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 of generally less than 5 mm during the past week has led to notable soil moisture decreases for nearly all locations and expansion of existing hotspot coverage. The largest decreases were observed in the Far North as well as central areas from Taranaki eastward to Hawke’s Bay. The driest soils across the North Island, when compared to normal for this time of the year, are found in the Far North and northern Waikato, while the wettest soils for this time of the year are located in western Bay of Plenty and Kapiti Coast.
A very large hotspot now covers nearly all of Northland, Auckland, northern Waikato, and the Coromandel Peninsula. Additional hotspots are in place across most of Wairarapa, East Cape, and central Manawatu-Whanganui.
Across the South Island this past week, minor to moderate drying of soils occurred in northern areas, while the lower South Island observed minor soil moisture increases. The driest soils across the South Island compared to normal for this time of the year are located in coastal Hurunui District, while the wettest soils for this time of the year include Stewart Island and a region extending from Dunedin to Invercargill.
The only official hotspot currently located in the South Island is in coastal Hurunui District, although much of northern and central Canterbury is nearing hotspot status.
Outlook and soil moisture
In the North Island, consistent high pressure will result in another week with minimal rainfall for nearly all locations. Scattered showers developing over Bay of Plenty and Gisborne on Thursday (9 January) may produce a few millimetres of rainfall in these regions. After several dry days, a decaying front may deliver 5 mm or less to southern and western areas on Monday (13 January). The majority of the North Island is likely to receive 10 mm or less of rain in the next week, with significantly less rain (<5mm) expected in the upper North Island and Wairarapa.
With minimal rainfall expected in the North Island during the next week, widespread soil moisture decreases are expected to continue. Existing hotspots are anticipated to strengthen and expand, and new hotspots will most likely emerge across Hawke’s Bay and Gisborne.
In the South Island, rainfall is expected for the West Coast this weekend (11-12 January). Rainfall amounts of 50-75 mm will be possible, while less than 20 mm is expected in Southland. However, little if any of this rain is expected to reach the eastern South Island. High pressure will then bring dry weather during early-to-mid next week. Although significant rainfall is expected for the West Coast, weekly rainfall in eastern areas is likely to be less than 10 mm.
Minimal rainfall amounts are expected in the eastern South Island during the next week, which means new hotspots may emerge across much of northern and central Canterbury, and perhaps eastern Marlborough as well.
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.
New Zealand Drought Index (NZDI)
As of 5 January, the New Zealand Drought Index (NZDI) map below shows that the upper North Island now has widespread very dry to extremely dry soils, and some locations could approach meteorological drought conditions with additional dry weather in the coming week. Widespread dry to very dry conditions also exist in the Wairarapa.
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.
