Hotspot Watch 11 December 2020
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
Once again, significant rainfall occurred in the western North Island during the past week, with widespread amounts of greater than 50 mm from southern Waikato to Kapiti Coast. In fact, some locations in this region received more than 100 mm. While Opotiki District also received more than 30 mm, the remainder of the North Island generally saw lighter rainfall totals. The upper North Island generally received less than 25 mm, with much of the East Coast receiving less than 10 mm. This resulted in small to moderate soil moisture decreases across much of the North Island, although small to moderate increases were observed from southern Waikato to Kapiti Coast. The driest soils across the North Island, when compared to normal for this time of the year, are now found in East Cape and much of Northland. Meanwhile, the wettest soils for this time of the year are located in coastal Manawatu-Whanganui.
Hotspots are now in place in East Cape, Hauraki District, Aupouri Peninsula, and near the Northland-Auckland border.
In the South Island, widespread rainfall amounts of 75 mm or more were observed in much of Tasman and the West Coast, with several areas receiving more than 100 mm. While much of Otago and Southland received 20-30 mm, Marlborough and Canterbury generally saw meagre rainfall amounts of less than 10 mm. This resulted in soil moisture decreases in much of the South Island, although some increases were observed in the West Coast. The driest soils in the South Island compared to normal for this time of the year are now located in central Westland, while the wettest soils for this time of the year are found in Nelson and nearby parts of Tasman.
In the past week, small hotspots have emerged in coastal Hurunui District, Banks Peninsula, and interior southern Canterbury.
Outlook and Soil Moisture
A weak front will bring a few scattered showers to the western and southern North Island on Saturday, but rainfall amounts are likely to be less than 5 mm. Similar rainfall amounts are expected from a few showers occurring in Hawke’s Bay, Gisborne, and Bay of Plenty from Sunday afternoon to early Monday morning. Otherwise, a large area of high pressure is expected to be in control of the North Island’s weather over the next week or more. This will result in extremely meagre rainfall for most if not all of the North Island during the next week, with nearly all locations receiving less than 10 mm, and some locations seeing no rainfall at all.
Due to the extremely meagre rainfall expected during the next week, at least moderate soil moisture decreases are expected for nearly all North Island locations. This will result in strengthening and expansion of all current hotspots in the upper North Island and East Cape.
Showers will continue to affect the West Coast and lower South Island from Friday afternoon into early Saturday morning (12 December), with rainfall totals reaching 25 mm in some areas. Thereafter, high pressure will overspread the South Island by Sunday and remain in place for several days. A front may reach the lower South Island by Thursday (17 December), but rainfall amounts are not expected to be significant at this time. Weekly rainfall totals may reach 30 mm in much of the West Coast and lower South Island, but the remainder of the South Island will likely see minimal amounts of 10 mm or less.
Expect additional soil moisture decreases across nearly all of the South Island during the next week, with substantial decreases possible in eastern areas. This will likely result in the strengthening and expansion of all current 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.
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 9 December, the New Zealand Drought Index (NZDI) map below shows that drier than normal soils are located in upper Northland and parts of coastal Canterbury and Otago, but meteorological drought is not currently found in New Zealand. 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.
