Hotspot Watch 1 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 generally decreased slightly during the past week. The most significant decreases were observed from Northland to Bay of Plenty, with smaller decreases in most other locations. The driest soils across the North Island compared to normal for this time of the year are found in southernmost Wairarapa, while the wettest soils for this time of the year are located near Napier and Hastings.

No hotspots are currently in place across the North Island, although soils in southern Wairarapa are approaching hotspot status.

In the South Island, soil moisture levels decreased slightly in most locations during the past week, although slight increases were observed in coastal Otago and Southland. The driest soils across the South Island compared to normal for this time of the year are found across far southern Canterbury and interior Otago, while the wettest soils for this time of the year are found in and around Dunedin.

The only currently active hotspot in the South Island is located in the Queenstown-Lakes District, although other locations in interior Otago and far southern Canterbury are nearing hotspot status.

Outlook and soil moisture

In the North Island, strong high pressure centred in the Tasman Sea will result in little if any rainfall over the next several days. In fact, many locations across the North Island may see no rainfall through to at least Wednesday (6 November). In addition, unusually warm temperatures are expected from Sunday through to early next week, which will enhance soil moisture loss. It is possible that a front will cross the North Island on Thursday to Friday (7-8 November), producing 10-20 mm in northern areas, but likely 10 mm or less in southern portions of the North Island. Overall, rainfall during the next week will generally be below to well below normal in the North Island, with many locations receiving no more than 10 mm.

Due to the anticipated below normal rainfall and above average temperatures in the next week, soil moisture levels are expected to decrease farther in many locations across the North Island. These decreases may lead to the formation of a hotspot in southern Wairarapa.

In the South Island, strong high pressure will bring dry weather through to Monday (4 November). In addition, unusually high temperatures are expected over the same time period, with eastern areas likely to see temperatures reaching or exceeding 30°C this weekend. Such temperatures will enhance soil moisture loss in the coming few days. A front will affect the West Coast on Tuesday and Wednesday (5-6 November), bringing 30-60 mm west of the Southern Alps, and generally 5-10 mm east of the mountains. Another front on Friday (8 November) could deliver an additional 10-20 mm to the West Coast, but little if any rain east of the divide.

Due to anticipated rainfall amounts in the coming week, soil moisture levels are likely to increase along the West Coast, while levels may remain fairly constant across Southland, Tasman, and Nelson. However, soil moisture levels are likely to decrease farther across the eastern half of the South Island. Thus, hotspot coverage may increase across interior Otago and far southern Canterbury, while a new hotspot may be close to emerging 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.

As of 29 October, the New Zealand Drought Index (NZDI) map below shows that drier than normal conditions are located across far southern Canterbury, a portion of interior Otago, and coastal Wairarapa. These areas may expand in the coming week as drier than normal conditions are anticipated in these locations. 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.