Hotspot Watch 28 January 2021

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

Minimal rainfall was observed across nearly all of the North Island during the past week, with most locations receiving 5 mm or less, and parts of the east coast receiving no rainfall at all. Meanwhile, parts of western Waikato, interior Taranaki, and northern Manawatu-Whanganui received 10-25 mm. This resulted in small-to-moderate soil moisture decreases across nearly all of the North Island during the past week. The driest soils across the North Island, when compared to normal for this time of the year, are found in much of Northland, northern and western Waikato, and parts of the east coast. Meanwhile, the wettest soils for this time of year for the North Island are located in southern Manawatu-Whanganui.
Hotspots are currently in place in most of Northland, parts of Auckland, northern Waikato, western Bay of Plenty, and most of the east coast from East Cape to Wairarapa. The New Zealand Drought Index (NZDI) map below shows that meteorological drought and severe meteorological drought are in place in the northern half of the Far North District. Widespread dry-to-extremely dry soils are in place in the remainder of Northland, Auckland, northern Waikato, western Bay of Plenty, and East Cape. Substantial rainfall occurred in the past week across the West Coast and Fiordland with widespread amounts of 75 mm or more. Much of Southland received 15-30 mm, but the remainder of the South Island received only minimal rainfall. In addition, very hot temperatures this week in the eastern South Island resulted in increased evaporation rates. This led to small-to-moderate soil moisture decreases across the entire South Island. The driest soils in the South Island compared to normal for this time of year are located in coastal Hurunui District, while the wettest soils for this time of the year for the South Island are found from southern Canterbury to central Southland.
A small hotspot remains in place in coastal Hurunui District, while new hotspots have formed in Banks Peninsula, Nelson, and nearby parts of Tasman.

Outlook and Soil Moisture

A front moving up the North Island today and tonight (28-29 January) will bring 5 mm or less to southern and eastern areas, with little if any rainfall elsewhere. A southeasterly wind flow will continue to produce light to moderate showers on Friday and Saturday along the east coast, with the remainder of the North Island remaining dry. High pressure will be located overhead during much of next week, resulting in widespread dry weather. Weekly rainfall totals may exceed 20 mm in parts of Hawke’s Bay and Gisborne, with 10 mm or less for the remainder of the eastern and lower North Island. However, the upper North Island may receive little if any rainfall during the next week.
With the exception of parts of Hawke’s Bay and Gisborne, where soil moisture changes may be minimal, moderate to even large soil moisture decreases are likely during the next week across a majority of the North Island. This will likely result in most current hotspots strengthening and expanding, particularly those in the upper North Island. Showers and isolated thunderstorms will occur in the eastern South Island today and tonight (28-29 January), with localised amounts that could reach 15 mm. However, on Friday, high pressure will arrive, bringing dry weather to the South Island for about the next week or so. Weekly rainfall totals of 10-15 mm will be observed in the eastern and southern South Island, but only minimal rainfall is expected elsewhere.
Due to the expected rainfall in the next week, at least moderate soil moisture decreases are likely across a majority of the South Island. This will likely strengthen and expand the current South Island hotspots, while new hotspots may emerge in Marlborough and central Canterbury.

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 25 January, the New Zealand Drought Index (NZDI) map below shows that meteorological drought and severe meteorological drought are in place in the northern half of the Far North District. Widespread dry-to-extremely dry soils are in place in the remainder of Northland, Auckland, northern Waikato, western Bay of Plenty, and East Cape. 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.