Hotspot Watch 22 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

Moderate-to-heavy rainfall was observed across the western North Island during the past week, with areas in southern Auckland, Waikato, Taranaki and northern Manawatu-Whanganui receiving about 30-60mm. Light rainfall was observed in the upper North Island, generally less than 25mm. However, eastern parts of the North Island received meagre rainfall, with most locations receiving 10mm or less. This resulted in minor-to-moderate soil moisture increases in the western North Island and minor-to-moderate soil moisture decreases in eastern parts of the North Island. Little change was observed elsewhere. The driest soils across the North Island, when compared to normal for this time of the year, are found in the Far North, Coromandel and small parts of the east coast. Meanwhile, the wettest soils for this time of year for the North Island are in New Plymouth District and southern Manawatu-Whanganui.

Hotspots are currently in place in much of Northland, parts of Auckland, northern Waikato and a small portion of East Cape, while hotspots have strengthened in eastern Wairarapa and expanded to include the eastern Tararua District and coastal Hawke’s Bay. 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 and northern Waikato.

Widespread heavy rain fell across the west and south of the South Island during the past week, with parts of the West Coast and western Southland receiving over 150mm, and parts of Otago and eastern Southland receiving over 50mm. However, rainfall was generally below 20mm across the Canterbury and Marlborough regions. This resulted in large soil moisture increases in the west and south of the South Island, with small soil moisture increases in parts of the Canterbury and Marlborough regions. 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 in southern Canterbury and interior Otago.

A small hotspot remains in place in coastal Hurunui District.

Outlook and Soil Moisture

Saturday will be a mostly dry day across the North Island before a band of showers brings 5mm or less to western areas on Sunday. Dry weather is expected nearly everywhere from Monday to Wednesday (25-27 January), while hot temperatures will contribute to increased evapotranspiration. Rain and showers then look to arrive on Thursday as a front moves across the North Island. Weekly rainfall totals look to generally below 20mm for most of the North Island, with some areas of the north and east receiving 10mm or less.

Due to the light rain in the next week and the hot temperatures expected on Tuesday and Wednesday, most of the North Island will see small-to-moderate soil moisture 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, particularly those in the east.

A band of rain will bring 20-50mm to the West Coast and western Southland on Saturday. The west of the South Island will remain at least showery from Sunday to Tuesday (24 – 26 January), while the eastern South Island remains dry. On Tuesday, eastern areas of the South Island will become very hot, which will contribute to increased evapotranspiration. A front will bring a band of rain and relief from hot temperatures on Wednesday and early Thursday, with the heaviest rain expected across the west of the South Island. Weekly rain totals may exceed 75-100mm for parts of the West Coast and western Southland, with 20-30mm in parts of eastern Southland and Otago. The remainder of the South Island looks to see less than 20mm of rain.

Due to the expected rainfall in the next week, small-to-moderate soil moisture increases are likely in the west and south of the South Island. However, additional soil moisture decreases are likely in Canterbury and Marlborough. This will at least slightly strengthen the current hotspot in the Hurunui District, 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 19 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 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.