Saturday, February 24, 2024

Weather Hotspot Watch - NZ Drough Index


Soil moisture anomaly (mm) at 9am on 22/02/2024

Hotspot Watch 23 February 2024

23 February 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:

  • Rainfall amounts of 20-40 mm affected the West Coast in the past week, although Fiordland received 75-150 mm.
  • However, the rest of the South Island saw meagre rainfall amounts of 10 mm or less, with some locations receiving no rainfall at all.  
  • This resulted in moderate soil moisture decreases across nearly the entire South Island.  
  • The driest soils across the South Island, when compared to normal for this time of the year, are found in Nelson, Marlborough and northern 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, Marlborough, northern and southern Canterbury, and eastern Otago. In addition, central Canterbury is close to hotspot status.  
  • As of 21 February, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in eastern Tasman, Nelson, Marlborough, northern and far southern Canterbury, and northern Otago. Very dry to extremely dry conditions are located in Nelson, Marlborough, northern Canterbury, and northern Otago.

Soil moisture anomaly (mm) at 9am on 15/02/2024

Soil moisture anomaly (mm) at 9am on 22/02/2024

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

As of 21 February, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in parts of Northland, Auckland, eastern Bay of Plenty, East Cape, much of the lower North Island, eastern Tasman, Nelson, Marlborough, northern and far southern Canterbury, and northern Otago. 

Very dry to extremely dry conditions are located in eastern Northland, far southern Manawatū-Whanganui, Wellington, Nelson, Marlborough, northern 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) - 21 February 2024 [NIWA]

The week ahead:

South Island:

  • A front moving up the South Island on Saturday (24 February) will bring moderate to heavy rain to the West Coast, Southland, and Otago.
  • After dry weather on Sunday and Monday, an additional round of showers or rain may arrive on Tuesday (27 February).
  • Thereafter, generally dry weather is expected through late next week.  
  • Weekly rainfall totals of 70-100 mm are possible in the lower West Coast, including Fiordland, with 30-40 mm in the upper West Coast and lower South Island.
  • However, regions such as Nelson, Marlborough, and Canterbury will likely see lighter rainfall totals of less than 15 mm.   
  • Due to the expected rainfall in the next week, additional soil moisture decreases are likely in the upper and eastern South Island, but the lower West Coast and lower South Island may see small increases.
  • Current hotspots may strengthen in the next week, while additional hotspots may form in central Canterbury.

Long-term outlook (through late March):

  • The drier (25th percentile) and middle (50th percentile) rainfall scenarios show drier or much drier than normal conditions across much of the country.
  • In the wetter (75th percentile) scenario, below normal rainfall is still forecast in isolated areas, but near normal rainfall is most favoured.
  • Very dry soil conditions could affect the lower North Island and eastern South Island in all three rainfall scenarios, along with the upper North Island in the drier scenario. The drier scenario also shows the possibility for a small area of meteorological drought to form in the lower South Island.



A drought is defined as a rainfall deficit which restricts or prevents a human activity – for example, farming or power generation.

Extended dry periods are experienced in New Zealand most years, but whether or not they are classed as drought depends on a range of different factors.

Soil moisture deficit, measured in mm, is used as one index of drought. It measures the amount of soil moisture available to the roots of pasture plants, and is generally higher in summer, when evaporation rates are higher.

Due to the impact on agriculture, the Ministry  for Primary Industries (MPI) declare when a dry period is classed as an adverse event. This declaration takes into account the climatic conditions as well as a number of impact-based criteria.

Causes of drought

Drought is caused by insufficient precipitation – for example rainfall – over an extended period of time. In turn, this can lead to the ground drying up, and can also reduce the volume of water in rivers/streams, lakes/dams and subterranean reservoirs.

Potential consequences of drought

The impacts of drought can be economic, environmental and social. Drought can affect farmers by shrivelling crops or drying out grass so that livestock don't have enough to eat (or drink). These losses then flow through into downstream production and other sectors, such as retail, where shortages can cause price increases.

Drought also increases the risk of fire, as well as depleting water flow in storage: this, in turn, can cause problems for the production of hydroelectric power. Drought can also mean that rural and urban populations have water restrictions imposed on them.

Drought is one of the major causes of malnutrition and famine in many parts of the world.

Recent examples of drought around New Zealand

  • 2010 – The worst drought in Northland in 60 years occurred when record low rainfall levels were recorded between Nov 2009 and April 2010. Instead of the 748mm which fell during the previous year, only 253 mm fell, leading to parched soils, significantly reduced pasture growth, and decreased farm productivity.
  • 2008 – Much of New Zealand encountered very dry conditions at the start of 2008, with the Waikato experiencing its driest January in a century. Severe moisture deficits continued throughout the north island until April/May, with the estimated cost to agriculture exceeding $1 billion, and an 11% fall in sheep numbers.
  • 2007 – Low rainfall and significant soil moisture deficits persisted throughout summer and autumn with record low rainfall totals occurring in many northern and eastern areas. This resulted in a severe shortage of feed for livestock, and lower than normal spring lambing and beef numbers, costing more than $500 million.

Drought research at NIWA

NIWA updates its soil moisture maps, which cover all of New Zealand, daily. In addition, NIWA monitors and archives rainfall and river flow data for all of the country.

This information is then provided to end-users such as Regional Councils and the MPI-led National Adverse Events Committee, to help them make informed decisions about drought planning, response and recovery.

Current NIWA research projects

  • Risk of drought under climate change – Of all of the threats posed to New Zealand by climate change, drought is the one which could have the largest effect on New Zealand's economy. This project has significantly advanced previous estimates of how drought severity and frequency are likely to change as global warming effects New Zealand.
  • Continuous monitoring of soil moisture levels throughout New Zealand – These measurements are used, together with a number of other variables, by farmers when assessing the status of the soil moisture in their region compared with other parts of the country and by authorities when deciding whether dry periods are classed as drought.

NEWA Communications.

Pigeon Post News, Richmond.

No comments:

Post a Comment

Looming changes forecast for Tasman District Council's 10 Year Plan

  Changes looming for Tasman District Council's 10 Year Plan   5 May, 2024 Tasman District Council’s Chief Financial Officer Mike Drum...