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, most areas were either dry or saw light rainfall totals of 5-10 mm during the past week. This generally resulted in soil moisture decreases across the North Island. The driest soils across the North Island, when compared to normal for this time of the year, are found in western Northland, western Waikato, northern Taranaki and northern Manawatū-Whanganui, while the wettest soils for this time of the year are found in coastal Gisborne.
Hotspot conditions are now located across much of the North Island, except for coastal Gisborne, and isolated pockets of the Manawatū, the Tararua Ranges, coastal Taranaki, Northland, the Bay of Plenty and the Central Plateau. As of 2 February, the New Zealand Drought Index (NZDI) map below shows that dry-to-extremely dry conditions extend across the northern half of the North Island, and across parts of Wellington. Additionally, pockets of meteorological drought have emerged across the Central Plateau, Waikato, a small area of western Auckland, while it has expanded slightly in parts of western Northland.
In the South Island, after a dry January, exceptionally heavy rainfall fell over the west, with widespread rainfall over 200 mm stretching from northern Fiordland to Arthurs Pass, and about the Buller, Tasman and Nelson ranges. Multiple areas locations have seen totals between 700-1000 mm. East of the Southern Alps, rainfall was sparser, with less than 5 mm for the upper Canterbury and Marlborough. Meanwhile the Mid-Canterbury, Southland and Otago received 10-50mm, with the largest totals on the Otago ranges. This resulted in significant soil moisture increase across the west, some slight decreases for the Marlborough and upper Canterbury, and near no significant changes elsewhere. The driest soils in the South Island, when compared to normal for this time of year, are in parts of Southland and Rakiura Stewart Island.
Hotspots have eased in the Tasman but remain in southern Otago and parts of Southland. As of the 2 February, the NZDI map below shows that dry conditions extend from Tasman, Nelson and Marlborough, across the Southern Alps and ranges, and down to much of Otago and Southland, including Stewart Island. In addition, very dry or extremely dry conditions are in parts of Tasman, Nelson, Marlborough, the Southern Alps, elevated Otago and coastal Southland. Meteorological drought has emerged for parts of the interior elevated Otago. These conditions may ease across western areas and the South Alps due to the recent heavy rainfall.
Outlook and soil moisture
In the North Island, some of the most significant rain to fall in more than a month looks to occur this weekend. Areas of moderate-to-heavy rain will begin to fall on Saturday (5 February) about the south, west and north, with the chance of isolated thunderstorms bringing locally higher totals to northern and central regions. Areas of rain and showers then look to extend over eastern areas on Sunday and Monday. The rain looks to persist until about Tuesday or Wednesday, when isolated coastal showers may affect northern and eastern areas. Widespread 20-60mm looks to fall, with some of the elevated areas receiving in excess of 100 mm.
Due to the expected of rainfall in the next week, soil moisture levels should increase across much of the North Island. This could see an abatement of some hotspots, particularly about southern and eastern areas.
In the South Island, the exceptionally heavy rain that has been occurring looks to clear the west on Saturday (5 February), but some eastern areas should see rain and showers with a southerly change. Areas of rain may linger over the upper North Island on Monday, but the rest of the Island looks largely dry. There is the chance of more rain about Fiordland and the West Coast around Wednesday or Thursday, there remains much uncertainty at this time. The weekend’s rainfall may bring an additional 20-50 mm for the upper North Island, with the southerly change bringing 10-20 mm about parts of the upper and middle Canterbury. More significant totals may then develop Wednesday or Thursday, with western areas potentially seeing 50-120 mm.
Due to the expected of rainfall in the next week, soil moisture levels look to increase about western areas and the upper north Island, and may increase slightly about the upper Canterbury. Soil moisture decreases may occur in the Otago and Southland, causing an exacerbation of existing hotspots.
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.
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Soil Moisture Anomaly Map – 4 February 2022 [NIWA]
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Soil Moisture Anomaly Maps, relative to this time of year. The maps show soil moisture anomaly (mm) for the past two weeks.
As of 2 February, the New Zealand Drought Index (NZDI) map below shows that dry-to-extremely dry conditions extend across the northern half of the North Island, and across parts of Wellington, and extending across the upper South Island, across the ranges, and throughout the inland Otago and Southland region. Meteorological drought continues in western Northland, while small pockets of meteorological drought have emerged in western Auckland, the Waikato, the Central Plateau, and about parts of the elevated Otago. 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.
Drought Index map 4 February 2022 [NIWA]
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