Regions experiencing significant soil moisture deficits are deemed “hotspots”. Persistent hotspot regions have the potential to develop into drought.
Recent rainfall and current soil moisture conditions:
North Island:
- Most of the North Island saw meagre rainfall totals of less than 10 mm in the past week.
- Higher amounts of 15-30 mm were observed in parts of southern Waikato, Taranaki, and northern Manawatū-Whanganui.
- Small to moderate soil moisture decreases were observed across nearly the entire North Island in the past week.
- The driest soils across the North Island, when compared to normal for this time of the year, are found near Cape Reinga and along much of the east coast, while the wettest soils for this time of the year are found in southern Waikato and northern Taranaki.
- Hotspots remain in place in a small portion of southern Gisborne, southern Hawke’s Bay, and Wairarapa, while a new hotspot has formed in Cape Reinga.
- As of 28 April, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in eastern Northland, northern Auckland, Coromandel Peninsula, coastal Bay of Plenty, coastal Gisborne and Hawke’s Bay, south coastal Taranaki, eastern Manawatū-Whanganui, and Wairarapa. Very dry to extremely dry conditions are located in eastern Northland, northern Coromandel, coastal Gisborne, southern Hawke’s Bay, and Wairarapa.
South Island:
- Rainfall amounts of 50-125 mm were generally observed in Fiordland and the West Coast in the past week.
- Meanwhile, the lower South Island received amounts of 15-30 mm, while the upper and eastern South Island again received less than 5 mm.
- This resulted in moderate soil moisture decreases across the upper and eastern South Island, but the West Coast and lower South Island saw little change.
- The driest soils across the South Island, when compared to normal for this time of the year, are found in northern Canterbury, while the wettest soils for this time of the year are found in western Southland.
- Hotspots remain in place across much of coastal Canterbury.
- As of 28 April, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in northern Tasman, Nelson, and northern and central Canterbury. Very dry to extremely dry conditions are located in parts of northern Canterbury.
Pictured above: Soil Moisture Anomaly Maps, relative to this time of year. The maps show soil moisture anomalies over the past two weeks.
As of 28 April, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in eastern Northland, northern Auckland, Coromandel Peninsula, coastal Bay of Plenty, coastal Gisborne and Hawke’s Bay, south coastal Taranaki, eastern Manawatū-Whanganui, Wairarapa, northern Tasman, Nelson, and northern and central Canterbury. Very dry to extremely dry conditions are located in eastern Northland, northern Coromandel, coastal Gisborne, southern Hawke’s Bay, Wairarapa, and parts of northern Canterbury. Please note: some hotspots in the text above may not correspond with the NZDI map. 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.
The week ahead:
North Island:
- A period of rain will move across the North Island tonight and early Thursday morning (1-2 May), followed by a drying trend.
- Several days of mostly dry weather are expected from Friday (3 May) into the middle of next week, although the upper North Island may see scattered showers on Sunday (5 May).
- Weekly rainfall totals are again likely to be meagre across the upper and eastern North Island with amounts of less than 15 mm, although totals of 15-30 mm will be possible from Taranaki to Wellington.
- Due to the expected rainfall in the next week, additional soil moisture decreases will be likely across much of the North Island, although the lower North Island may not see much change.
- Current hotspots located along the east coast and near Cape Reinga will likely strengthen in the next week.
South Island:
- Rain will continue moving through the upper and central South Island today and tonight, followed by a drying trend on Thursday morning (1-2 May).
- From Friday (3 May), generally dry weather is expected through at least the middle of next week, although the lower South Island may see showers on Friday and Monday (3 May and 6 May).
- Weekly rainfall totals of 10-30 mm are expected in the upper South Island, while areas farther south will generally see 15 mm or less.
- Due to the expected rainfall in the next week, small soil moisture decreases may occur in many regions, with the best chance in the lower South Island.
- The current hotspots located in Canterbury may strengthen at least slightly in the next week.
Long-term outlook through early June:
- The drier (25th percentile) and middle (50th percentile) rainfall scenarios both show below normal to well below normal rainfall widespread across both islands.
- Even in the wetter (75th percentile) scenario, below normal rainfall is signalled across large portions of both islands, increasing confidence in a below normal rainfall outcome.
- Very dry soil conditions may affect parts of the North Island and eastern South Island, particularly in the drier scenario.
Pictured above: 35-day forecast rainfall anomaly scenarios (Top), and 35-day forecast dryness and drought scenarios (Bottom). These maps are updated daily at https://niwa.co.nz/climate/seasonal-climate-outlook
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.
