Source: NIWA – National Institute of Water and Atmospheric Research
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, ex-tropical cyclone Dovi delivered heavy rainfall across central and southern regions, with widespread weekly totals of 50-125 mm. Pockets of rainfall greater than 150 mm were observed in these areas as well. Meanwhile, weekly rainfall was much more sparse in the rest of the North Island, with Northland, Auckland, northern Waikato, western Bay of Plenty, and Gisborne generally receiving 15 mm or less. This rainfall resulted in large soil moisture increases across the central and southern North Island, but moderate decreases were observed in Northland and Auckland. The driest soils across the North Island, when compared to normal for this time of the year, are found in northwestern Northland, while the wettest soils for this time of the year are found in eastern Bay of Plenty, Gisborne, and western Waikato south to Wellington and Wairarapa.
Due to the last week’s relative dryness, the existing hotspot in western Northland strengthened and expanded northward to include Cape Reinga. As of 15 February, the New Zealand Drought Index (NZDI) map below shows that dry conditions are now located across the upper and central North Island. In addition, very dry or extremely dry conditions are located in Northland, Auckland, Waikato, and the Central Plateau.
In the South Island, ex-tropical cyclone Dovi brought heavy rainfall to the northern half of the island, with widespread amounts of 50-125 mm, and areas of more than 150 mm located in Tasman and the upper West Coast. Meanwhile, it was another week with light rainfall in the lower South Island, where most locations received less than 20 mm, and parts of Southland received less than 5 mm. This resulted in large soil moisture increases in the upper South Island and much of Canterbury, while Otago and Southland saw small soil moisture decreases. The driest soils in the South Island, when compared to normal for this time of the year, are located in western Southland and Stewart Island, while the wettest soils for this time of the year are found from Tasman east to Marlborough, and south to central Canterbury.
The existing hotspots covering much of coastal Southland and Stewart Island remained in place with little change this week. As of 15 February, the New Zealand Drought Index (NZDI) map below shows that dry conditions are now located across the upper and lower South Island. In addition, very dry or extremely dry conditions are located in the interior upper South Island, interior Otago, much of Southland and Stewart Island. Small areas in western Southland and northern Stewart Island are on the cusp of meteorological drought.
Outlook and Soil Moisture
In the North Island, high pressure will bring mostly dry weather through Saturday, although the Central Plateau could see a few afternoon showers on both Friday and Saturday. On Sunday (20 February), a front moving north will bring a period of rain to the southern and central North Island, with a few more scattered showers on Monday. During the middle and end of next week, moisture dropping south from the subtropics may bring rainfall to the upper and eastern North Island. Weekly rainfall amounts could reach 25-40 mm from the Coromandel Peninsula to Hawke’s Bay, with most other areas receiving 15-25 mm.
Due to the expected rainfall in the next week, soil moisture levels may decrease slightly in the western and southern North Island, with little change expected elsewhere. This may result in a small expansion of the current hotspot located in Northland.
In the South Island, a front will move north from late Friday through to early Sunday (18-20 February), bringing moderate rainfall amounts to much of the island. Thereafter, high pressure will result in mostly dry weather, although a weak front could bring light showers to the lower South Island on Tuesday. Weekly rainfall totals are expected to reach 20-40 mm across a majority of the South Island, although higher amounts nearing 100 mm will be possible in Fiordland and the lower West Coast.
Due to the expected rainfall in the next week, soil moisture levels will likely not change significantly across the South Island. However, slight soil moisture increases may occur in Southland and Stewart Island. This could result in a slight weakening of the hotspots currently located in those areas.
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.
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Pictured above: Soil Moisture Anomaly Maps, relative to this time of year. The maps show soil moisture anomaly for the past two weeks.
New Zealand Drought Index (NZDI)
As of 15 February, the New Zealand Drought Index (NZDI) map below shows that dry conditions are now located across the upper and central North Island, along with the upper and lower South Island. In addition, very dry or extremely dry conditions are located in Northland, Auckland, Waikato, the Central Plateau, the interior upper South Island, interior Otago, much of Southland and Stewart Island. Small areas in western Southland and northern Stewart Island are on the cusp of meteorological drought.
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.
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