Soil moisture patterns for Tasman and Nelson and New Zealand Drought Index (NZDI) - NIWA

Soil moisture anomaly (mm) at 9am on 14/03/2024

Hotspot Watch 15 March 2024

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.

Recent rainfall and current soil moisture conditions:

South Island:

• Much like the North Island, a majority of the South Island saw only meagre rainfall in the past week, with 5 mm or less observed in most of the upper and eastern South Island.
• Pockets of the upper West Coast, central Canterbury, and Southland received up to 20 mm.
• Weekly rainfall totals in the lower West Coast and Fiordland were much higher, however, with generally 50-100 mm observed.  
• This resulted in small to moderate soil moisture decreases across most of the upper and eastern South Island, with little change observed elsewhere.    
• The driest soils across the South Island, when compared to normal for this time of the year, are found across parts of Tasman, Nelson, Marlborough Sounds, and Canterbury, while the wettest soils for this time of the year are found in western Southland.  
• Hotspots are currently located across much of the upper and eastern South Island, including eastern Tasman, Nelson, parts of Marlborough, nearly all of Canterbury, and eastern Otago.
• As of 13 March, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in eastern Tasman, Nelson, Marlborough, northern and southern Canterbury, and eastern Otago. Very dry to extremely dry conditions are located in Nelson, Marlborough, parts of northern and far southern Canterbury, and northern Otago.

Soil moisture anomaly (mm) at 9am on 07/03/2024

Soil moisture anomaly (mm) at 9am on 14/03/2024

Pictured above: Soil Moisture Anomaly Maps, relative to this time of year. The maps show soil moisture anomalies over the past two weeks.

New Zealand Drought Index (NZDI) 

As of 13 March, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in Northland, Auckland, part of the Coromandel Peninsula, eastern Bay of Plenty, much of the lower North Island, eastern Tasman, Nelson, Marlborough, northern and southern Canterbury, and eastern Otago. 

Very dry to extremely dry conditions are located in much of Northland, Wellington, Nelson, Marlborough, parts of northern and far southern Canterbury, and northern Otago. 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.

 

New Zealand Drought Index (NZDI) - 13 March 2024 [NIWA]

The week ahead:

South Island:

• Areas of light to moderate rain are moving north across the island today (15 March), but high pressure will begin moving in on Saturday (16 March).
• A few showers will affect the eastern and lower South Island on-and-off from Saturday to Tuesday (16-19 March), but other regions will remain mostly dry.  
• A better chance for some rain may reach the lower West Coast by late next week.
• Weekly rainfall totals of 25-50 mm will be possible along the West Coast, with pockets of 15-25 mm in the eastern and lower South Island.
• However, the upper South Island again looks to see meagre rainfall totals that will likely be less than 10 mm.   
• Due to the expected rainfall in the next week, moderate soil moisture decreases are likely in the upper South Island, but rest of the island may only see small decreases.
• The current hotspots in the upper South Island will likely strengthen and expand in the next week, with those in Canterbury and Otago likely seeing little change overall.

Key Messages for the Top of the South Island:

• From October 2023 through April 2024, Blenheim recorded just 88 mm of rain – the lowest rainfall over that period since at least 1941.
• Many parts of the region experienced six or seven consecutive months with below normal rainfall, depending on the location.
• This severe rainfall deficit culminated in a precipitation drought for parts of Marlborough, Nelson, and Tasman.

NIWA.

Pigeon Post News, Richmond.