Hotspot Watch 22 October 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

In the North Island, moderate rainfall amounts of generally 25-50 mm were observed in central areas spanning from Waikato eastward to Hawke’s Bay and Gisborne. Conversely, meagre rainfall amounts less than 10 mm occurred in the upper and lower North Island. This resulted in notably improved soil moisture levels in Hawke’s Bay and southern Gisborne, while moderate soil moisture decreases occurred across Northland, Auckland, northern Waikato, Bay of Plenty, and East Cape. Little change was observed elsewhere. The driest soils across the North Island, when compared to normal for this time of the year, are found near Cape Reinga, coastal Wairarapa, and the northern tip of East Cape. Meanwhile, the wettest soils for this time of the year are located in northern Hawke’s Bay.
No hotspots are currently found in the North Island, although Cape Reinga is on the cusp of hotspot formation. In the South Island, heavy rainfall again occurred along the West Coast and Tasman during the past week, with widespread rainfall of 50-100 mm. Moderate amounts up to 30 mm occurred in northern Marlborough and parts of coastal Otago, with up to 20 mm in the rest of the lower South Island. However, central and northern Canterbury saw little if any rainfall during the past week. This resulted in small soil moisture decreases across much of the South Island, with more significant decreases observed in Banks Peninsula, while small increases occurred in Tasman and coastal Otago. The driest soils in the South Island compared to normal for this time of the year are located in Banks Peninsula and coastal southern Canterbury, while the wettest soils for this time of the year are found in northern Tasman and in the Clutha District.
A hotspot remains in place in southern Canterbury, while new hotspots have emerged this week near Christchurch and in coastal Selwyn District.

Outlook and Soil Moisture

In the North Island, each afternoon through to Monday (26 October) will feature scattered showers popping up across northern and central areas. Isolated areas may see substantial rainfall amounts from these showers (up to 25 mm), while most locations will receive light to moderate amounts. However, little if any rainfall is expected in the lower North Island on these days. A front is likely to move across the lower and central North Island on Tuesday and Wednesday (27-28 October), producing up to 35 mm in some areas.
Due to the localised nature of the expected showers in the upcoming days, rainfall amounts will also be quite localised. However, most locations in the upper and central North Island will likely see either little change or small decreases in soil moisture levels during the next week. Meanwhile, parts of the lower North Island may observe slight increases. This may result in the formation of new hotspots in Cape Reinga and East Cape during the next week. In the South Island, high pressure will result in dry conditions for most areas through Saturday (24 October), although a few showers will impact Fiordland and Southland. By late Sunday and Monday, a moisture-rich front will bring at least moderate rain to the West Coast and lower South Island. Then by Tuesday (27 October), moderate rainfall may overspread the east and top of the South Island. Dry weather then returns for the middle of next week. Weekly rainfall totals may exceed 100 mm along the West Coast, with up to 30 mm in the lower South Island and 20 mm along the east coast.
At least small soil moisture increases may be observed in much of the western and northern South Island during the next week, with isolated increases also possible in the lower South Island. Meanwhile, eastern areas are likely to see either little change or minor soil moisture decreases. This may result in slight strengthening of the existing hotspots in the eastern South Island.

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.

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 19 October, the New Zealand Drought Index (NZDI) map below shows that drier than normal soils are located in coastal Canterbury, northern Otago, and are beginning to emerge near East Cape, but meteorological drought is not currently found in New Zealand. 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.