Hotspot Watch 10 February 2022

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, most locations from the Coromandel Peninsula and south received substantial rainfall of at least 50-75 mm in the past week, with some areas receiving more than 100 mm. In addition, parts of the Coromandel, east coast, and Taranaki saw more than 150 mm in the past week. Amounts were a bit more muted in Northland, Auckland, northern Waikato, and Wellington, where amounts were generally 30-70 mm. This rainfall resulted in large soil moisture increases across the entire North Island. The driest soils across the North Island, when compared to normal for this time of the year, are found in western Northland, while the wettest soils for this time of the year are found in eastern Bay of Plenty, Gisborne, and western Waikato south to Wellington and Wairarapa.  

Due to the last week’s heavy rainfall, hotspot conditions in the North Island have almost completely dissipated. The one exception is a small portion of western Northland where a small hotspot still exists. As of 7 February, the New Zealand Drought Index (NZDI) map below shows that dry conditions are located across nearly all of the North Island. In addition, very dry or extremely dry conditions are located in Northland, Auckland, Waikato, and the Central Plateau. Pockets of meteorological drought are located in western Northland, a small portion of western Auckland, and western and southern Waikato. However, these conditions will likely ease somewhat as the NZDI incorporates the recent heavy rainfall.

In the South Island, very heavy rainfall impacted much of the West Coast, Tasman, and Marlborough Sounds, with many locations receiving several hundred millimetres of rain. Meanwhile, 30-70 mm was observed across much of the eastern South Island. However, Southland and Stewart Island generally received minimal rainfall. This resulted in moderate to large soil moisture increases in the upper and eastern South Island, while Southland saw additional soil moisture decreases. The driest soils in the South Island, when compared to normal for this time of the year, are located in western Southland and Stewart Island, while the wettest soils for this time of the year are found in far northern Canterbury and Marlborough Sounds.

Much like the North Island, the week’s heavy rainfall caused most South Island hotspots to dissipate. However, hotspots remain in place in much of coastal Southland and Stewart Island. As of 7 February, the New Zealand Drought Index (NZDI) map below shows that dry conditions are now located across the upper and lower South Island. In addition, very dry or extremely dry conditions are located in the interior upper South Island, interior Otago, coastal Southland and Stewart Island. However, these conditions may ease somewhat as the NZDI incorporates the recent rainfall.

Outlook and Soil Moisture

In the North Island, heavy rainfall ahead of Tropical Cyclone Dovi will begin to impact areas from Taranaki south to Wellington on Friday afternoon and evening. Very heavy rain may continue in the lower North Island on Saturday and Sunday as Dovi approaches, with the chance for flooding to occur. By Monday and Tuesday (14-15 February), much lighter showers will continue to impact parts of the North Island. By the middle of the week, high pressure will arrive with generally dry weather. Weekly rainfall amounts could be very substantial from Taranaki south to Wellington-Wairarapa, with widespread amounts above 100 mm, and some locations possibly receiving 200 mm or more. Rainfall totals will gradually decrease farther north, with amounts of less than 25 mm possible in Northland and Auckland.

Due to the expected rainfall in the next week, soil moisture levels could increase substantially in the lower half of the North Island. However, some soil moisture decreases may occur in Auckland and Northland, where the current small hotspot may expand.

In the South Island, after a brief lull in the rain on Friday, heavy rain associated with Tropical Cyclone Dovi will impact northern areas on Saturday and Sunday (12-13 February), along with the possibility of flooding. This could also include northern Canterbury. By Monday, high pressure will return, providing generally dry weather through the end of next week. Weekly rainfall totals will be substantial in the upper South Island, with widespread amounts of 100-150 mm, and more localised amounts of more than 200 mm. However, the lower South Island will again see generally light rainfall of less than 20 mm.

Due to the expected rainfall in the next week, soil moisture levels will increase substantially in the upper South Island. However, small additional decreases may occur in the Deep South. This could result in the expansion of the hotspots currently located in Southland and Stewart Island.

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 at https://www.niwa.co.nz/climate/nz-drought-monitor/droughtindicatormaps)

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 7 February, the New Zealand Drought Index (NZDI) map below shows that dry conditions are now located across nearly all of the North Island, along with the upper and lower South Island. In addition, very dry or extremely dry conditions are located in Northland, Auckland, Waikato, the Central Plateau, the interior upper South Island, interior Otago, coastal Southland and Stewart Island. Pockets of meteorological drought are located in western Northland, a small portion of western Auckland, and western and southern 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.