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Hotspot Watch 22 April 2022

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, many locations received minimal rainfall of 5 mm or less during the past week. However, localised heavier rainfall occurred in much of Northland and the Coromandel Peninsula, with amounts of 50-100 mm, and localised higher amounts. This resulted in soil moisture decreases across much of the western and southern North Island, although increases were observed in Northland and the Coromandel Peninsula. The driest soils across the North Island, when compared to normal for this time of the year, are found in northern Waikato, while the wettest soils for this time of the year are found near Napier.Isolated, small hotspots are currently located in parts of northern and central Waikato. As of 19 April, the New Zealand Drought Index (NZDI) map below shows that dry conditions are now located near Cape Reinga. In the South Island, heavy rainfall impacted much of the West Coast during the past week, with widespread amounts of 50-100 mm, and amounts of more than 150 mm commonplace in higher terrain. Much of Southland and Stewart Island saw beneficial rainfall of more than 30 mm. However, only light rainfall was observed in the eastern South Island, with much of Marlborough and Canterbury receiving less than 10 mm. This resulted in some soil moisture increases in the West Coast, Otago, Southland, and Stewart Island, while small decreases were observed in Canterbury. The driest soils in the South Island, when compared to normal for this time of the year, are located in southeastern Otago, while the wettest soils for this time of the year are found near Kaikōura.South Island hotspots are currently located in coastal Selwyn and Ashburton districts, along with parts of southern Canterbury and interior Otago. As of 19 April, the New Zealand Drought Index (NZDI) map below shows that dry conditions are now located in parts of the West Coast, central Otago, and northern Southland. In addition, very dry to extremely dry conditions are located in southern Otago and coastal Southland, with meteorological drought located across Stewart Island.

Outlook and Soil Moisture

In the North Island, moderate to heavy rain passed through northern and eastern areas today (22 April), bringing beneficial rainfall. However, a large area of high pressure will move over the North Island beginning on Saturday (23 April), resulting in several consecutive days of generally dry weather. Weekly rainfall totals could reach 25-30 mm in parts of the western North Island and in Gisborne, while elsewhere amounts are likely to be less than 20 mm. Due to the expected rainfall in the next week, soil moisture levels will likely decrease across much of the North Island. This may result in strengthening and enlargement of the small hotspots located in Waikato.In the South Island, high pressure will be the dominant feature over the next week, leading to mostly dry weather for many locations. However, a front will bring some rain to Fiordland and the lower South Island on Saturday night and early Sunday (23-24 April). Weekly rainfall totals could reach 30-40 mm in Fiordland, Southland, and Stewart Island, while other locations may only see meagre amounts of 15 mm or less. Due to the expected rainfall in the next week, soil moisture levels may improve slightly in the lower South Island, while other areas are likely to see soil moisture decreases. Existing hotspots in Canterbury and Otago may therefore strengthen at least slightly.

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.

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 19 April, the New Zealand Drought Index (NZDI) map below shows that dry conditions are now located near Cape Reinga, parts of the West Coast, central Otago, and northern Southland. In addition, very dry to extremely dry conditions are located in southern Otago and coastal Southland, with meteorological drought located across Stewart 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.