecast confidence is generally low during this time, with showers possibly lingering across the South Island on Thursday. Weekly rainfall totals of 100 to 150 mm are expected about Fiordland, Westland, and about the Kaikoura Ranges, with isolated totals of up to 200 mm. For Tasman, Nelson, Marlborough, the Canterbury High Country near the divide, and Canterbury coast about Kaikōura, weekly rainfall totals of 50 to 100 mm are expected. However, there is a low probability of higher amounts of rain of 100 to 200 mm for portions of Canterbury depending on the track of the slow-moving low pressure system over the next week. The ranges and foothills about Otago will likely see 30 to 50 mm of rain, with a low potential of up to 100 mm of rain. 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 decrease slightly about Southland and inland Otago. For the remainder of the South Island, soil moisture will likely see moderate to large increases over the next seven days. The hotspot in Canterbury about Christchurch is likely to dissipate, while additional hotspots may form in Otago.
Long-term outlook (through to mid-May)
- In the drier (25th percentile) scenario, drier than normal conditions are signalled for the south and southwest of the South Island, parts of Tasman, and much of the upper North Island, with wet conditions for lower Wellington and parts of North Canterbury, and near normal conditions elsewhere.
- The middle (50th percentile) scenario shows drier than normal conditions for small portions of Southland, wet to very wet conditions east of the South Island and for the south and east of the North Island, and near normal conditions for the remainder of the country.
- In the wetter (75th percentile) scenario, wet to very wet conditions are expected for the east and north of the South Island and east and south of the North Island, wet conditions signalled for the remainder of the North Island except Auckland and Northland, 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.
