Hotspot Watch 17 March 2023

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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, the past week (through 16 March) saw rainfall totals of generally 30-70 mm in Taranaki, southern Waikato, northern Manawatū-Whanganui, and the higher terrain north of Wellington City. However, nearly all other locations in the North Island received rainfall totals of 25 mm or less. This resulted in most of the North Island seeing small to moderate soil moisture decreases in the past week, although small increases were observed in the west where the heavier rainfall occurred. The driest soils across the North Island, when compared to normal for this time of the year, are found in western Northland, Horowhenua, and Kapiti Coast, while the wettest soils for this time of the year are found in Hawke’s Bay and Wairarapa.

No hotspots are currently located in the North Island.

In the South Island, moderate to heavy rain was observed in the past week (through 16 March) across Tasman, the West Coast, and Fiordland, where amounts of 50-120 mm were widespread. Amounts of 25-40 mm were observed in Southland and Stewart Island, while the rest of the South Island generally received less than 25 mm. This resulted in small to moderate soil moisture increases along the West Coast and lower South Island, while decreases occurred in Canterbury, Marlborough, and Nelson. The driest soils in the South Island, when compared to normal for this time of the year, are located in Buller District, Southland, and Stewart Island, while the wettest soils for this time of the year are found in northern Canterbury.

Most previous hotspots dissipated in the past week, although one remains in a small part of interior Southland. In addition, a new hotspot has formed in the greater Nelson region. As of 14 March, the New Zealand Drought Index (NZDI) map below shows that dry to extremely dry conditions are currently located in Buller District, with dry to very dry conditions in southern Tasman, southern Canterbury, and much of Otago, Southland, and Stewart Island.

Outlook and Soil Moisture

In the North Island, showers and thunderstorms (which may be locally heavy) are expected to continue today and tonight (17 March), with a drying trend on Saturday morning (18 March). After generally dry weather on Sunday and Monday, a front will bring showers on Tuesday (21 March), followed by a few more days of dry weather. Weekly rainfall totals of 30-50 mm are expected in much of the western and central North Island, with generally 25 mm or less in Northland and the east coast.

Due to the expected rainfall in the next week, soil moisture levels may increase slightly in the western and central North Island, while at least minor decreases may occur in Northland and the east coast. There is a small chance that a hotspot may form in Horowhenua in the next week.

In the South Island, rain and thunderstorms will continue moving north today and tonight (17 March), with generally dry weather in place for the weekend. However, moderate to heavy rain will again impact most of the South Island on Monday and Tuesday (20-21 March), before dry weather returns for the mid-week period. Weekly rainfall totals could again exceed 100 mm for most of the West Coast to Fiordland, with much of the eastern and lower South Island receiving 30-50 mm.

Due to the expected rainfall in the next week, soil moisture levels may increase across a majority of the South Island, including the driest areas of Buller District and Southland. The current hotspot in Southland may fully dissipate in the next week, along with the new hotspot in the Nelson area.

For more information on the potential for dryness in the weeks to come, please check out the drought forecasting tool, a collaboration between NIWA and the Ministry for Primary Industries.

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 anomalies over the past two weeks.

New Zealand Drought Index (NZDI)

As of 14 March, the New Zealand Drought Index (NZDI) map below shows that dry to extremely dry conditions are currently located in Buller District, with dry to very dry conditions in southern Tasman, southern Canterbury, and much of Otago, Southland, and Stewart Island. 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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