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, scattered showers and storms resulted in some heavy rainfall over parts of the west, while the east remained mostly dry. Much of the Far North, Waikato, western Bay of Plenty, Taranaki, Manawatū-Whanganui and Wellington region recorded over 30 mm, with some isolated spots picking up over 100 mm. However, other parts of Northland, Hawke’s Bay and Gisborne missed out on this rain, collecting less than 15 mm, while some portions of these regions remained completely dry. This resulted in soil moisture increases for parts of Northland, Waikato, Taranaki, western Bay of Plenty and Manawatū-Whanganui. Meanwhile further soil moisture decreases were observed in northern Gisborne and eastern Bay of Plenty. The driest soils across the North Island, when compared to normal for this time of the year, are found in western Northland, East Cape and spots of the Tararua District, while the wettest soils for this time of the year for the North Island are found in the Far North, inland Waikato, Taranaki and southern Manawatū-Whanganui.
Hotspot conditions are have emerged in Cape Reinga, while hotspot conditions remain southern Northland, parts of Auckland, northern Waikato, parts of Gisborne, southern Hawke’s Bay, southern Manawatū-Whanganui and parts of the Wairarapa. The New Zealand Drought Index (NZDI) map below shows that widespread dry to very dry conditions extend from southern Northland to Wellington, excluding Taranaki, and across the east of the North Island. A small area of extremely dry conditions is also located in coastal Gisborne and southern Wairarapa.
In the South Island, rain affected western and northern areas, while the east saw little rain. Marlborough, Nelson, Tasman, West Coast and western Southland received widespread over 50 mm of rain, with isolated locations collecting over 150 mm. Meanwhile, the majority of Canterbury, Otago and Southland received less than 15 mm, with some areas remaining completely dry. This resulted in large soil moisture increases in the north and modest soil moisture increases along the west. Soil moisture levels decreased across the east. The driest soils in the South Island compared to normal for this time of year are located in eastern Hurunui District and southeastern Southland, while the wettest soils for this time of the year for the South Island are found in lower Fiordland.
Hotspots have expanded to be scattered across much of the Canterbury, while hotspots remain in far eastern Marlborough and much of Clutha District. A hotspot has eased near Nelson. The New Zealand Drought Index (NZDI) map below shows that widespread dry to very dry conditions are now in place across much of the Marlborough and eastern and coastal Canterbury as well as parts of Otago and Southland. Extremely dry conditions are also found in coastal northern Canterbury and far eastern coastal Marlborough.
Outlook and Soil Moisture
On Friday (2 April) and Saturday, a weakening front brings the chance of rainfall to most of the North Island, but totals look to be generally less than 5mm, although some isolated areas about the northern Hawke’s Bay or Gisborne ranges could receive up to 10 mm. Chiefly dry conditions will then develop for Sunday and Monday, except for a few brief, light showers in the west and southwest. A few showers may occur for eastern areas on Wednesday (7 April). After drying up again, some rainfall is expected along the west late next week or even the following weekend.
Due to the lack of substantial rainfall over the next week, small to moderate moisture decreases are expected for large swaths of the North Island. The current hotspots in the upper, eastern and southwestern North Island are likely to strengthen at least slightly.
In the South Island, a cold front will bring widespread rainfall of 10 mm or less to most areas on Friday (2 April), with some spots in Southland collecting as much as 10-25 mm. A gusty westerly flow will become established across the South Island by Sunday (4 April), bringing a period of showery weather to the west until at least Tuesday, with widespread 50 mm or more extending from the Westland District Fiordland, and totals of 5 mm or less for the rest of Southland. During this period, eastern areas will remain largely dry. While brief showers may then continue in Fiordland from Wednesday, there is the chance of more significant rainfall late next week or early next weekend for much of the west.
Due to the expected weekly rainfall, at least moderate soil moisture decreases are expected across the eastern South Island in the week, while small soil moisture increases are possible in the west. Current hotspots in the east are expected to strengthen and may even expand further.
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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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 24 March, the New Zealand Drought Index (NZDI) map below shows that widespread dry to very dry conditions extend from southern Northland to Wellington, excluding Taranaki, and across the east of the North Island, as well as Marlborough, eastern and coastal Canterbury, parts of Otago and Southland. Extremely dry conditions are also found in coastal Gisborne, southern Wairarapa, coastal northern Canterbury and far eastern coastal Marlborough. However, meteorological drought is not currently found in New Zealand. 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.
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