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.
Facts: Soil Moisture
There was little rain, but plenty of opportunity for evaporation in the North Island last week due to high pressure firmly in place over the Tasman Sea. As a result soil moisture levels across the entire island decreased when compared to this time last week. Slightly below normal soil moisture levels for this time of year are now present for the majority of the island with the only exceptions being western Waikato, Gisborne and Hawke’s Bay where soil moisture is near normal. The driest soil in the North Island exists in South Wairarapa and Lower Hutt where soil moisture levels have continued to drop over the past few weeks.
In the South Island, soil moisture remained near normal along the West Coast and western Southland as well as in the Kaikoura and Christchurch regions. Elsewhere drier than normal soils for this time of year became even drier. In particular, parts of Marlborough, northeastern Otago as well as northern and southern sections of eastern Canterbury are now on the cusp of severely drier than normal soil moisture conditions. Additionally, drier than normal soils for this time of year have continued to develop in the Nelson and Tasman regions.
Outlook
For the North Island, rain forecast over the upcoming week is set to be largely contained to lower western regions of the island - from Waitomo to Porirua. As a result, soil moisture levels in these areas are likely to remain the same or show slight improvement. Conversely, eastern parts of the island will be sheltered from significant rainfall and it is expected that soil moisture levels along the east coast from Northland to Gisborne will become drier by this time next week.
For the South Island, a series of active fronts are forecast over the week ahead. Rain associated with these fronts will be concentrated west of the Divide and soil moisture levels there should remain in the near normal range. Depending on the amount of rain spill-over to areas east of the divide, some hot spot areas including central Otago and southern Canterbury as well as Tasman and Nelson may see some alleviation to low soil moisture levels. Although some rain is also forecast for the other hotspot region of northern Canterbury, the amount of rain required to improve soil moisture levels in the area is looking unlikely.
Overall, a hotspot persists and grows in north and northeast Otago through to southern sections of east Canterbury. Another hotspot is located in north-central parts of eastern Canterbury, roughly north of Christchurch and south of the Waiau River which has intensified over the past week. Hotspot development is also being monitored in South Wairarapa and Lower Hutt.
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 at https://www.niwa.co.nz/climate/nz-drought-monitor/droughtindicatormaps)
Pictured below: Soil Moisture Anomaly Maps, relative to this time of year. On the left are values this time last week. On the right are the most recent values.
