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.
Facts: Soil Moisture
In the North Island, isolated pockets of moderate to heavy rainfall occurred in the west during the last week. Mt Ruapehu collected 27 mm, the wettest daily rainfall total observed in over a month and about 1/5th of the April monthly normal. Parts of Taranaki, the western Taraura Ranges and the central plateau all collected around 10-30 mm. Wellington received 18mm of rainfall, also about 1/5th the monthly normal. Elsewhere in the North Island, 1-10 mm of rainfall was observed. This resulted in moderate soil moisture decreases across most of the North Island. The driest soils across the North Island, when compared to normal for this time of the year, are found in the Hauraki Plains and western parts of Northland, while the wettest soils for this time of the year are found in along the east coast.
Hotspots remain in the Firth of Thames and Hauraki Plains as well as northern Kaipara. As of 5 April, the New Zealand Drought Index (NZDI) map below shows that small dry patches are located in northern Northland.
In the South Island, areas across the West Coast, Southland and Otago received much-needed rainfall on 5 and 6 April. Invercargill observed 32 mm of rainfall, their wettest day in one-and-a-half years. Overall, Invercargill collected over one-third of their normal April rainfall in the last week. Meanwhile, Milford Sound collected 75 mm of rainfall and Mueller Hut recorded 85 mm on 5 April, their wettest daily rainfall totals since February. Although widespread 50-100 mm fell across the West Coast and Fiordland, across the Southern Alps rainfall totals were more meagre, with 10-25 mm for parts of Otago, Southland and Canterbury and less than 10 mm for Nelson and much of Marlborough. This resulted in soil moisture decrease across the east of the South Island, but modest soil moisture increases for the west of the South Island. The driest soils across the South Island, when compared to normal for this time of year, are found in Southland and southern Otago, while the wettest soils are found in northern coastal Canterbury, about Kaikoura.
Hotspots have expanded over much of Otago, and remain in much of Southland excluding Fiordland as well as eastern Rakiura/Stewart Island. As of 5 April, the New Zealand Drought Index (NZDI) indicated that meteorological drought has worsened in Southland and Stewart Island, where meteorological drought or severe meteorological drought is present. For the rest of southern Southland and southern Otago, the NZDI indicates that conditions are dry-to-extremely dry. However, the recent rainfall in Southland may have alleviated some of these conditions.
Outlook and Soil Moisture
In the North Island, it will be a generally dry weekend. During mid-next week, the remnants of Tropical Cyclone Fili may bring areas of heavy rain and thunderstorms to some regions, most likely starting in northern areas, then moving to eastern and southern areas late next week. However, the nature of the system brings a high degree of uncertainty. If the remnants of Fili were to impact the North Island, weekly totals could exceed 50-150 mm for some parts of Northland, Auckland, Bay of Plenty, Gisborne, Hawke’s Bay and Wellington. Alternatively, should the system take a more northward track, much lighter rainfall totals will be observed.
If the remnants of Fili were to impact the North Island, soil moisture levels would increase substantially for the regions that receive heavy rain. Given that the Hauraki Plains and northern Kaipara are in the area which could receive this rainfall, hotspots may abate. However, this is highly dependent on the track of the system.
In the South Island, while a few showers should bring isolated and light rainfall totals to Fiordland and the lower West Coast on the weekend, a burst of heavier rain looks to develop on Monday. Daily rainfall totals of 50-100 mm are possible extending from Fiordland to the central West Coast, with 10-25 mm possible for inland regions. However, only 10-20 mm rainfall looks to make it to drought-afflicted regions in Southland. Following this, a southerly change will ring some showers to eastern areas, with up to 10-30 mm predicted over the next week about the Canterbury foothills and ranges. Additionally, if the remnants of Fili track far enough south, areas of heavy rainfall may develop for the coastal upper Canterbury.
Soil moisture levels may remain the same for western areas, remain the same or decrease slightly for eastern areas, and decrease slightly for northern areas. Hotspots in Southland and Otago are unlikely to ease, while a hotspots near Nelson and Marlborough may develop.
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.
Pictured above: Soil Moisture Anomaly Maps, relative to the time of year. The maps show soil moisture anomaly for the past two weeks.
New Zealand Drought Index (NZDI)
As of 6 April, the New Zealand Drought Index (NZDI) below indicates meteorological drought or severe meteorological drought in lower Southland and Stewart Island, with dry-to-extremely dry conditions in southern Southland and southern Otago. Meanwhile, isolated spots of dry conditions are present in northern Northland.
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.
