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 generally 25-50 mm were observed in central areas spanning from
Waikato eastward to Hawke’s Bay and Gisborne. Conversely, meagre rainfall amounts less than 10 mm occurred in the upper
and lower North Island. This resulted in notably improved soil moisture levels in Hawke’s Bay and southern Gisborne,
while moderate soil moisture decreases occurred across Northland, Auckland, northern Waikato, Bay of Plenty, and East
Cape. Little change was observed elsewhere. The driest soils across the North Island, when compared to normal for this
time of the year, are found near Cape Reinga, coastal Wairarapa, and the northern tip of East Cape. Meanwhile, the
wettest soils for this time of the year are located in northern Hawke’s Bay.
No hotspots are currently found in the North Island, although Cape Reinga is on the cusp of hotspot formation.
In the South Island, heavy rainfall again occurred along the West Coast and Tasman during the past week, with widespread
rainfall of 50-100 mm. Moderate amounts up to 30 mm occurred in northern Marlborough and parts of coastal Otago, with up
to 20 mm in the rest of the lower South Island. However, central and northern Canterbury saw little if any rainfall
during the past week. This resulted in small soil moisture decreases across much of the South Island, with more
significant decreases observed in Banks Peninsula, while small increases occurred in Tasman and coastal Otago. The
driest soils in the South Island compared to normal for this time of the year are located in Banks Peninsula and coastal
southern Canterbury, while the wettest soils for this time of the year are found in northern Tasman and in the Clutha
District.
A hotspot remains in place in southern Canterbury, while new hotspots have emerged this week near Christchurch and in
coastal Selwyn District.Outlook and Soil Moisture
In the North Island, each afternoon through to Monday (26 October) will feature scattered showers popping up across
northern and central areas. Isolated areas may see substantial rainfall amounts from these showers (up to 25 mm), while
most locations will receive light to moderate amounts. However, little if any rainfall is expected in the lower North
Island on these days. A front is likely to move across the lower and central North Island on Tuesday and Wednesday
(27-28 October), producing up to 35 mm in some areas.
Due to the localised nature of the expected showers in the upcoming days, rainfall amounts will also be quite localised.
However, most locations in the upper and central North Island will likely see either little change or small decreases in
soil moisture levels during the next week. Meanwhile, parts of the lower North Island may observe slight increases. This
may result in the formation of new hotspots in Cape Reinga and East Cape during the next week.
In the South Island, high pressure will result in dry conditions for most areas through Saturday (24 October), although
a few showers will impact Fiordland and Southland. By late Sunday and Monday, a moisture-rich front will bring at least
moderate rain to the West Coast and lower South Island. Then by Tuesday (27 October), moderate rainfall may overspread
the east and top of the South Island. Dry weather then returns for the middle of next week. Weekly rainfall totals may
exceed 100 mm along the West Coast, with up to 30 mm in the lower South Island and 20 mm along the east coast.
At least small soil moisture increases may be observed in much of the western and northern South Island during the next
week, with isolated increases also possible in the lower South Island. Meanwhile, eastern areas are likely to see either
little change or minor soil moisture decreases. This may result in slight strengthening of the existing hotspots in the
eastern South Island.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.