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, substantial rainfall was observed for most northern and central regions in the past week. Amounts of 30-60 mm were common across southern Northland, Auckland, Waikato, Bay of Plenty, Taranaki, and interior Manawatū-Whanganui. However, it was a much drier week in the Far North, Gisborne, Hawke’s Bay, and Wairarapa, where weekly rainfall totals were generally less than 20 mm. This resulted in a notable increase in soil moisture levels across most of the upper and central North Island, while small decreases were observed along the east coast. The driest soils across the North Island, when compared to normal for this time of the year, are found in the eastern Coromandel Peninsula and southern Manawatū-Whanganui, while the wettest soils for this time of the year are found in eastern Northland, Auckland, northern Waikato, and parts of coastal Gisborne and Hawke’s Bay.
No hotspots are currently located in the North Island.
In the South Island, heavy rain impacted the West Coast during the past week, with widespread amounts greater than 100 mm, and some areas receiving more than 200 mm. Amounts of 75 mm or more were also observed in Marlborough Sounds and Tasman, with 30 mm or more found in interior Otago and Southland. Lesser amounts were observed in eastern Marlborough and Canterbury, with rainfall totals of 10-25 mm. This resulted in soil moisture increases in Marlborough Sounds, Tasman, West Coast, Otago, and Southland, with small decreases along the east coast. The driest soils in the South Island, when compared to normal for this time of the year, are located in Nelson, central Canterbury, and the lower West Coast, while the wettest soils for this time of the year are found in interior Otago and parts of Southland.
No hotspots are currently located in the South Island. As of 29 October, the New Zealand Drought Index (NZDI) map below shows that dry conditions are located in southwest Fiordland.
Outlook and Soil Moisture
In the North Island, a front will move through on Thursday (3 November), bringing up to 10 mm to most northern and central regions. Thereafter, a large area of high pressure moving overhead will bring mostly dry weather to the North Island through early next week. However, a few regions may see spotty light showers from Friday through the weekend with minimal accumulations. Weekly rainfall totals will likely be 15 mm or less for most of the North Island, with isolated pockets up to 20 mm for locations that see heavier showers.
Due to the expected rainfall in the next week, small soil moisture decreases are likely across a majority of the North Island. However, hotspots are unlikely to form.
In the South Island, extremely heavy rain will impact the West Coast from Wednesday through midday Thursday (2-3 November). Rainfall totals of more than 300 mm will be possible during this time, bringing the chance for flooding and slips to the West Coast. Meanwhile, amounts nearing 100 mm will be possible in Tasman and Fiordland, with 20-40 mm in Southland and interior Otago. After much lighter rainfall in the West Coast on Friday (4 November), high pressure will bring generally dry weather through early next week. Outside of the western South Island where the extreme rainfall will occur, weekly rainfall totals may reach 20-40 mm in Nelson, Otago, and Southland, with generally less than 20 mm in coastal Marlborough and Canterbury.
Due to the expected rainfall in the next week, soil moisture levels will increase dramatically in the West Coast, with smaller increases in Nelson and the lower South Island. However, further soil moisture decreases may occur along the east coast. There is a small chance that a hotspot may form in central Canterbury in the next week.
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 this time of year. The maps show soil moisture anomaly for the past two weeks.
New Zealand Drought Index (NZDI)
As of 29 October, the New Zealand Drought Index (NZDI) map below shows that dry conditions are located in southwest Fiordland. 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.
