Weekly update to help media assess likelihood of extremely dry weather preceding a drought. Regions experiencing severely to extremely drier than normal soils conditions are deemed “hotspots”.
Facts: Soil Moisture
For the North Island, soil moisture levels are drier than normal for this time of year for much of the lower part of the North Island as well as western and northern areas of the island. Isolated pockets of severely drier than normal soil moisture exist in the same aforementioned areas.
For the South Island, soil moisture levels remain drier than normal for this time of year for large sections along and east of the Divide as well as southwest parts of the island. Severely to extremely drier than normal soils for this time of the year exist north and south of the Banks Peninsula (though not present over Banks Peninsula) as well as the far northwest parts of Otago.
Week-to-Week Comparison
For the North Island, when compared to this time last week, the coverage and severity of drier than normal soils have decreased significantly over nearly the entirety of the island. This is largely due to the rainfall associated with ex-Tropical Cyclone Pam. The most dramatic change in soil moisture content occurred from eastern Bay of Plenty, Gisborne and Hawke’s Bay. Soils have dried a bit from the Kapiti Coast to near Wanganui.
For the South Island, there has not been significant change in soil moisture content when compared to last week. Severely to extremely drier than normal soils for this time of year for areas over parts of eastern Canterbury from roughly Ashburton to Oamaru then again from north of Christchurch to near Kaikoura. Isolated severely drier than normal soils also exist over central and western parts of Southland, west and northwest of Invercargill and in the Queenstown-Lakes District.
Commentary
For the North Island, when considering the current soil moisture anomalies for this time of year, scattered to isolated hotspots exist in Northland, western Auckland, western and central Waikato, Manawatu-Wanganui and Wairarapa, and extreme southern Hawke’s Bay regions.
For the South Island, hotspot areas persist in sections of eastern Canterbury as well as isolated pockets of southern and western Southland and farther north into interior Otago. Overall, the driest soils relative to this time of year are present over the hotspot areas of eastern Canterbury, between Christchurch and Kaikoura.
For hotspot regions, sustained rainfall over an extended period of time is needed to return conditions back to normal.
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)
Pictured below: Soil Moisture Anomaly Maps, relative to this time of year. Above are values this time last week. Below are the most recent values.
