Hotspot Watch 16 December 2022

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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, most northern and central regions received moderate to heavy rainfall during the past week. Rain amounts of 30-60 mm were widespread in these areas, with 75 mm or more observed in the Central Plateau, Bay of Plenty, the Coromandel, and East Cape. In fact, a few areas in these regions received more than 150 mm. However, rainfall was much lighter in the Far North, east coast, and lower North Island, with amounts generally 20 mm or less. This resulted in significant soil moisture increases from southern Northland to the Central Plateau, along with East Cape and Taranaki. However, some soil moisture decreases were observed in the Far North and east coast. The driest soils across the North Island, when compared to normal for this time of the year, are found in coastal Wairarapa, while the wettest soils for this time of the year are found in southern Northland, Auckland, the Coromandel, and western Bay of Plenty.

New hotspots have formed in the past week in Māhia Peninsula and coastal Wairarapa.

In the South Island, moderate to heavy rain occurred in Tasman and the West Coast, where rainfall totals generally reached 50-100 mm. Totals of 30-50 mm occurred in Marlborough Sounds and small parts of interior Otago and Southland. Elsewhere, weekly rainfall totals were generally less than 20 mm. This generally resulted in small soil moisture increases in Tasman, the West Coast, and Marlborough Sounds, but moderate soil moisture decreases elsewhere. The driest soils in the South Island, when compared to normal for this time of the year, are located in the lower West Coast, while the wettest soils for this time of the year are found in Tasman and lower Fiordland.

A small hotspot has emerged in the past week in Banks Peninsula.

Outlook and Soil Moisture

In the North Island, rain is likely to develop in western and central regions on Saturday (17 December), with afternoon showers and thunderstorms possible on Sunday. Next week will generally be unsettled, with afternoon showers and thunderstorms expected to be a nearly daily occurrence, especially in central and eastern portions of the North Island. However, at least isolated showers and thunderstorms will be possible in the rest of the island as well. Weekly rainfall totals will be highly dependent on where exactly the heaviest showers and thunderstorms occur in the coming days. However, it does appear likely that the Central Plateau and east coast could see the highest amounts of 50-100 mm, with localised totals of 20-50 mm elsewhere.

Due to the expected rainfall in the next week, central and eastern portions of the North Island will likely see additional soil moisture increases, although small decreases may occur in the top of the island. The newly-formed hotspots along the east coast may dissipate during the next week.

In the South Island, moderate rainfall is occurring today (16 December) in Otago and Southland, and will likely occur again on Saturday. On Sunday, (18 December), the upper South Island may see widespread showers and some thunderstorms. Beginning on Monday, most afternoons will feature scattered showers and isolated thunderstorms, mostly in the Alps and West Coast. Weekly rainfall totals of 30-50 mm are likely across most of the South Island, with areas of the West Coast potentially seeing slightly higher amounts, and coastal Canterbury perhaps slightly less.

Due to the expected rainfall in the next week, soil moisture levels may increase at least slightly for most areas, although the Canterbury coast may see slight additional decreases. The current hotspot in Banks Peninsula is likely to remain in place over the next week, and may strengthen slightly.

Pictured above: Soil Moisture Anomaly Maps, relative to this time of year. The maps show soil moisture anomaly for the past two weeks.

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.

New Zealand Drought Index (NZDI) 

As of 14 December, the New Zealand Drought Index (NZDI) map below shows that no dry conditions are currently located across the country. 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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