North Otago aquifer hits 10-year peak

Groundwater levels in the North Otago volcanic aquifer have hit a 10-year peak, because of recent high rainfall.

Before the recent flood, underground water levels in the Deborah area had been dropping under the influence of the autumn dry spell, causing concern to many in the district.

They were centimetres away from breaching the 25 percent restriction level that triggers irrigation restrictions, Otago Regional Council (ORC) director of environmental information and science John Threlfall said.

Dr Threlfall said the rapid changes began late on the evening of May 24 when the Kakanui River spiked upwards under the torrential rain of the easterly airstream. Within four hours the groundwater level had responded to the recharge, such was the intensity of the rainfall.

The groundwater rose sharply under the two bouts of heavy rain, and finally peaked last Wednesday as the drainage of overlying soils reduced and the recharge intensity eased.

The 1.7 m rise was the sharpest and largest in the previous 10 years since August 2000, which is still the largest single recharge event recorded in a quarter-century at 2.9 m.

However, that level was eclipsed this morning when the third heavy burst of rain in the area since May 22 pushed groundwater levels up by a further 300mm, taking the peak to 131m, a total increase of two metres.

Dr Threlfall said this had the positive effect of significantly replenishing the aquifer and forestalling the possibility of irrigation restrictions for several months.

The groundwater level rose in a staircase fashion with each step punctuated by a new burst of rain.

The total rainfall at The Dasher rain gauge from May 22 until today was 423 mm, about half of which soaked through the soil into the aquifer below, Dr Threlfall said.

The rest of the rain would have run off and contributed to flooding, he said.

Further information about historical groundwater trends in North Otago is available at www.orc.govt.nz