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NIWA's Hotspot Watch

Wednesday, 13 March 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
In the North Island, from Wellington-Wairarapa to the lower Waikato, soil moisture levels increased over the past week from due to beneficial rainfall. In the north and east of the North Island, soil moisture levels generally remained the same. The driest soils for the time of year are found across the northern Waikato and Bay of Plenty. The wettest soils for the time of year are located in Gisborne, Central Hawke’s Bay, and Wellington-Wairarapa.

In the South Island, soil moisture levels are similar to this time last week. The driest soils for the time of year are across the Buller and Tasman District as well as Nelson, where the Ministry for Primary Industries recently classified the combination of impacts of fire and drought as a medium-scale adverse event. The wettest soils for the time of year are now located in northeast Marlborough.

Hotspots are located in Northland, Auckland, northern and central Waikato, central and western Bay of Plenty, South Taranaki, central and southern Manawatu-Wanganui, and northern Hawke’s Bay in the North Island and in the Buller District, Tasman District, Nelson, western Marlborough, southern Otago, and central Southland.

Outlook and Soil Moisture
A slow-moving weather pattern with moist, northerly air flows near New Zealand will most likely cause normal or above normal rainfall across the central and northern South Island and western North Island over the next week. This may result in appreciable increases in soil moisture.

From Thursday into Friday, a band of rain will be focused over Westland before shifting northward on Saturday. Rainfall totals will be heavy across the Alps, potentially exceeding 125 mm. Some of the rain will cross over the ranges into Otago and Canterbury, totalling 10 to 25 mm, highest across the interior. Rainfall over the drought-affected areas in the north of the South Island are forecast to total 10 to 25 mm, highest in the west. Additional rain is expected early next week across the aforementioned regions, although timing and placement is unclear at this time. The intensity and amount of rain that is anticipated may lead to localized flooding and slips.

For the central and northern South Island, soil moisture levels are most likely to increase over the next week. Levels in the lower South Island may remain the same or decrease slightly.

Rainfall over the North Island will be less abundant with only light amounts forecast through Saturday. Rainfall chances will begin to increase from Sunday into early next week in the western part of the island and then the north. While rainfall amounts are uncertain, some could be heavy due to the moist northerly winds. Meanwhile, normal or below normal rainfall is the most likely outcome for eastern areas of the island.

Soil moisture levels are most likely to increase in the western North Island over the next week, have about equal chances of remaining the same or increasing in the northern North Island, and have about equal chances of remaining the same or decreasing in the east.

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.
ends

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