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, dry weather was commonplace over the past week, with most locations receiving less than 10 mm of rain. In fact, parts of the upper and central North Island received no rainfall at all. However, a persistent onshore wind flow did bring notable rainfall to parts of Gisborne and northern Hawke’s Bay, where some locations received 50-100 mm. This resulted in moderate to large soil moisture decreases across nearly the entire North Island, with the exception of parts of Gisborne and northern Hawke’s Bay. The driest soils across the North Island, when compared to normal for this time of the year, are found across much of Northland, Auckland, northern Waikato, interior Bay of Plenty, and East Cape, while the wettest soils for this time of the year are found in Kāpiti Coast and Wairarapa.
Due to last week’s dryness in most locations, hotspots have expanded, and are now located across nearly all of Northland, Auckland, and northern Waikato, with smaller hotspots in southern Waikato, interior Bay of Plenty, and parts of East Cape. As of 15 March, the New Zealand Drought Index (NZDI) map below shows that dry or very dry conditions are now located in Northland, Auckland, and northern Waikato. In addition, extremely dry conditions are located in parts of Auckland.
In the South Island, most locations saw generally light rainfall of 15 mm or less during the past week. However, the western South Island saw 25 mm or more, with heavier amounts of 30-75 mm across most of the West Coast Region. This resulted in moderate soil moisture decreases across most of the island, although small soil moisture increases were observed in parts of the West Coast. The driest soils in the South Island, when compared to normal for this time of the year, are located in coastal Southland and Stewart Island, while the wettest soils for this time of the year are found in Marlborough Sounds and northern Canterbury.
South Island hotspots are currently located along the south coast of Otago, much of coastal Southland, and much of Stewart Island. As of 15 March, the New Zealand Drought Index (NZDI) map below shows that dry or very dry conditions are now located in coastal Otago and much of Southland. In addition, extremely dry conditions are located in coastal Southland and nearly all of Stewart Island, with meteorological drought near Tiwai Point and in northern Stewart Island.
Outlook and Soil Moisture
In the North Island, locally heavy rain may impact parts of Northland and Auckland on Friday night or Saturday morning (18-19 March). A moisture-rich front is likely to spread moderate to heavy rain across much of the North Island from late Sunday through to early Tuesday (20-22 March). Thereafter, scattered showers and thunderstorms will be possible during the middle of next week. Weekly rainfall totals could reach or exceed 50 mm across large portions of the North Island, with pockets of higher totals possible in Bay of Plenty and the east coast.
Due to the expected rainfall in the next week, soil moisture levels could increase a small to moderate amount, especially in the upper and eastern North Island. While this may help to weaken some of the hotspots currently in place, they are unlikely to dissipate entirely.
In the South Island, moderate to heavy rain may impact Fiordland and the lower West Coast on Saturday (19 March), with the upper West Coast and Tasman seeing moderate rain on Sunday. A plume of moisture may then affect the upper South Island and parts of Canterbury from late Monday into Tuesday (21-22 March), bringing moderate rainfall amounts. Mostly dry weather is likely during the middle of next week, before another low possibly brings rain to the lower South Island on Thursday (24 March). Weekly rainfall totals could reach 50-70 mm in parts of the western and upper South Island, and perhaps in parts of Canterbury as well. Substantially lower amounts are expected in Otago and Southland, however.
Due to the expected rainfall in the next week, soil moisture levels may improve slightly in the upper South Island and parts of Canterbury. However, additional soil moisture decreases may occur in Otago and Southland. This may result in a further expansion of current hotspots in the lower South Island.
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 March, the New Zealand Drought Index (NZDI) map below shows that dry or very dry conditions are now located in Northland, Auckland, northern Waikato, coastal Otago, and much of Southland. In addition, extremely dry conditions are located in parts of Auckland, coastal Southland and nearly all of Stewart Island, with meteorological drought near Tiwai Point and in northern Stewart Island.
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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