Thursday, 23 February 2017
NIWA's Hotspot Watch
A weekly update describing soil moisture across the country to help assess whether severely to extremely dry conditions are occurring or imminent. Regions experiencing these soil
moisture deficits are deemed “hotspots”. Persistent hotspot regions have the potential to develop into drought.
Facts: Soil Moisture
Across the North Island, soil moisture levels have generally increased during the past week primarily due to heavy rain
that occurred late last week and the weekend. This is especially true from the western Bay of Plenty to Hawke’s Bay and
Wairarapa, as these areas have seen the most substantial soil moisture increases. In fact, the previous hotspot in
coastal Wairarapa has dissipated due to the heavy rain last week. Smaller but still significant increases also occurred
in the Coromandel Peninsula. Meanwhile, the most substantial soil moisture decreases were observed in East Cape and
Opotiki District, with smaller decreases also occurring in Taranaki and southern Northland. The driest soils across the
North Island compared to normal for this time of the year are found in East Cape and Opotiki District, while the wettest
soils for this time of the year are found in the Far North as well as from western Bay of Plenty to Hawke’s Bay and
coastal Wairarapa.
Hotspots remain in place in East Cape, the Opotiki District, as well as on the Mahia Peninsula. In these areas soils
continue to be severely to extremely drier than normal for this time of year.
Across the South Island, soil moisture levels have generally decreased slightly during the past week. This was most
evident across central Canterbury, the northern West Coast as well as in Tasman. Meanwhile, moderate soil moisture
increases were observed in Marlborough and Kaikoura District, with slight improvements in Hurunui District as well. The
driest soils across the South Island compared to normal for this time of the year are found in the Selwyn and Ashburton
Districts as well as in southwestern Southland, while the wettest soils for this time of the year are found in coastal
Marlborough and northern Fiordland.
The hotspot located in Selwyn and Ashburton Districts has increased in size a bit during the past week, while the
previous hotspot in coastal Hurunui District has weakened enough to no longer qualify as a hotspot, although soils
remain drier than normal there.
Outlook and Soil Moisture
For the North Island, predominate high pressure during the next week will likely lead to only minimal rainfall for most
locations. Isolated showers will be possible along the eastern coast and in Northland on Friday (24th February), but any
rainfall will be 5 mm or less. After nearly completely dry weather over the weekend, isolated showers will return on
Monday, but amounts will again be meagre. There is the possibility for a few locally heavy showers in the central North
Island on Tuesday (28th February), but amounts will likely remain under 15 mm. For the next week as a whole, most North
Island locations will only see rainfall totals of 5-15 mm, but a few places in southern and western areas may not see
any precipitation at all.
With minimal rainfall expected across the entire North Island during the next week, soil moisture levels are expected to
decrease nearly everywhere. This will cause the hotspots in Opotiki District, East Cape, and the Mahia Peninsula to
strengthen, with the possibility for additional hotspots to form in central and southern Gisborne. A new hotspot may
also form in western Taranaki during the next week.
For the South Island, rainfall amounts are also expected to be meagre during the next week. A weak front will bring a
few showers to Southland and Otago on Saturday (25th February), but generally amounts will be less than 10 mm. A few
more millimetres may fall across Canterbury on Sunday. However, high pressure is expected to bring nearly completely dry
weather to the South Island for much of next week. Rainfall totals across the South Island will be minimal during the
next week, with many locations expected to receive less than 5 mm. Only a few locations in eastern areas may receive as
much as 10 mm.
Just like the North Island, minimal rainfall during the next week will likely cause soil moisture levels to decrease
almost everywhere in the South Island. This will allow the hotspot in Ashburton and Selwyn Districts to strengthen and
also to increase in size across central Canterbury. The previous hotspot in Hurunui District will likely return as well.
In addition, new hotspots may form in Nelson and southwestern Southland during the next week.
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 above: Soil Moisture Anomaly Maps, relative to this time of year. The maps show soil moisture anomaly for the
past two weeks.