NIWA's Hotspot Watch 1 April 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
Across the North Island, soil moisture levels have generally remained the same or increased when compared to this time last week. The most notable increases were observed across central and western Waikato as well as across Bay of Plenty and northern Hawke’s Bay. Increases also occurred in parts of Northland and far western Taranaki. The main exception was across Wellington-Wairarapa, where soil moisture levels decreased slightly. The wettest soils compared to normal for this time of the year are found along the Coromandel Peninsula, northwestern Bay of Plenty, and in northern Northland while the driest are found in Masterton, Lower and Upper Hutt, and in far northeastern Gisborne.
Across the South Island, soil moisture changes have been variable over the last week with increases in the north and decreases in the south. The most significant increases occurred in Tasman, Nelson, and Marlborough whereas substantial decreases happened in eastern Southland and eastern Otago. The driest soils compared to normal for this time of the year are found in eastern Southland to Otago and in central and northern Canterbury while the wettest are in Nelson.
Outlook
Low pressure passing over the country from Friday into Saturday will bring a soaking rainfall to much of the North Island. The heaviest falls are expected in Northland, the eastern Bay of Plenty, Gisborne and Taranaki with localized amounts in excess of 50 mm; elsewhere, 10-25 mm are expected through Saturday. Amounts may be lower across parts of Auckland where downpours will be hit and miss. Rain will subside to showers on Sunday, with daily amounts generally less than 10 mm across the western and northern parts of the island. Isolated showers will accompany a southerly change from Monday into Tuesday. Amounts will generally be 5-10 mm from Wellington to Gisborne and Northland (though locally higher amounts are possible) and less than 5 mm across the remainder of the North Island. High pressure is then forecast to build over the island on Wednesday and Thursday before some rain may approach from the south and west on Friday.
Across the South Island, locally heavy rainfall will move across the north from Friday into Saturday; some of the highest amounts may fall in the regions that saw flooding this time last week. Rainfall through Saturday may exceed 50 mm across parts of the northern West Coast and Tasman with a general 10-25 mm for the rest of the northern and central South Island. Isolated showers on Saturday across the far southwest will amount to less than 10 mm. A fresh push of southerlies on Sunday will be accompanied by rain across Southland and the West Coast. Falls will be on the order of 10-25 mm for western Southland and the West Coast but generally less than 10 mm for the dry areas of eastern Southland and Otago. Isolated showers will move northward on Sunday, affecting the central and northern West Coast across to Canterbury; amounts will mostly be between 5 and 10 mm. Showers will dot the central and northern part of the island, as well as the far south, on Monday before high pressure causes a mostly dry day on Tuesday. Rain, some heavy, is forecast to return to the far southwest on Wednesday while the rest of the island is dry. Rain will then extend up the West Coast on Thursday with lighter falls across Southland and Otago. Patchy rainfall may expand into Canterbury and the northern portion of the island to end the week.
Soil Moisture
The North Island has great variance in soil moisture for this time of the year, with areas of much wetter and drier than normal soil. Hotspot areas remain present across southern and western Hawke’s Bay, much of Wellington-Wairarapa, interior Manawatu-Whanganui, northern and southeastern Waikato, far southwestern Northland, western Auckland, northeastern Bay of Plenty, and northeast coastal Gisborne. Over the past week, the hotspot areas in Taranaki and central Waikato have been eliminated. A reduction in hotspot coverage is expected this weekend into early next week before stabilizing in the mid-week. Despite this, hotspot elimination is not anticipated for the driest soils in Wellington-Wairarapa.
On the South Island, hotspot areas remain present in northern and central Canterbury and central Southland. Hotspot activity has expanded to include parts of eastern Otago over the last week but has been eliminated in far northern Marlborough. Over the next week, hotspot activity is forecast to remain the same or worsen slightly across the far south and along the east coast. New hotspot development is not expected across the remainder of the South Island.
NB: due to the timing and reporting of daily 24 hour rainfall amounts (9 am each day) and time required to compute soil moisture deficit values, the rain that occurred on Friday 1 April will not be fully reflected in the soil moisture anomaly maps until Saturday 2 April.
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).
Soil moisture anomaly maps
