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 less than 25 mm during the past week, especially in the Central Plateau and Wairarapa where rainfall totals were generally less than 10 mm. However, pockets of heavier rainfall were observed in the Coromandel Peninsula, Bay of Plenty, and Taranaki, where isolated amounts of 50 mm or more occurred. This resulted in small soil moisture decreases across a majority of the North Island, although some increases were observed in Bay of Plenty and Taranaki. 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 in coastal Gisborne and Hawke’s Bay.
A small hotspot is currently located in northern Waikato. As of 26 April, the New Zealand Drought Index (NZDI) map below shows that the only areas in the North Island with dry conditions are a small area near Cape Reinga as well as one in central Waikato.
In the South Island, moderate rainfall impacted much of the West Coast, Southland, and Stewart Island, with widespread amounts up to 30 mm, and isolated pockets up to 50 mm. Elsewhere however, rainfall amounts were much lower, with most locations in the upper and eastern South Island receiving 10 mm or less. This resulted in small soil moisture decreases for many areas, although Southland and Stewart Island saw notable soil moisture increases. The driest soils in the South Island, when compared to normal for this time of the year, are located in coastal central Canterbury and eastern 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 26 April, the New Zealand Drought Index (NZDI) map below shows that dry conditions are located in parts of the upper South Island, West Coast, Otago, and Southland. In addition, very dry to extremely dry conditions are located in southern Otago, eastern Southland, and Stewart Island.
Outlook and Soil Moisture
In the North Island, persistent high pressure will result in mostly or completely dry weather for many locations during the next week. However, an easterly wind flow will bring scattered showers to places such as Northland, Auckland, the Coromandel Peninsula, Bay of Plenty, and Gisborne. These areas could see weekly rainfall totals of 10-15 mm, while other locations in the North Island may see little if any rainfall.
Due to the expected rainfall in the next week, soil moisture levels will likely decrease across much of the North Island. This will likely result in strengthening and enlargement of the hotspot currently located in Waikato.
In the South Island, a front will reach the lower West Coast, Southland, and Stewart Island on Sunday (1 May), bringing a period of moderate to heavy rainfall to these areas. By Wednesday and Thursday (4-5 May), much lighter showers and rain will impact these same areas. The rest of the South Island will generally see high pressure during the next week. Weekly rainfall totals may exceed 100 mm in parts of the West Coast, with up to 30 mm in Southland and Stewart Island. However, only meagre rainfall of less than 10 mm is expected elsewhere.
Due to the expected rainfall in the next week, soil moisture levels may improve slightly in the lower South Island and West Coast, while other areas are likely to see soil moisture decreases. Existing hotspots in Canterbury and Otago may therefore strengthen and expand.
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 26 April, the New Zealand Drought Index (NZDI) map below shows that dry conditions are located near Cape Reinga, a small area in central Waikato, parts of the upper South Island, West Coast, Otago, and Southland. In addition, very dry to extremely dry conditions are located in southern Otago, eastern Southland, and 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.
