Hotspot Watch 23 November 2022

Source: NIWA – National Institute of Water and Atmospheric Research

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 was observed in most locations during the past week. In much of Northland, Auckland, and the lower North Island, rainfall totals generally exceeded 50 mm, while much of the western and central North Island received 100 mm or more, with isolated areas of 150 mm. However, rainfall amounts were lower near Cape Reinga along with coastal Gisborne and Hawke’s Bay, where amounts were less than 25 mm. This resulted in significant soil moisture increases for nearly all of the island, although small soil moisture decreases were observed in coastal Gisborne and Hawke’s Bay. The driest soils across the North Island, when compared to normal for this time of the year, are found in southern Manawatū-Whanganui, while the wettest soils for this time of the year are found in eastern Northland, Auckland, and coastal Bay of Plenty.

No hotspots are currently located in the North Island.

In the South Island, many regions received moderate to heavy rain during the past week. Amounts of 75-150 mm occurred in much of Tasman and the West Coast, with 50 mm or more observed in most other regions. However, amounts of less than 25 mm were observed in eastern Marlborough, parts of Otago and Southland. This resulted in large soil moisture increases across the upper and central South Island, with minor increases observed in interior Southland. Conversely, some soil moisture decreases occurred in coastal Otago and Southland, as well as in Fiordland. The driest soils in the South Island, when compared to normal for this time of the year, are located in northern Fiordland and the lower West Coast, while the wettest soils for this time of the year are found in Tasman, Marlborough Sounds, parts of coastal Canterbury, and interior Southland.

In the past week, moderate to heavy rainfall caused the hotspot located in central Canterbury to dissipate. Therefore, no hotspots are currently located in the South Island. As of 21 November, the New Zealand Drought Index (NZDI) map below shows that dry and very dry conditions are located in portions of central Canterbury, with dry conditions also emerging near Nelson.

Outlook and Soil Moisture

In the North Island, unsettled weather is likely to continue over the next several days. On Thursday (24 November), periods of moderate to heavy rain are likely, especially in western and central regions. Conditions may become somewhat drier on Friday and Saturday before another round of at least moderate rain on Sunday (27 November). High pressure may deliver a period of more tranquil weather early next week before additional showers or rain occur on Wednesday (30 November). Weekly rainfall totals may again be substantial in western and central portions of the North Island, where amounts may reach 75-110 mm. 40-60 mm may be found in Northland and Auckland, although lower amounts of 25 mm or less may occur in coastal Gisborne and Hawke’s Bay.

Due to the expected rainfall in the next week, additional soil moisture increases are likely in large portions of the North Island, although slight decreases may occur along the east coast. No hotspots are expected to form in the North Island in the next week.

In the South Island, a round of heavy rain will impact the West Coast later today and Thursday morning (23-24 November), with some lighter rain for Otago and Southland. Friday and Saturday may feature some showers or lighter rain in the western and southern portions of the island. High pressure is likely to bring quieter weather on Sunday and Monday (27-28 November), followed by another chance for rain in the West Coast on Tuesday. Weekly rainfall totals may reach 75-125 mm in much of the West Coast and Tasman, with 20-40 mm possible for most of the rest of the South Island. However, coastal Canterbury may receive less than 20 mm in the next week.

Due to the expected rainfall in the next week, soil moisture levels will likely increase at least slightly in the West Coast, with generally little change expected elsewhere. However, small soil moisture decreases may occur in coastal Canterbury. There is a small chance that the previous hotspot in central Canterbury may begin to re-emerge in the next week. No other hotspots are expected to form in the South 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.

MIL OSI