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, many locations received moderate rainfall amounts of up to 25 mm in the past week, especially in central and southern areas. In addition, parts of northern Hawke’s Bay and Taranaki received more than 30 mm. However, it was another mostly dry week in Northland and Auckland, where less than 10 mm was observed. This resulted in generally little change to soil moisture levels across most of the North Island, although small additional decreases were observed in Northland. The driest soils across the North Island, when compared to normal for this time of the year, are found in western Northland, southern Auckland, and much of Waikato, while the wettest soils for this time of the year are found in coastal Gisborne and Hawke’s Bay.
Small, isolated hotspots are currently located in parts of Waikato. As of 10 May, the New Zealand Drought Index (NZDI) map below shows that dry conditions are located in parts of Waikato and Manawatū-Whanganui. In addition, very dry conditions are located in central Waikato.
In the South Island, heavy rain impacted the West Coast during the past week, with widespread amounts of 50-100 mm, and 150 mm or more in higher terrain. Elsewhere, most locations generally received 15-30 mm, although slightly higher amounts were observed in interior Canterbury. This resulted in small soil moisture increases for most areas, although small additional decreases occurred near Nelson, Banks Peninsula, and interior Otago. The driest soils in the South Island, when compared to normal for this time of the year, are located along Tasman Bay and in Banks Peninsula, while the wettest soils for this time of the year are found near Kaikōura.
South Island hotspots are currently located in coastal Selwyn and Ashburton districts, along with parts of southern Canterbury and interior Otago. As of 10 May, the New Zealand Drought Index (NZDI) map below shows that dry conditions are located in much of the upper South Island and Otago. In addition, very dry conditions are located in parts of the upper South Island and eastern Otago, with an area of extremely dry conditions along the Marlborough-Canterbury border.
Outlook and soil moisture
In the North Island, a front will bring showers to western areas on Saturday night and Sunday morning (14-15 May), with another round of potentially heavier rain arriving on Sunday night and Monday. By late Tuesday (17 May), a separate front will bring more showers to western and northern areas. After mostly dry conditions on Wednesday and Thursday, another front may reach the North Island with additional moderate rainfall on Friday (20 May). Weekly rainfall totals could reach 40-60 mm in the western North Island, with lower amounts of 15-20 mm expected in most other locations.
Due to the expected rainfall in the next week, small to moderate soil moisture increases are likely in the western North Island, while other areas are likely to see little change. This may cause the current small hotspots in Waikato to ease or dissipate completely.
In the South Island, a series of fronts will move up the West Coast during the next week, bringing periodic moderate to heavy rain to areas west of the divide from Saturday (14 May) through Thursday (19 May). Moderate rainfall amounts will also reach the lower South Island, with lesser amounts elsewhere. Weekly rainfall totals may reach 100-200 mm in much of the West Coast, with 30-50 mm possible in Southland and interior Otago. However, from Marlborough south to coastal Otago, weekly totals may not exceed 20 mm.
Due to the expected rainfall in the next week, soil moisture levels will likely improve in the lower South Island and West Coast, while additional soil moisture decreases may occur farther east. Existing hotspots in Canterbury may slightly strengthen and expand, while those in interior Otago may improve somewhat.
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).
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.
Pictured above: Soil Moisture Anomaly Maps, relative to this time of year. The maps show soil moisture anomaly for the past two weeks to 12 May 2022.
As of 10 May, the New Zealand Drought Index (NZDI) map below shows that dry conditions are located in parts of Waikato and Manawatū-Whanganui, and much of the upper South Island and Otago. In addition, very dry conditions are located in central Waikato, parts of the upper South Island, and eastern Otago, with an area of extremely dry conditions along the Marlborough-Canterbury border. 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.
