Hotspot Watch 26 March 2020

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

Across the North Island, substantial rainfall greater than 30 mm was observed in many western areas along with East Cape and small parts of northern Wellington during the past week. Moderate rainfall was observed in many other areas as well. This rainfall led to moderate to large soil moisture increases across much of the upper North Island, the western coast, East Cape, and Gisborne. Conversely, soil moisture levels remained largely unchanged across Manawatu-Whanganui, much of Wellington, and Wairarapa. The driest soils across the North Island, when compared to normal for this time of the year, are located in the Coromandel Peninsula, Wairarapa, and parts of Manawatu-Whanganui. Meanwhile, the wettest soils for this time of the year are located near Cape Reinga. The New Zealand Drought Index (NZDI) shows that severe meteorological drought coverage has receded significantly in the past week, but it is still found in parts of Northland, Auckland, the Coromandel Peninsula, and East Cape. Meteorological drought remains widespread in the upper North Island (see NZDI map).
Hotspot coverage has decreased substantially in the past week, but hotspots still remain across parts of Northland and Auckland, the Coromandel Peninsula, Mahia Peninsula, Wairarapa, and much of the lower North Island. In the South Island during the past week, heavy rainfall occurred along the West Coast and Stewart Island, with moderate rainfall in the lower South Island and parts of Marlborough. Elsewhere, rainfall amounts were minimal. Soil moisture increases were observed in the West Coast and Tasman, with minor increases in southern Canterbury. No substantial changes were noted elsewhere. The driest soils in the South Island compared to normal for this time of the year are located in coastal Hurunui District, while the wettest soils for this time of the year are found in Clutha District and Fiordland. The NZDI shows that meteorological drought continues to recede across much of Marlborough (see NZDI map).
Current hotspots in the South Island are found near Nelson, northeastern Marlborough, and in broken pockets from coastal Hurunui south to near Ashburton.

Outlook and Soil Moisture

In the North Island, a passing front on Friday (27 March) will bring up to 15 mm of rain for southern areas, with isolated amounts possibly greater than 25 mm in Wairarapa. However, the front will weaken across the central and upper North Island, where amounts will likely be less than 10 mm. While rainfall will be minimal for many areas over the upcoming weekend, a developing easterly flow will bring showers and periods of steadier rain to the east coast from Sunday (29 March) into early next week before high pressure returns by the middle of the week. Total weekly rainfall may exceed 30 mm in eastern areas, but 15 mm or less is possible from Northland to Waikato.
Substantial rainfall in the next week will likely help to reduce hotspot coverage in the east coast and Wairarapa, although lighter rainfall amounts in the upper North Island could result in hotspots there beginning to expand once again. In the South Island, low pressure will bring moderate to heavy rain to the West Coast today, with amounts likely to exceed 30 mm. On Friday (27 March), moderate rain is likely to affect Canterbury and Marlborough also, with localised areas exceeding 30 mm. After a mostly dry weekend, a developing easterly flow may bring more moderate rain to the top of the South Island early next week.
Moderate rainfall over the next week should allow for soil moisture increases across much of the eastern and northern South Island, although minor decreases may occur in Otago and Southland. This rainfall distribution may help to ease hotspots currently located in the top of the South Island and Canterbury.

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.

New Zealand Drought Index (NZDI)

As of 23 March, the New Zealand Drought Index (NZDI) map below shows that severe meteorological drought coverage has receded significantly across the upper North Island during the past week, although meteorological drought remains fairly widespread. In the South Island, meteorological drought and severe meteorological drought have also decreased significantly in Marlborough. 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.