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Soils holding extra moisture

Wairarapa soils have been significantly more moist than normal this year, due in large part to two cyclones.

All 11 National Institute for Water and Atmospheric Research [Niwa] hotspot watch reports this year have reported the region as having between 20mm and 60mm more rain than average.

In the most recent and final hotspot watch for the season, Niwa said the wettest soils are currently found in Hawke’s Bay, coastal Manawatu-Whanganui, and Wairarapa.

The driest Wairarapa got during the summer season was around December 16, when coastal soils were between 20mm and 30mm drier than normal.

At that point, Wairarapa’s coastal soils were the driest in the North Island for the time of year – the only time a hotspot was declared for Wairarapa this summer.

But by January 12, the hotspot had entirely dissipated, and Wairarapa had some of the wettest soils in the North Island.

A hotspot is declared by Niwa if soils are “severely drier than normal”, which happens when the soil moisture deficit is less than -110mm, and the soil moisture anomaly is less than -20mm.

Soil moisture deficit is defined as 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.

A soil moisture anomaly is a difference between the historical normal soil moisture deficit or surplus for a given time of year and actual soil moisture deficits.

Cyclone Hale made landfall on January 11 and left Riversdale farms with metre-deep river sediment covering entire paddocks, and kilometres of fence lines ruined.

The January 12 hotspot watch said while the Coromandel Peninsula, Gisborne, and the east coast experienced rainfall of more than 200mm and flooding was observed in some areas, almost all of the remainder of the North Island saw substantial rainfall – generally 50mm or more.

The February 16 hotspot watch said Cyclone Gabrielle brought severe to catastrophic flooding to parts of the upper and eastern North Island, and all of Wairarapa had a soil moisture anomaly of more than 60mm.

Another front could bring additional showers by tomorrow, Niwa said.

“Weekly rainfall totals of 15-30mm are likely for much of the island, but 15mm or less may occur in the upper North Island.

“The expected rainfall could cause soil moisture levels to decrease at least a small amount along the east coast, with generally little change expected elsewhere.”

Grace Prior
Grace Prior
Grace Prior is a senior reporter at the Wairarapa Times-Age with a keen interest in environmental issues. Grace is the paper’s health reporter and regularly covers the rural sector, weather, Greater Wellington Regional Council, and coastal stories.

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