Hotspot Watch 1 May 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

In the North Island, generally minimal rainfall of 10 mm or less occurred in most areas during the past week. However, isolated locations near East Cape and eastern Northland received 20 mm or more. This resulted in nearly universal minor to moderate soil moisture decreases across the entire island. The driest soils across the North Island, when compared to normal for this time of the year, stretch across the upper North Island, Bay of Plenty, Hawke’s Bay, and southern Gisborne. Meanwhile, the wettest soils for this time of the year are located in Wairarapa. The New Zealand Drought Index (NZDI) shows that meteorological drought and severe meteorological drought remain in place across parts of the Coromandel Peninsula and a small part of northern Auckland. Meteorological drought has now re-emerged along the southern coast of Hawke’s Bay (see NZDI map).
Hotspots are currently found across parts of Aupouri Peninsula, western Northland, much of Auckland, the Coromandel Peninsula, southern Hastings, Central Hawke’s Bay, and a part of coastal Manawatu-Whanganui. In the South Island, heavy rainfall of 70 mm or more was observed across large parts of the West Coast during the past week, while Southland received up to 25 mm. However, locations from Otago north to Marlborough and Nelson once again received only minimal rainfall (generally 5 mm or less). Although moderate soil moisture increases were observed in western Southland, moderate decreases were common across Canterbury and the top of the South Island. The driest soils in the South Island compared to normal for this time of the year are located in Nelson, nearby parts of Tasman and Marlborough, and much of central Canterbury, while the wettest soils for this time of the year are found near Kaikoura. The NZDI shows that meteorological drought is not found in the South Island, although drier than normal conditions are still present in the top of the South Island (see NZDI map).
Currently, the only official hotspot in the South Island is located from around Christchurch south to coastal Selwyn and Ashburton districts (including parts of Banks Peninsula).

Outlook and soil moisture

In the North Island, dry conditions are expected on Saturday, but beneficial rain will arrive thereafter as a moisture-rich front arrives on Sunday. After the heaviest rain departs by Monday morning, continuing showers on Monday and Tuesday will bring additional light to moderate rainfall amounts to many areas. By mid-to-late next week, high pressure will begin to build back in with drier conditions expected. Total rainfall amounts in the next week are likely to be substantial, and quite beneficial to dry portions of the North Island. Amounts of 30-50 mm will be possible in the upper North Island, with up to 30 mm along the east coast. Even more significant totals will be possible from Taranaki to Bay of Plenty, where 50-75 mm may accumulate.
The moderate to heavy rainfall expected in the next week will result in widespread soil moisture increases across the North Island. While the largest increases are likely in the central portions of the island, at least minor improvements are expected in the very dry parts of the upper North Island and the east coast. This should result in a weakening of all current hotspots across the North Island. In the South Island, heavy rain will impact the West Coast on Saturday, with a few showers reaching the eastern part of the island. Wet conditions will continue along the West Coast on Sunday and Monday, but little of this moisture will move farther east. There will be a possibility for more widespread light to moderate rain to move through Otago, Canterbury, and Marlborough on Tuesday (5 May), but at this time there remains moderate forecast uncertainty. While weekly rainfall totals may exceed 100 mm in much of the West Coast, amounts of 25 mm or less are most likely for the eastern South Island. Areas in the far north such as Tasman, Nelson, and parts of Marlborough could also receive significant rainfall of at least 30-40 mm.
Soil moisture increases will be likely in the next week along the West Coast and in parts of the upper South Island. In eastern areas from Canterbury to Otago, little change in soil moisture levels will be the most likely outcome.

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.

NZ soil moisture anomaly (mm) 9am 22 April 2020 [NIWA]

New Zealand soil moisture anomaly (mm) at 9am on 30 Apr 2020 [NIWA]

Soil moisture anomaly maps, relative to this time of year. The maps show soil moisture anomaly for the past two weeks.

As of 29 April, the New Zealand Drought Index (NZDI) map below shows that meteorological drought remains in place across parts of the Coromandel Peninsula and a small part of northern Auckland, while meteorological drought has now re-emerged along the southern coast of Hawke’s Bay. Meteorological drought is no longer found in the South Island. 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.

NZ Drought Index map for 29 April 2020 [NIWA]