Hotspot Watch 11 March 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

Across the North Island, rainfall during the past week was quite variable. Amounts were generally less than 10 mm in the lower North Island, while conversely there were scattered areas with more than 30 mm in parts of Gisborne, the Central Plateau, Waikato, and eastern Northland. This caused soil moisture levels to increase slightly in the top half of the North Island, while generally decreasing slightly in the lower North Island. The driest soils across the North Island, when compared to normal for this time of the year, are generally found in scattered areas from Northland to Gisborne, along with Wairarapa. Meanwhile, the wettest soils for this time of the year are located in Kapiti Coast. The New Zealand Drought Index (NZDI) shows that severe meteorological drought currently encompasses Northland, Auckland, much of Waikato, western Bay of Plenty, East Cape, and portions of interior Manawatu-Whanganui, with meteorological drought in place from Bay of Plenty through the Central Plateau (see NZDI map).   

A majority of the North Island remains in official hotspot status, although scattered areas in eastern Northland, Bay of Plenty, western Waikato, and Taranaki have fallen below hotspot thresholds due to rainfall in the past week.

In the South Island during the past week, moderate rainfall occurred in the West Coast and Southland, where many locations received more than 30 mm of rain. However, rainfall was generally meagre from Marlborough to southern Canterbury, where rainfall was generally less than 10 mm. The driest soils in the South Island compared to normal for this time of the year are located in Nelson, Tasman, coastal Hurunui, and Banks Peninsula, while the wettest soils for this time of the year are found in Clutha District. The New Zealand Drought Index (NZDI) shows that severe meteorological drought is now found in southern Marlborough (see NZDI map).   

Current hotspots in the South Island are found in Nelson and nearby parts of Tasman and Marlborough, coastal Hurunui, and Banks Peninsula.

Outlook and soil moisture

In the North Island, scattered showers and isolated thunderstorms will bring a few millimetres of rain to some locations today (11 March), but rainfall will not be widespread. On Thursday and Friday, a few showers will impact eastern Northland, Coromandel Peninsula, Gisborne, and Hawke’s Bay, while other areas remain dry. It currently appears that mostly dry weather will be in place for the upcoming weekend (14-15 March), although there is a small chance that nearby low pressure could bring light to moderate rain to the Far North. By the middle of next week, there is some indication that an ex-tropical cyclone could pass near the North Island, although at this time there is a large amount of uncertainty regarding its possible track and impacts.

There remains uncertainty about total rainfall amounts in the next week, particularly in the upper North Island. However, at least minor soil moisture increases will be possible in Northland, Gisborne, and Hawke’s Bay, while further decreases are possible elsewhere. However, should an ex-tropical cyclone impact parts of the North Island next week, soil moisture increases would be much more significant.

In the South Island, high pressure overhead will produce mostly if not entirely dry weather from Thursday through Saturday (12-14 March). After isolated showers on Sunday, more dry weather is expected heading into next week. Overall, minimal rainfall is expected in the South Island during the next week, with many locations receiving less than 10 mm.

Due to the expected minimal rainfall in the next week, soil moisture decreases are anticipated across a majority of the South Island. This will likely lead to the strengthening and expansion of current South Island hotspots.

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.

Soil moisture anomaly (mm) at 9am on 10 March 2020 [Infographic: NIWA]

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Soil moisture anomaly maps, relative to this time of year. The maps show soil moisture anomaly for the past two weeks.

As of 8 March, the New Zealand Drought Index (NZDI) map below shows that all of the upper North Island is experiencing severe meteorological drought, along with East Cape and interior Manawatu-Whanganui, with meteorological drought in many other locations. In the South Island, meteorological and severe meteorological drought are now found in much of Marlborough, Tasman, and far northern Canterbury. 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.

MIL OSI