Hotspot Watch 18 March 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, rainfall during the past week was minimal for most locations, with amounts generally 5 mm or less. However, moderate rainfall amounts of 20 mm or more were observed in much of Hawke’s Bay, Gisborne, and eastern Northland. This resulted in slight soil moisture increases in the aforementioned areas, although the rest of the North Island generally saw slight soil moisture decreases. The driest soils across the North Island, when compared to normal for this time of the year, are generally scattered across most regions. Meanwhile, the least dry soils for this time of the year are located along the western coast of Waikato. The New Zealand Drought Index (NZDI) shows that severe meteorological drought currently encompasses most of Northland, Auckland, northern Waikato, western Bay of Plenty, East Cape, and small portions of interior Manawatu-Whanganui, with meteorological drought in place from Bay of Plenty through the Central Plateau (see NZDI map).   

Large portions of the North Island remain in official hotspot status, although scattered areas in eastern Northland, Bay of Plenty, western Waikato, Taranaki, and Kapiti Coast have fallen below official hotspot thresholds.

In the South Island during the past week, rainfall of 30 mm or more was observed in Fiordland and lower Westland, while meagre rainfall of generally less than 5 mm occurred in nearly all other locations. This rainfall distribution resulted in most areas seeing at least minor soil moisture decreases. The driest soils in the South Island compared to normal for this time of the year are located in coastal Hurunui, while the wettest soils for this time of the year are found in Clutha District. The New Zealand Drought Index (NZDI) shows that meteorological drought and severe meteorological drought is found across much of Marlborough (see NZDI map).   

Current hotspots in the South Island are found in Nelson and nearby parts of Tasman, northeastern Marlborough, and coastal Hurunui south to near Timaru.

Outlook and Soil Moisture

In the North Island, primarily dry weather is expected through Friday (20 March), although western areas may see a few showers producing up to 5 mm of rain. A slow-moving front between late Saturday and Monday will have the chance to produce 15-30 mm for many areas before high pressure brings a return to dry weather for the middle of next week.  

With rainfall amounts that could exceed 20 mm in many areas during the next week, soil moisture levels will have the chance to increase at least slightly across much of the North Island, with localised larger increases possible. This would result in at least a small decrease in hotspot coverage.

In the South Island, a front on Thursday (19 March) will bring moderate rainfall to the West Coast and small amounts to the lower South Island. Low pressure arriving on Saturday has a chance to bring significant rainfall to the West Coast, but amounts of only 10 mm or less are expected east of the Alps. A few light showers may bring small accumulations early next week before high pressure brings more tranquil conditions.

Substantial rainfall during the next week could bring moderate soil moisture increases to the West Coast, while minor increases will be possible in the lower South Island. However, with weekly rainfall amounts that may be less than 15 mm, eastern portions of the South Island are likely to see additional soil moisture decreases and strengthening of current hotspots. 

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 16 March, the New Zealand Drought Index (NZDI) map below shows that nearly all of the upper North Island is experiencing severe meteorological drought, along with East Cape and small portions of interior Manawatu-Whanganui, with meteorological drought in many other locations. In the South Island, meteorological drought and severe meteorological drought are found in much of Marlborough and far northern Canterbury.

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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Meteorologist/Forecaster
Research subject: Climate