Hotspot Watch for 8 December 2016

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

A lack of decent rainfall over the past week in the North Island has meant that most places saw a decrease in soil moisture levels. The exceptions were inland parts of the Bay of Plenty, Gisborne and northern Hawke’s Bay where soil moisture levels slightly increased. The soils in this area remain slighter wetter than normal for the time of year. The driest soils across the North Island compared to normal for this time of the year are found along the coast in the Central Hawke’s Bay and Tararua districts, East Cape, as well as northern Waikato and Northland. In the North Island, hotspots continue to stay in place in coastal areas of Central Hawke’s Bay and Tararua districts, East Cape, and in the Mahia Peninsula.

Soil moisture levels decreased right across the South Island over the past week.  Northern Canterbury and eastern Marlborough were the only locations to receive more than 10mm of rain in the last 7 days and despite this, high evaporation rates still saw soil moisture levels drop in this area. Soil moisture levels continue to be slightly wetter than normal in coastal Otago, southern coastal Canterbury and about Invercargill. The driest soils in the South Island compared to normal for this time of the year are still found in northern Canterbury.

Outlook and soil moisture

For the North Island, after widespread showers on Thursday, dry weather briefly returns on Friday. An active front is set to pass over the North Island on Saturday bringing widespread rain, particularly for Waikato, Bay of Plenty and northward. While showers are set to continue for western areas throughout Sunday and Monday, little rainfall is expected in the North Island during the middle of next week. Overall 15-50mm of rain is expected to fall across the North Island over the next week with Hawke’s Bay and the Wairarapa expected to stay drier with 10mm or less expected.

Soil moisture levels across the North Island are generally expected to remain unchanged or slightly decrease. The exception is in Hawke’s Bay where soil moisture levels are set to decrease. This will likely lead to an expansion of the hotspot located from southern Hawke’s Bay to the northern Wairarapa.

For the South Island, a dry day on Friday will be followed by wet weather on Saturday, particularly for western and northern locations where up to 30mm of rain is expected. Showers are set to continue across the South Island during Sunday and Monday with drier weather returning on Tuesday and Wednesday.

With generally low rainfall amounts compared to normal expected in the western South Island, soil moisture levels in this area are expected to decrease. The hotspot in coastal Hurunui district will likely stay about the same.

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).