Hotspot Watch 11 November 2021

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, heavy rainfall affected Gisborne and Hawke’s Bay late last week, with some locations in Gisborne receiving 150-200 mm since last Thursday, and event totals exceeding 300 mm in a few spots. This resulted in flash flooding across parts of Gisborne. Elsewhere, pockets of rainfall greater than 30 mm were observed in northern Wairarapa, southern Waikato, Bay of Plenty, Coromandel Peninsula, and Northland. However, much of the North Island received less than 25 mm in the last week, with coastal Taranaki and Manawatū-Whanganui receiving no rain at all. This resulted in moderate to large soil moisture increases across the eastern North Island, while the rest of the North Island generally saw small to moderate soil moisture decreases. The driest soils across the North Island, when compared to normal for this time of the year, are found in coastal Taranaki and Manawatū-Whanganui, along with interior Wairarapa, while the wettest soils for this time of the year are found across coastal Gisborne.  

Currently, no official hotspots are located in the North Island. The New Zealand Drought Index (NZDI) map below shows that no dry conditions are located in the North Island as of 8 November.

In the South Island, the past week was very dry with only minimal rainfall observed. Most locations received less than 5 mm, with some parts of Canterbury, Otago, and the lower West Coast receiving no rainfall at all. This resulted in moderate to large soil moisture decreases across the entire South Island. The driest soils in the South Island, when compared to normal for this time of the year, are located in parts of Canterbury, coastal Southland, and the lower West Coast, while the wettest soils for this time of the year are found in far northern Canterbury, much of Marlborough, and areas near Dunedin.

In the past week, official hotspots have emerged in coastal Ashburton and Waimate districts of Canterbury. The New Zealand Drought Index (NZDI) map below shows that no dry conditions are located in the South Island as of 8 November.

Outlook and Soil Moisture

In the North Island, a few showers may be found on Friday, before periods of moderate to heavy rain affect northern and western parts of the island during Saturday and early Sunday (13-14 November). In fact, localised flooding will be possible in the upper North Island this weekend due to heavy rain. Conversely, the eastern North Island will likely only see minimal rainfall this weekend. Monday through Wednesday of next week will feature showery conditions in western areas, with mostly dry weather elsewhere. By Thursday (18 November), high pressure will bring widespread dry weather. Weekly rainfall totals could reach or exceed 50 mm in the upper and western North Island, with pockets of higher amounts possible where the heaviest rain occurs this weekend. However, the east coast will see significantly less rainfall, with 10 mm or less expected in Gisborne, Hawke’s Bay, and Wairarapa during the next week.

The substantial rainfall expected in the upper and western North Island this week will result in small to moderate soil moisture increases there. However, moderate soil moisture decreases are likely along the east coast due to light rainfall and warm temperatures. There is a small chance that a hotspot may form in Wairarapa during the next week, with no hotspots expected elsewhere.

In the South Island, heavy rain will reach the West Coast on Friday. This weekend (13-14 November), two more rounds of rain will affect the West Coast, while moderate rainfall will also reach Southland, Otago, and southern Canterbury on these days. More rain will affect the West Coast during early-to-mid next week, although this rain is unlikely to cross over into the eastern South Island. Mostly dry weather will arrive on Thursday (18 November). Weekly rainfall totals could easily reach 150-200 mm in the West Coast, with pockets of higher amounts possible. Meanwhile, 30-50 mm is likely to accumulate in the lower South Island and in southern Canterbury. Lighter amounts are expected from central Canterbury to Marlborough, however, where weekly totals may remain below 15-20 mm.

Due to the expected weekly rainfall, significant soil moisture increases are likely to occur in the western South Island, while the lower South Island may see moderate increases. However, areas from coastal Marlborough to central Canterbury will likely see additional soil moisture decreases during the next week due to light rainfall and warmer than average temperatures. The existing hotspot in Waimate District may dissipate during the next week, while the one in Ashburton District may remain in place. Meanwhile, additional hotspots may form in central or northern Canterbury 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.

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 8 November, the New Zealand Drought Index (NZDI) map below shows that no dry conditions are located across the country at this time. 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.