Hotspot Watch 24 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, substantial rainfall of 30-70 mm occurred in most western and central regions in the past week, including Taranaki, Waikato, Manawatū-Whanganui, and Bay of Plenty. However, much lighter rainfall was observed in the upper North Island and east coast, where amounts were generally less than 20 mm. This resulted in many areas in the North Island seeing small to moderate soil moisture increases in the past week, especially in western and central regions. The driest soils across the North Island, when compared to normal for this time of the year, are found in southern Northland, while the wettest soils for this time of the year are found in the Coromandel, Bay of Plenty, and parts of the east coast.

No hotspots are currently located in the North Island.

In the South Island, heavy rain once again occurred along the West Coast, with amounts of 75-150 mm, and some high terrain receiving 200 mm or more. Amounts of 30-70 mm were widespread in the rest of the West Coast, Otago, Southland, and Stewart Island, while amounts of 25 mm or less occurred in Nelson, Marlborough, and most of Canterbury. This resulted in moderate to substantial soil moisture increases in the West Coast and lower South Island, while minor decreases occurred in northern Canterbury. The driest soils in the South Island, when compared to normal for this time of the year, are located in Nelson and a small portion of eastern Otago, while the wettest soils for this time of the year are found in central Canterbury.

Due to the past week’s rainfall, all previous hotspots in the South Island have now dissipated. As of 21 March, the New Zealand Drought Index (NZDI) map below shows that dry to very dry conditions are currently located in Buller District, southern Otago, and eastern Southland.

Outlook and soil moisture

In the North Island, dry weather is expected on Saturday, but a weak front will slowly move up the island on Sunday and Monday (26-27 March), bringing scattered showers and isolated thunderstorms. A few more showers may occur on Wednesday (29 March) as a cold front moves through. While high pressure is expected to bring generally dry weather thereafter, Northland may continue to see showers on Wednesday and Thursday. Weekly rainfall totals of 15-30 mm are likely in the western and central North Island, with 15 mm or less most likely in Bay of Plenty and the east coast. Rainfall totals in Northland could be somewhat higher, but the confidence levels at this time are low.

Due to the expected rainfall in the next week, soil moisture levels will likely decrease at least a small amount in Bay of Plenty and the east coast, with generally little change expected elsewhere. No hotspots are expected to form in the North Island in the next week.

In the South Island, moderate to heavy rain will move up the West Coast on Saturday (25 March), followed by generally dry weather through Monday night. On Tuesday (28 March), a potent cold front will move up the South Island, bringing a period of moderate to heavy rain and high-elevation snow. High pressure then returns beginning on Wednesday (29 March). Weekly rainfall totals of 50-100 mm will be possible for the West Coast and Fiordland, while the remainder of the South Island is most likely to receive 15-25 mm.

Due to the expected rainfall in the next week, soil moisture levels along the West Coast may increase slightly, while minor decreases could occur elsewhere. There is a small chance that the previous hotspot in the Nelson area could re-emerge in the next week.

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.

Soil moisture anomaly (mm) at 9am on 16/03/2023 [NIWA]

Soil moisture anomaly (mm) at 9am on 23/03/2023 [NIWA]

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

As of 21 March, the New Zealand Drought Index (NZDI) map below shows that dry to very dry conditions are currently located in Buller District, southern Otago, and eastern Southland. 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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