Hotspot Watch 28 November 2019

A weekly update describing soil moisture across the country to help assess whether severely to extremely dry conditions are occurring or imminent. Regions experiencing these soil moisture deficits are deemed “hotspots”. Persistent hotspot regions have the potential to develop into drought.

Facts: Soil Moisture

Across the North Island, soil moisture levels decreased island-wide during the past week due to meagre rainfall and above average temperatures. The most significant decreases occurred across the upper North Island including Waikato, Auckland, and Northland. The driest soils compared to normal for this time of the year are found in coastal Wairarapa, where the drying trend continued during the past week. The wettest soils for this time of the year are located in parts of inland Gisborne, western portions of Taranaki, Manawatu-Wanganui and Wellington, where soil moisture levels are near normal for this time of year.
During the past week, several hotspots emerged in eastern coastal Wellington-Wairarapa northbound to coastal Central Hawke’s Bay District, along with isolated hotspots in the Hauraki District in the Coromandel, western coastal Auckland, and in the Far north. In the South Island, soil moisture levels decreased island-wide during the past week as well. Significant soil moisture decreases were observed in many places, including Marlborough, Nelson and Tasman, along with Westland. The driest soils compared to normal for this time of the year are found in eastern Marlborough and interior southern Canterbury. The wettest soils for this time of the year continues to be found near Dunedin.

During the past week, a hotspot emerged in eastern coastal Marlborough.

Outlook and Soil Moisture

In the North Island, scattered showers are expected in central and northern regions on Friday and Saturday (29-30 November) with rainfall totals up to 10-20 mm possible around the Central Plateau but likely below 10 mm elsewhere. Isolated to scattered showers are set to continue for the aforementioned regions through to early next week. On Wednesday (4 December) an incoming front may bring more widespread rain to big parts of the island with additional rainfall of 20-30 mm possible. Weekly rainfall totals may exceed 50 mm in central North Island also including Taranaki and about the ranges in western Wellington Region, 10-20 mm in the upper North Island, while coastal Gisborne southbound to coastal Wairarapa may see rainfall totals below 10 mm. In addition, unusually warm temperatures will continue over the upcoming week.
Due to the anticipated rainfall in the next week, soil moisture levels are expected to increase in central portions of the island, slightly increase or remain constant in the north, while slightly decrease or remain constant in costal eastern areas. The hotspots in coastal Wellington-Wairarapa northbound to coastal Central Hawke’s Bay District may expand in the upcoming week. In the South Island, isolated to scattered showers are expected Thursday-Saturday (28-30 November). A series of fronts will bring moderate to heavy rain to western areas on Sunday-Wednesday (1-4 December). Total weekly rainfall in Fiordland, Westland, and Tasman may exceed 200-300 mm, and rainfall totals in the interior may exceed 50 mm. The Alps will shield the east coast from most of the rain with weekly rainfall totals below 20 mm expected.
Soil moisture levels are likely to increase in western areas, in the interior and in the upper South Island (excluding eastern Marlborough) during the next week. Eastern coastal areas may see soil moisture decreases due to lighter rainfall amounts.

The current hotspot in eastern coastal Marlborough may expand over 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 )

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.

As of 25 November, the New Zealand Drought Index (NZDI) map below shows that drier than normal conditions are located across coastal Wairarapa and in the Aupouri Peninsula. Please note: some hotspots in the text above may not correspond with the NZDI map, mainly 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 the instantaneous status maps of soil moisture anomaly.