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
Across the North Island, most locations received less than 15 mm of rainfall in the past week, particularly in northern
areas. This led to additional soil moisture decreases across a majority of the North Island, although an onshore flow
has delivered more substantial rainfall and slightly improved soil moisture conditions to Hawke’s Bay in recent days.
The driest soils across the North Island, when compared to normal for this time of the year, are found across Northland
and northern Waikato, while the wettest soils for this time of the year are located near Taupo and Kapiti Coast.
A very large hotspot continues to cover nearly all of Northland, Auckland, northern Waikato, and the Coromandel
Peninsula. Additional hotspots are in place across most of Wairarapa, East Cape, and central Manawatu-Whanganui.
Across the South Island this past week, very minimal rainfall amounts were observed in eastern areas (generally less
than 5 mm), while more substantial rainfall occurred along the West Coast. This resulted in nearly universal soil
moisture decreases, with the largest losses observed from Tasman to southern Canterbury. The driest soils across the
South Island compared to normal for this time of the year are located in coastal Hurunui District as well as interior
Selwyn and Ashburton, while the wettest soils for this time of the year include Stewart Island, southern Southland, and
Clutha District.
Hotspot coverage has expanded substantially across Canterbury during the past week, now covering from coastal Hurunui
District south to approximately Waimate District.
Outlook and Soil Moisture
In the North Island, another week of very dry conditions is expected for most locations as persistent high pressure
remains overhead. A few showers occurring today (15 January) in the Far North will produce 5-10 mm, but otherwise dry
weather is anticipated for the bulk of the coming week. In fact, much of the northern and western North Island may see
weekly totals of 5 mm or less. Conversely, an easterly wind flow will bring several days of light to briefly moderate
rainfall to Hawke’s Bay and Gisborne this week, producing totals of 25-35 mm.
With meagre rainfall expected in much of the North Island, widespread soil moisture decreases are expected to continue.
Existing hotspots are anticipated to strengthen and expand, although conditions may not change significantly in
Wairarapa. Meanwhile, higher rainfall amounts may lead to soil moisture increases in Hawke’s Bay and Gisborne.
In the South Island, very minimal rainfall amounts are expected everywhere in the next week as high pressure sits
directly overhead. In fact, many locations may receive no rainfall in the next seven days, with only a few places seeing
up to 5 mm.
With little if any rainfall expected in the South Island in the next seven days, hotspot coverage will increase and
strengthen in eastern and northern areas. Much of Canterbury will be covered by a hotspot, which may also reach
southward into interior Otago. Additionally, new hotspots are likely to emerge across Nelson and Marlborough.
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.