Source: NIWA – National Institute of Water and Atmospheric Research
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.
Recent rainfall and current soil moisture conditions:
North Island:
- The upper and central North island generally saw rainfall totals of 25-50 mm in the past week, with localised amounts of 75 mm or more.
- However, generally meagre rainfall occurred along the east coast, where totals were less than 10 mm.
- Moderate to large soil moisture increases were observed across the western and central North Island in the past week, but additional decreases occurred along the east coast.
- The driest soils across the North Island, when compared to normal for this time of the year, are found in the Coromandel Peninsula, Gisborne, and Hawke’s Bay, while the wettest soils for this time of the year are found in western Northland along with western and southern Waikato.
- Most previous hotspots in the upper North Island, Manawatū-Whanganui and western Wellington have now dissipated, but hotspots are still located in parts of the Coromandel Peninsula and much of the east coast from East Cape to Wairarapa.
- As of 12 April, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in Northland, northern Auckland, Coromandel Peninsula, coastal Gisborne and Hawke’s Bay, coastal Taranaki, southern Manawatū-Whanganui, and Wellington. Very dry to extremely dry conditions are located in much of Northland, northern Coromandel, southern Hawke’s Bay, and Wellington, while parts of eastern Northland are experiencing meteorological drought.
South Island:
- Very heavy rain affected the West Coast in the past week, with high elevations receiving 500 mm or more in many instances. The upper South Island also received substantial rainfall totals of 60-120 mm.
- Interior Canterbury, Otago, and Southland received 30-60 mm, but the east coast generally saw 10-25 mm.
- This resulted in moderate to large soil moisture increases across much of the South Island, although northern Canterbury 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 Marlborough and western Southland.
- Previous hotspots in the upper South Island have all dissipated due to the recent rainfall, but hotspots are still located in coastal northern and central Canterbury.
- As of 12 April, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in northern Tasman, Nelson, northern Marlborough, parts of Canterbury, and northern Otago. Very dry to extremely dry conditions are located in northern Tasman, Nelson, western Marlborough, and 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 12 April, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in Northland, northern Auckland, Coromandel Peninsula, coastal Gisborne and Hawke’s Bay, coastal Taranaki, southern Manawatū-Whanganui, Wellington, northern Tasman, Nelson, northern Marlborough, parts of Canterbury, and northern Otago. Very dry to extremely dry conditions are located in much of Northland, northern Coromandel, southern Hawke’s Bay, Wellington, northern Tasman, Nelson, western Marlborough, and parts of northern Canterbury. In addition, parts of eastern Northland are experiencing meteorological drought. 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:
- Daily rounds of showers will affect the northern and western North Island from Monday to Friday (15-19 April), while the east coast will remain generally dry during this time.
- A more substantial area of rain may move across the island on Friday night and Saturday morning, followed by more isolated showers on Sunday (21 April).
- Weekly rainfall totals could reach 25-50 mm across the upper and western North Island. However, rainfall totals along the east coast may be 15 mm or less.
- Due to the expected rainfall in the next week, small soil moisture increases may occur in the western North Island, but small decreases could again occur along the east coast.
- Current hotspots located along the east coast could strengthen slightly in the next week.
South Island:
- Showers will move through the West Coast and lower South Island on Tuesday (16 April), followed by mostly dry weather across the island on Wednesday and Thursday.
- However, a more substantial band of rain will move across the island on Friday (19 April). More showers will be possible on Sunday (21 April).
- Weekly rainfall totals could reach 50-120 mm along the West Coast, with 25-50 mm possible in the lower South Island.
- However, lighter rainfall totals of less than 25 mm are likely in the upper and eastern South Island.
- Due to the expected rainfall in the next week, some soil moisture decreases may occur in the upper and eastern South Island, with other regions generally seeing little change.
- The current hotspots located in Canterbury may strengthen at least slightly in the next week.
Long-term outlook (through mid-May):
- The drier (25th percentile) and middle (50th percentile) rainfall scenarios both show near normal to below normal rainfall across much of both islands.
- In the wetter (75th percentile) scenario, near normal to above normal rainfall is signalled across much of the country.
- Very dry soil conditions may affect parts of eastern NZ in the middle and drier scenarios, but this is not forecast to be widespread.
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.
