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Source: NIWA – National Institute of Water and Atmospheric Research

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

Meagre rainfall was observed across much of the northern, western and lower North Island during the past week, with most locations receiving 10 mm or less. However, eastern Bay of Plenty and a couple of localised areas in Hawke’s Bay received 30 mm or more. This resulted in minor to moderate soil moisture decreases across nearly all of the North Island, although slight increases were observed in northern Waikato. The driest soils across the North Island, when compared to normal for this time of the year, are found in the Far North and Aupouri Peninsula. Meanwhile, the wettest soils for this time of the year for the North Island are located in coastal Bay of Plenty and Horowhenua.

Hotspots are currently in place in much of Northland, northern Auckland, the northern Coromandel Peninsula, a small portion of East Cape, and coastal 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, with widespread dry to extremely dry soils in the rest of Northland, Auckland, and northern Waikato.

Minimal rainfall was observed across all of the South Island during the past week, with nearly all locations receiving less than 10 mm. This resulted in moderate to large soil moisture decreases throughout the South Island. The driest soils in the South Island compared to normal for this time of the year are located in southeastern Southland, coastal Hurunui District, and Buller District, while the wettest soils for this time of the year for the South Island are found in southern Canterbury and much of eastern Otago.

During the past week a new, small hotspot has formed in coastal Hurunui District.

Outlook and Soil Moisture

Saturday will be a dry day across the North Island, before a front arrives on Sunday, bringing showers and thunderstorms, particularly to the western North Island. After a mostly dry day on Monday, another front will arrive Tuesday night and Wednesday (19-20 January) with periods of rain and isolated thunderstorms. Western portions of the North Island could receive up to 20 mm from this event, although amounts will be much lower along the east coast. Mostly dry conditions then return on Thursday and Friday. Weekly rainfall totals may reach 40-50 mm in the western North Island, with totals of 20 mm or less for much of the upper North Island. Meanwhile, the east coast is only expected to receive 10 mm or less.

Due to the expected rainfall in the next week, western portions of the North Island may see at least small soil moisture increases, while the upper North Island may observe small decreases. However, moderate to large soil moisture decreases will be possible along the east coast. This will likely result in all current North Island hotspots strengthening at least a small amount, while the hotspot in coastal Wairarapa may strengthen substantially.

After a dry Saturday, a front will bring moderate rainfall amounts to the West Coast and lower South Island on Sunday. The West Coast will remain at least showery on Monday before a strong front brings heavy rainfall to the western South Island on Tuesday (19 January). However, little of this rain is expected to reach the eastern South Island. After scattered showers on Wednesday, late next week will be mostly dry as high pressure arrives. Weekly rainfall totals may exceed 150 mm along much of the West Coast, with 40-50 mm in Southland and Otago. However, amounts will be much lower from Marlborough to Canterbury, where weekly totals may only reach 20 mm or so.

Due to expected rainfall in the next week, soil moisture increases are likely in the west and south of the South Island. However, at least small soil moisture decreases will be possible in Marlborough and Canterbury. The current hotspot in Hurunui District may strengthen at least slightly during the next week, while a new hotspot may emerge in Marlborough.

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.

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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 13 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, with widespread dry to extremely dry soils in the rest of Northland, Auckland, and northern Waikato. 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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MIL OSI