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, heavy rain in excess of 100 mm was observed during the past week in Wairarapa, with other parts of
Wellington seeing 50 mm or more. Other areas including coastal Gisborne to Tararua and Taranaki to northern
Manawatu-Whanganui received 30 mm or more.
Conversely, generally meagre rainfall was observed in the upper North Island. This rainfall resulted in massive soil
moisture increases in the lower North Island and moderate increases along much of the east coast. Meanwhile, small soil
moisture decreases occurred in the upper North Island, although decreases in the Far North were more substantial. The
driest soils across the North Island, when compared to normal for this time of the year, are located in the Coromandel
Peninsula and a small portion of Central Hawke’s Bay. Meanwhile, the wettest soils for this time of the year are located
in Wairarapa. The New Zealand Drought Index (NZDI) shows that severe meteorological drought coverage has again receded
significantly in the past week, although it is still widespread in the Coromandel Peninsula. Meteorological drought
remains in place across parts of Northland, Auckland, northern Waikato, and East Cape (see NZDI map at bottom).
Hotspot coverage decreased substantially in the past week in the lower North Island, but hotspots still remain across
parts of the Far North, Auckland, much of the Coromandel Peninsula, southern Hastings, Central Hawke’s Bay, and parts of
coastal Manawatu-Whanganui.
In the South Island during the past week, heavy rainfall of 50 mm or more occurred in northern and central Canterbury
along with Marlborough Sounds. Moderate rainfall also occurred along much of the West Coast, although minimal rainfall
was observed in Tasman and the lower South Island. This resulted in significant soil moisture increases across much of
Marlborough and Canterbury.
However, slight soil moisture decreases occurred along the West Coast and the lower South Island.
The driest soils in the South Island compared to normal for this time of the year are located in Nelson and nearby parts
of Tasman, while the wettest soils for this time of the year are found in Kaikoura. The NZDI shows that meteorological
drought has completely dissipated across the upper South Island, although widespread dry soils are still present (see
NZDI map at bottom).
Current hotspots in the South Island are found in Nelson and nearby parts of Tasman, as well as a small portion of
interior Selwyn District.Outlook and Soil Moisture
In the North Island, high pressure will bring mostly dry weather over the next several days. However, an onshore wind
flow through the upcoming weekend (4-5 April) may produce up to 10 mm across Gisborne and Hawke’s Bay. The only chance
for notable widespread rainfall will occur next Wednesday (8 April), as a passing front may produce 5-10 mm for much of
the North Island.
Generally light rainfall amounts across a majority of the North Island during the next week will likely result in soil
moisture decreases in many locations, with most existing hotspots strengthening and expanding at least slightly.
In the South Island, high pressure will produce dry weather through at least Saturday (4 April).
Beginning on Sunday a front will deliver moderate to heavy rain to the West Coast, and by Tuesday (7 April) some rain
may cross into northern, eastern, and lower portions of the South Island as well.
Total weekly rainfall may exceed 75 mm in much of the West Coast, 15-25 mm in northern areas, 20-30 mm in the lower
South Island, and 15 mm or less in much of Canterbury.
While weekly rainfall may result in little change or even slight increases in soil moisture levels for many locations,
in much of Canterbury soils are likely to dry at least slightly. This could result in an expansion of the hotspot
currently located in interior Selwyn District.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.
As of 30 March, the New Zealand Drought Index (NZDI) map below shows that severe meteorological drought and
meteorological drought have both receded significantly in the North Island, although the Coromandel Peninsula remains
heavily affected. In the South Island, meteorological drought and severe meteorological drought have both completely
dissipated. 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.