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, moderate rainfall amounts of 15-30 mm were generally observed in northern and western areas during the past week, although isolated areas of more than 30 mm were also observed. However, it was substantially drier in the eastern and southern North Island, where amounts were generally less than 10 mm. This resulted in additional soil moisture decreases in the eastern and southern North Island, while other areas generally saw little change. The driest soils across the North Island, when compared to normal for this time of the year, are found from East Cape to Hawke’s Bay, as well as in southern Waikato, while the wettest soils for this time of the year for the North Island are found in the Far North.
Hotspot conditions continue to be widespread across the North Island, including western Northland, southern Auckland, much of Waikato, eastern Bay of Plenty, much of Gisborne, Hawke’s Bay south to Wairarapa, much of Manawatū-Whanganui, and Kapiti Coast. The New Zealand Drought Index (NZDI) map below shows that meteorological drought is in place in a small portion of East Cape, while widespread dry-to-extremely dry conditions are in place across nearly all of the North Island.
In the South Island, moderate to heavy rain was observed across Tasman, Nelson, and the West Coast during the past week, with widespread amounts of 50-75 mm, and some areas receiving more than 100 mm. However, once again rainfall amounts were meagre in the eastern South Island, with most locations receiving less than 10 mm, and some receiving none at all. While additional soil moisture decreases occurred in the eastern South Island, substantial soil moisture increases were observed in Tasman, Nelson, and the West Coast. The driest soils in the South Island compared to normal for this time of year are located in Coastal Hurunui, Banks Peninsula, eastern Southland, and Clutha District, while the wettest soils for this time of the year for the South Island are found in Fiordland.
Hotspots are currently in place in coastal Hurunui District, Banks Peninsula, Selwyn and Ashburton districts, eastern Southland, and Clutha District. However, previous hotspots located in Tasman and Nelson have now dissipated. The New Zealand Drought Index (NZDI) map below shows that dry to very dry conditions are widespread in the northeastern South Island.
Outlook and Soil Moisture
A front arriving on Saturday (6 March) will bring moderate rainfall amounts to the western North Island, with some locations receiving up to 20 mm. However, eastern areas will see much lighter amounts. After a few more scattered showers or isolated thunderstorms on Sunday, high pressure will bring dry weather from Monday through about Wednesday. However, by late next week there will be the chance for another area of low pressure to produce at least light rainfall. Weekly rainfall totals across the North Island could be quite variable, with western areas receiving 30-50 mm, but the Far North and east coast may receive 15 mm or less.
Due to the expected weekly rainfall, at least small soil moisture increases will be possible in the western North Island. However, further soil moisture decreases will be likely along the east coast, and may also occur in the upper North Island. While current hotspots in the western North Island (such as Manawatū-Whanganui) may weaken at least slightly, those in the east coast may further strengthen and expand.
In the South Island, the West Coast will continue to see heavy rain tonight and Saturday (6 March), with amounts up to 75 mm possible. However, only small amounts will occur in the eastern South Island. After dry weather on Monday and Tuesday, another front will bring rain to the West Coast around Wednesday (10 March), with showers possible in eastern areas. Weekly rainfall totals are likely to exceed 100 mm in parts of the West Coast and perhaps Tasman, while 20-25 mm will be possible near Nelson and in the lower South Island.
Due to the expected weekly rainfall, further soil moisture increases will be possible in the western South Island, while little change is anticipated in southern areas and near Nelson. However, at least small additional soil moisture decreases are likely along the east coast. Current hotspot located in Canterbury may slightly strengthen and expand during the next week.
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 3 March, the New Zealand Drought Index (NZDI) map below shows that meteorological drought is in place in a small portion of East Cape. Widespread dry-to-extremely dry conditions are in place across nearly all of the North Island and the northeastern 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.
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