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
Recent background: In the North Island and much of the South Island, soil moisture levels were near normal during the winter and early spring. An exception was southern Canterbury and northern Otago, where soil moisture levels have been below normal for the last several weeks. More recently, notable drying has been observed in the North Island since late September.
In the North Island, rainfall amounts were generally meagre during the past week, with nearly all locations receiving less than 10 mm, and some spots receiving no rainfall at all. This resulted in widespread 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 near Cape Reinga and coastal Wairarapa. Meanwhile, the wettest soils for this time of the year are located from Manawatu-Whanganui to Hawke’s Bay.
Despite the general drying of soils during the past week, no hotspots are currently found across the North Island.
In the South Island, heavy rainfall of 75 mm or more was observed in the past week across much of the West Coast, with Southland and Stewart Island receiving more than 40 mm. While much of Otago saw about 25 mm, the remainder of the eastern and northern South Island received generally meagre rainfall amounts. This generally resulted in slight soil moisture increases in the western and southern South Island, while minor to moderate decreases were observed elsewhere. The driest soils in the South Island compared to normal for this time of the year are located in coastal southern Canterbury, while the wettest soils for this time of the year are found in the lower West Coast.
Despite continued drying in the northern and eastern South Island, no hotspots are currently in place.
Outlook and Soil Moisture
In the North Island, high pressure sitting nearby will result in dry conditions through the upcoming weekend. However, beginning on Monday (12 October), weak low pressure with a subtropical connection will move across the North Island, delivering perhaps 15-25 mm to many locations, and isolated amounts up to 40 mm in Hawke’s Bay and Gisborne by the end of Tuesday. After more dry weather on Wednesday, additional light rainfall may affect the eastern North Island on Thursday (15 October).
The expected rainfall amounts in the next week will likely result in little change to the soil moisture situation in much of the North Island. However, the northern half of the east coast may see at least slight soil moisture increases, while the Far North and Wairarapa may observe slight decreases.
In the South Island, a front will bring heavy rainfall to much of the West Coast from Saturday through to early Monday (10-12 October), with amounts exceeding 100 mm in some places. However, little of this moisture will cross the Southern Alps, so rainfall in Canterbury and Otago will be minimal. Another much weaker front will move up the South Island on Wednesday (14 October), but only light rainfall amounts are expected. Across the eastern South Island, weekly rainfall totals are likely to be less than 15 mm.
Due to expected heavy rainfall, additional soil moisture increases will be likely in the western South Island during the next week. Conversely, further soil moisture decreases are expected in the eastern South Island. This may result in the formation of a hotspot in southern Canterbury or northern Otago, where the driest soils are currently located.
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 5 October, the New Zealand Drought Index (NZDI) map below shows that drier than normal soils are currently located in southern Canterbury and northern Otago, but 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.
