Hotspot Watch 15 January 2020

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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

Across the North Island, most locations received less than 15 mm of rainfall in the past week, particularly in northern areas. This led to additional soil moisture decreases across a majority of the North Island, although an onshore flow has delivered more substantial rainfall and slightly improved soil moisture conditions to Hawke’s Bay in recent days. The driest soils across the North Island, when compared to normal for this time of the year, are found across Northland and northern Waikato, while the wettest soils for this time of the year are located near Taupo and Kapiti Coast.

A very large hotspot continues to cover nearly all of Northland, Auckland, northern Waikato, and the Coromandel Peninsula. Additional hotspots are in place across most of Wairarapa, East Cape, and central Manawatu-Whanganui.

Across the South Island this past week, very minimal rainfall amounts were observed in eastern areas (generally less than 5 mm), while more substantial rainfall occurred along the West Coast. This resulted in nearly universal soil moisture decreases, with the largest losses observed from Tasman to southern Canterbury. The driest soils across the South Island compared to normal for this time of the year are located in coastal Hurunui District as well as interior Selwyn and Ashburton, while the wettest soils for this time of the year include Stewart Island, southern Southland, and Clutha District.

Hotspot coverage has expanded substantially across Canterbury during the past week, now covering from coastal Hurunui District south to approximately Waimate District.

Outlook and Soil Moisture

In the North Island, another week of very dry conditions is expected for most locations as persistent high pressure remains overhead. A few showers occurring today (15 January) in the Far North will produce 5-10 mm, but otherwise dry weather is anticipated for the bulk of the coming week. In fact, much of the northern and western North Island may see weekly totals of 5 mm or less. Conversely, an easterly wind flow will bring several days of light to briefly moderate rainfall to Hawke’s Bay and Gisborne this week, producing totals of 25-35 mm. 

With meagre rainfall expected in much of the North Island, widespread soil moisture decreases are expected to continue. Existing hotspots are anticipated to strengthen and expand, although conditions may not change significantly in Wairarapa. Meanwhile, higher rainfall amounts may lead to soil moisture increases in Hawke’s Bay and Gisborne.

In the South Island, very minimal rainfall amounts are expected everywhere in the next week as high pressure sits directly overhead. In fact, many locations may receive no rainfall in the next seven days, with only a few places seeing up to 5 mm.

With little if any rainfall expected in the South Island in the next seven days, hotspot coverage will increase and strengthen in eastern and northern areas. Much of Canterbury will be covered by a hotspot, which may also reach southward into interior Otago. Additionally, new hotspots are likely to emerge across Nelson and Marlborough.  

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.

New Zealand Drought Index (NZDI)

As of 12 January, the New Zealand Drought Index (NZDI) map below shows that the upper North Island continues to have widespread very dry to extremely dry soils, and some locations could approach meteorological drought conditions with additional dry weather in the coming week. Widespread dry to very dry conditions also exist in Wairarapa and East Cape.

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.

Research subject: Climate