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
In the North Island, September to date has featured near normal amounts of rain. The greatest rainfall amounts over the last seven days have fallen across Taranaki, the Central North Island, and the Waikato, where 75 to 150 mm of rain has fallen, with areas around Taranaki Maunga seeing nearly 200 mm of rain. Elsewhere, 30 to 50 mm of rainfall has been observed except for coastal Hawke’s Bay and the east coast of the Manawatu and Wellington regions. Portions of Central Hawke’s Bay and the coastal Wairarapa have recorded 25 mm or less of rain through to yesterday (25 September). Soil moisture levels generally increased slightly across the North Island in the past week, although a decrease was observed in the Napier area. The driest soils across the North Island, when compared to normal for this time of the year, are found around Napier, while the wettest soils for this time of the year are generally found in the upper North Island.
No hotspots are currently located in the North Island. However, as of 23 September, the New Zealand Drought Index (NZDI) map shows that drier than normal conditions are currently found in the Napier area.
In the South Island, well above normal rain amounts have occurred in inland Otago and Southland. 100 to 200mm of rain has fallen across most of the West Coast and portions of the Tasman district over the last seven days. The east coast and portions of Southland and interior Otago have been dry, with observed rainfall less than 30 mm for these areas, except along the immediate coast in South Canterbury and potions of Coastal Otago and Marlborough, where less than 10 mm of rain has fallen. Soil moisture levels generally increased slightly across the entire South Island in the past week. The driest soils across the South Island, when compared to normal for this time of the year, are found in a small portion of coastal North Otago, while the wettest soils for this time of the year are generally found in northern Canterbury and interior Otago.
No hotspots are currently located in the South Island.
Soil Moisture Anomaly Maps, relative to this time of year. The maps show soil moisture anomalies over the past two weeks.
As of 23 September, the New Zealand Drought Index (NZDI) map shows that drier than normal conditions are currently found in the Napier area. 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
A series of cold fronts will move across the North Island, bringing several rounds of showers or periods of rain through Thursday, 2 October. In between fronts there will be dry periods. Weekly rainfall totals will generally range from 20-40 mm for the northern and western North Island, with pockets up to 50 mm, while the eastern North Island will receive 10 to 30 mm of rain. Due to the expected rainfall in the next week, only small changes to soil moisture levels are likely for most of the North Island, except for the coastal portions of Hawke’s Bay and the east coast of the Manawatu and Wellington regions, where slight decreases in soil moisture may occur. In the South Island, moderate to heavy rain is forecast for western and southern areas, as an active weather pattern will bring several cold fronts through the South Island between today and Thursday, 2 October. Weekly rainfall totals will range between 75-150 mm for the West Coast. The eastern South Island, including Marlborough, will generally see weekly rainfall totals of 5 to 25 mm. Elsewhere, interior locations and Southland will receive 20-40 mm of rain.
Due to the expected rainfall in the next week, only small changes to soil moisture levels are likely for most of the South Island, except for the east coast and Marlborough region, where slight decreases in soil moisture may occur.
Long-term outlook (through late October)
- The drier (25th percentile) rainfall scenario shows below normal rainfall across the east and the top of the South Island and most of the east coast of the North Island, including very dry conditions in coastal Canterbury and Otago.
- The middle (50th percentile) and upper (75th percentile) rainfall scenarios show a notable wet signal for the upper North Island. This may indicate areas of low pressure moving south out of the subtropics in late October.
- Very dry soil conditions may begin to affect the east coasts of both islands 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 /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.
