Hotspot Watch 5 February 2020

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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

Across the North Island, most locations continued to receive little rain during the past week (generally less than 5 mm), leading to additional drying of soils across much of the island. The driest soils, when compared to normal for this time of the year, are found across parts of Northland, Auckland, northern and western Waikato, interior Taranaki and eastern Bay of Plenty. For the second week in a row, there are no areas in the North Island where near normal or above normal soil moisture levels are found. In addition, the New Zealand Drought Index (NZDI) shows that the upper North Island (northern Waikato northbound) has widespread meteorological drought conditions. Areas of extreme meteorological drought have expanded in the past week and are now found in parts of the Far North, southern Northland, parts of Auckland, Great Barrier Island and northern Coromandel.

The current hotspots continued to grow in the past week. A very large hotspot covers all of Northland, Auckland, much of Waikato, and the Coromandel Peninsula. Additional hotspots are in place across Wairarapa, East Cape, parts of Hawke’s Bay, central Manawatu-Whanganui to Taranaki, and portions of Bay of Plenty.

For the South Island, heavy rain was observed in the west and south during the past week and led to significant, rapid soil moisture increases. Many places in the east saw slight soil moisture improvements as well. The largest increases in the past week occurred across Otago, northern Fiordland and southern Westland. The driest soils compared to normal for this time of the year continue to be found across Tasman and interior Buller District. Meanwhile, the wettest soils for this time of the year are found in coastal Otago, around Dunedin. 

Most of the hotspots in the east decreased during the past week. Current hotspots are now covering western Marlborough, Nelson and nearby parts of Tasman, and much of Hurunui District south to the Waimate District.

Outlook and Soil Moisture

In the North Island, mostly dry conditions will continue during next week and rainfall is expected to be below normal for the time of year. However, an onshore flow may bring a few more showers to eastern areas such as eastern Bay of Plenty, Gisborne and Hawke’s Bay and weekly rainfall totals may reach 10-15 mm. For the remainder of the island, weekly rainfall totals will be meagre, generally 5 mm or less.

With minimal rainfall expected in much of the North Island, widespread soil moisture decreases are expected to continue in the next week, especially in the west and north. Most existing hotspots are anticipated to intensify and expand.

In the South Island, the heavy rain will ease on Wednesday (5 February) but showers may linger especially in the interior on Thursday (6 February). A weakening front may bring rain and showers, mainly to the west and South on Saturday (8 Feb) but will be followed by several days of mostly dry conditions with high pressure overhead. Weekly rainfall totals may exceed 50 mm in parts of Fiordland, while the rest of coastal Southland and southern Otago may record up to 20 mm. Weekly rainfall totals in Fiordland may reach 50 mm. Up to 25 mm of total rainfall is possible in Southland, Otago and over higher terrain in Westland. However, for the remainder of the island, weekly rainfall totals will generally be 10 mm or less.

In the upcoming week, soil moisture levels are expected to decrease in the majority of the island; however, minor increases are possible in Fiordland, Southland, Otago and over higher terrain in Westland. Current hotspots in northern Canterbury, Nelson, and Tasman are expected to strengthen and expand 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)

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 2 February, the New Zealand Drought Index (NZDI) shows that the upper North Island (northern Waikato northbound) has widespread meteorological drought conditions. Areas of extreme meteorological drought have expanded in the past week and are now found in parts of the Far North, southern Northland, parts of Auckland, Great Barrier Island and northern Coromandel. Meteorological drought coverage is expected to increase in these regions due to dry weather in the next week.

 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.


Research subject: Climate