Hotspot Watch 22 January 2020

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 little if any rain during the past week (generally less than 5 mm), leading to further drying of soils. The only exceptions were parts of Hawke’s Bay and Gisborne, where localised rainfall amounts greater than 30 mm were observed. The driest soils across the North Island, when compared to normal for this time of the year, are found across much of Northland and northern Waikato, while the wettest soils for this time of the year are located in Wairoa District (Hawke’s Bay). In addition, the New Zealand Drought Index (NZDI) shows that the upper North Island has widespread extremely dry soils, and some locations in Northland, Auckland, and northern Waikato have reached meteorological drought conditions (see map at bottom). 

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

Little if any rainfall was observed across the South Island during this past week, leading to universal soil moisture decreases. The largest decreases occurred across Tasman and the West Coast. The driest soils in the South Island compared to normal for this time of the year are located across Tasman, Buller, and Hurunui, as well as interior Selwyn and Ashburton. Meanwhile, the wettest soils for this time of the year are found near Invercargill.

Hotspot coverage has continued to expand during the past week, now covering much of Hurunui District south to approximately Waimate District, along with Nelson and nearby parts of Tasman. Much of Marlborough is also approaching hotspot status in recent days. 

Outlook and Soil Moisture

In the North Island, very dry conditions will continue during the next week as high pressure remains overhead. This will lead to little if any rainfall for the vast majority of the North Island, and any locations that do see rainfall will generally receive 5 mm or less. In addition, with temperatures expected to be above to well above average in the coming days, this will lead to further soil moisture loss through additional evapotranspiration. 

With minimal rainfall expected in much of the North Island, widespread soil moisture decreases are expected to continue in the next week. All existing hotspots are anticipated to strengthen and expand. 

In the South Island, fronts arriving on 23 January, 25 January, and 29 January will bring moderate to heavy rain to the West Coast, with weekly totals reaching 50-100 mm. Meanwhile, more modest totals of 15-30 mm will be observed in the lower South Island. Elsewhere, minimal rainfall totals are expected across Nelson, Marlborough, and Canterbury, with many locations receiving less than 5 mm. 

Moderate to heavy rainfall in the West Coast will lead to some soil moisture increases in that region in the next week, while minor increases may be observed across Southland and parts of Otago. However, the rest of the South Island is expected to see further soil moisture decreases due to meagre rainfall and above to well above average temperatures in the coming days. Current hotspots in Canterbury, Nelson, and Tasman are expected to strengthen and expand in the next week, while a new hotspot is likely to form in 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 20 January, the New Zealand Drought Index (NZDI) map below shows that the upper North Island has widespread extremely dry soils, and some locations in Northland, Auckland, and northern Waikato have reached meteorological drought conditions. Meteorological drought coverage is expected to increase in these regions due to dry weather in the next week.

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.