Hotspot watch 29 October 2020

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.

Facts: soil moisture

In the North Island, moderate rainfall amounts of generally 25-50 mm were observed across much of the southern half of the island, along with similar amounts in parts of Northland. Conversely, amounts of generally 10 mm or less were found in Bay of Plenty, much of Waikato, Auckland, and lower Northland. Soil moisture decreases were observed across most of the North Island during the past week, with the most substantial losses occurring from lower Northland through to Bay of Plenty and East Cape. The driest soils across the North Island, when compared to normal for this time of the year, are now found along a tier from lower Northland through to Auckland, northern Waikato, Bay of Plenty, and northern Gisborne, along with coastal Wairarapa. Meanwhile, the wettest soils for this time of the year are located in western Waikato and near Wellington City.

The North Island’s first hotspot has now formed in East Cape, with coastal Bay of Plenty nearing hotspot status.

In the South Island, heavy rain was again found in the West Coast and Tasman during the past week, with widespread amounts greater than 100 mm. Amounts at or above 30 mm also occurred in northern Marlborough, far northern Canterbury, interior Otago and western Southland. Elsewhere in the eastern South Island, amounts up to 25 mm were generally observed. This resulted in small to moderate soil moisture increases across Tasman, northern Canterbury, southern Canterbury, and Southland, while small decreases occurred in eastern Marlborough and central Canterbury. The driest soils in the South Island compared to normal for this time of the year are located in Banks Peninsula and coastal Selwyn District, while the wettest soils for this time of the year are found in Southland.

A hotspot now runs along coastal Canterbury from approximately Christchurch south to near the Otago border, while a new hotspot has formed in eastern Marlborough.

Outlook and soil moisture

In the North Island, showers will be found on Friday and Saturday as a couple of weak disturbances move through, with moderate accumulations possible in the Central Plateau. After generally dry conditions on Sunday and Monday, more substantial rainfall may impact the upper half of the North Island on Tuesday and Wednesday (3-4 November), where isolated amounts up to 50 mm may occur. Weekly rainfall totals may reach 25-50 mm across much of the central North Island, while amounts of 25 mm or less are favoured to occur in the Far North and along the east coast.

Due to the moderate rainfall expected in the central North Island during the next week, that area may see little change in soil moisture levels. However, further decreases are likely to occur in the Far North and the east coast, and this may result in new hotspots forming in these areas.  

In the South Island, heavy rain will impact much of the West Coast on Friday and Saturday (30-31 October), where amounts may exceed 100 mm in some areas. Southland and interior Otago may also receive up to 30 mm from this event, while amounts along much of the east coast will be relatively light. Scattered showers may impact the eastern South Island on Monday (2 November), but again amounts are likely to be light. High pressure will then bring dry weather through the middle of next week. Weekly rainfall amounts are expected to exceed 100 mm across much of the western South Island, with up to 35 mm in the lower South Island. Meanwhile, amounts may be 25 mm or less for much of Canterbury and Marlborough.

At least minor soil moisture decreases are likely to occur in the next week across much of Canterbury and Otago, while small increases are possible along the West Coast. Little change is likely in Southland and interior Otago. This may allow the current hotspots in Canterbury and Marlborough to expand and strengthen during the next week.    

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.

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