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Hotspot Watch 18 November 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

In the North Island, moderate to heavy rain generally affected western areas during the past week, while much less rain was observed along the east coast. Amounts of 25-50 mm were common from Northland south to the Kapiti Coast, with some areas seeing more than 50 mm. In addition, parts of southern Waikato, Taranaki, and the Kapiti Coast received 100 mm or more during the past week. Conversely, rainfall amounts were meagre in the eastern North Island, where Gisborne, Hawke’s Bay, and Wairarapa generally received less than 5 mm. This resulted in moderate soil moisture increases across the upper and western North Island. Meanwhile, eastern areas saw soil moisture decreases, particularly in Wairarapa. The driest soils across the North Island, when compared to normal for this time of the year, are found in Wairarapa, while the wettest soils for this time of the year are found across coastal Gisborne.Currently, no official hotspots are located in the North Island, although coastal Wairarapa is nearing hotspot status. The New Zealand Drought Index (NZDI) map below shows that no dry conditions are located in the North Island as of 15 November. In the South Island, heavy rainfall was observed in western areas during the past week, with widespread amounts of 100 mm or more from Tasman south to Fiordland. There were even pockets of rainfall above 200 mm along the West Coast. Moderate rainfall was observed in Otago and Southland, with many areas seeing 30-50 mm or more. However, much lighter rainfall occurred from eastern Marlborough south to central Canterbury, where amounts were generally less than 10 mm. This resulted in moderate to large soil moisture increases in western and southern parts of the South Island, while at least small soil moisture decreases occurred from Marlborough to central Canterbury. The driest soils in the South Island, when compared to normal for this time of the year, are located in parts of central Canterbury and coastal Southland, while the wettest soils for this time of the year are found along the West Coast and in parts of interior Otago.Hotspots are currently in place in coastal Hurunui, Selwyn, and Ashburton districts, while a previous hotspot in Waimate District dissipated in the past week. The New Zealand Drought Index (NZDI) map below shows that no dry conditions are located in the South Island as of 15 November.

Outlook and Soil Moisture

In the North Island, an area of high pressure will be in control through at least Sunday morning (21 November), leading to dry weather for most locations over the next few days. From late Sunday into Monday, a weak passing low may bring rain to the lower half of the North Island, with showers possible farther north. Thereafter, mostly dry weather returns beginning on Tuesday, with perhaps a few more showers for the lower North Island on Wednesday or Thursday. Weekly rainfall totals could reach 15-30 mm across much of the southern half of the North Island, although confidence levels are a bit lower than usual due to the uncertainty of the rainfall. In the upper North Island, rainfall totals are likely to be lower, in the 5-15 mm range.

Due to the expected rainfall in the next week, soil moisture levels are likely to remain generally constant or decrease slightly across the southern half of the North Island. Farther north, at least small soil moisture decreases are more likely. There continues to be a small chance that a hotspot may form in Wairarapa during the next week, with no hotspots expected elsewhere. In the South Island, moderate to locally heavy rain will reach the West Coast by late Friday through Sunday. Some of this rain may also reach the lower South Island during the upcoming weekend (20-21 November). Generally drier weather will reach the South Island by early next week, but a few showers can’t be ruled out by Wednesday or Thursday (24-25 November). Weekly rainfall totals could exceed 100 mm in the lower West Coast, with up to 50 mm farther north. Amounts of 15-25 mm will be possible across Southland and Otago, while amounts of less than 20 mm are likely from central Canterbury to Marlborough.Due to the expected rainfall, small soil moisture decreases are likely across the eastern South Island during the next week. Elsewhere, little change is expected. The existing hotspots in coastal Hurunui, Selwyn, and Ashburton districts are likely to remain in place, while there is a small chance for a new hotspot to form in eastern Marlborough during the next week.

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 15 November, the New Zealand Drought Index (NZDI) map below shows that no dry conditions are located across the country at this time. 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.