NIWA's Hotspot Watch 30 October 2015
A weekly update describing soil moisture across the country to help assess whether severely to extremely dry conditions are occurring or imminent.
Regions experiencing these soil moisture deficits are deemed “hotspots”. Persistent hotspot regions have the potential to develop into drought.
Soil Moisture
For the North Island, soil moisture levels have either increased or not changed significantly when compared to this time last week. Slightly below normal to below normal soil moisture levels for this time of year are now present for most of the island. However, one exception exists from southern Hawke’s Bay through south-central Manawatu-Wanganui, where soil moisture is near normal or above normal for this time of year. The driest soil in the North Island exists in the far lower part of the island, roughly south of a line from the Kapiti Coast to Carterton, as well as the far eastern Waikato to Bay of Plenty and northern Gisborne regions where soil moisture levels are much drier to severely drier than normal for this time of year.
NB: due to the time required to produce soil moisture maps, the effects of rainfall, some significant, over the past 24 hours over the central and northern North Island have not yet been reflected in the most recent soil moisture map.
In the South Island, soil moisture levels have not changed significantly for much of the island when compared to this time last week. Much of the island along and east of the Divide, north of a line from Dunedin to the Queenstown Lakes district, has significantly to severely drier than normal soils for this time of year. This is also true for northern areas of the island from the northeast Tasman region through to the northeast Marlborough region. Most other areas of the island have near normal soil moisture levels for this time of year, except from eastern Southland to southeast Otago, (roughly from Invercargill to Dunedin) where wetter than normal soils for this time of year are present.
Outlook
For the North Island, rainfall through early Tuesday is likely to be spotty and light, at most, for nearly the whole of the island as high pressure builds east and over the country. A cold front and associated area(s) of low pressure will move northeast and likely bring widespread rain to the island later Tuesday into Wednesday. Early indications are that 5-15 mm may fall over a sizeable part of the island, with locally higher amounts possible over central and western parts of the island, as well as about the Gisborne ranges. Thereafter, there is uncertainty as to what will unfold. Computer models are at odds regarding the potential for a low to travel just north of Northland and bring the threat for rain for far northern areas of the island or if high pressure will arrive from the Tasman and bring dry conditions.
With that in mind, it’s likely that soil moisture levels will remain nearly steady or decrease over much of the island by this time next week.
For the South Island, rainfall through early Monday is likely to be spotty and light, at most, for nearly the whole of the island as high pressure builds east. A cold front and associated area(s) of low pressure will move northeast and likely bring widespread rain to the island later Monday into Tuesday. Widespread rainfall will mean amounts of 25-50 mm along and west of the Divide, from top to bottom, with locally higher amounts. East of the Divide, as is usually the case, rainfall will be significantly less with most areas (except the eastern foothills) likely receiving 5-15 mm with the least amount of rain likely between Christchurch and Dunedin where amounts may be even less. Thereafter, high pressure and generally dry weather should end the work week for the island.
As a result, soil moisture levels west of the Divide will likely remain steady or increase, while the rest of the island sees moisture levels hold steady or decrease by this time next week. Eastern areas of Canterbury and northeast areas of Otago may possibly see enhanced soil moisture loss due to the drying effects of expected strong foehn winds ahead of the front on Monday and early Tuesday.
For the North Island, two hotspots exist: one in the far southern part of the island, roughly south of a line from the Kapiti Coast to Carterton, and another from eastern Waikato to the Bay of Plenty. For the South Island, a hotspots persist from far northeastern Otago through to nearly the whole of eastern Canterbury, as well as from northeast Marlborough through to eastern Tasman regions.
NB: Hotspot conditions are not perceived for the Auckland, Northland and Gisborne regions due to recent rainfall that has not yet been accounted in the soil moisture maps.
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
Soil moisture deficit is 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
Soil moisture anomaly is the difference between the historical normal soil moisture deficit (or surplus) for a given time of year and actual soil moisture deficits.
Dry and extremely dry soil
The definitions of “extremely” and “severely” dry soils are based on a combination of the current soil moisture status and the difference from normal soil moisture.
Check out NIWA's soil moisture maps
Soil moisture anomaly maps
Pictured below soil moisture anomaly maps, relative to this time of year. The maps compare last week’s soil moisture anomaly with today’s.
