Heavy rain localised, wider falls needed

Dave Stewart.
Dave Stewart.
Dunedin might have had an intense thunderstorm but more rain across a greater area is needed if Otago's rivers are to get back to normal, water scientists says.

Monday's thunderstorm was so heavy most of it would have run off, providing little benefit, hydrologist Dave Stewart said. About 18mm fell in the hour between 4pm and 5pm.

''Fifteen millimetres in 20 minutes - there is no way the ground can absorb that. It can be damaging rain,'' Mr Stewart said.

However, steady rain continued through the evening and 31.2mm was recorded in the central city on Monday, 23.4mm at Musselburgh and 10.2mm at Dunedin airport.

Niwa figures showed the rainfall was not record-breaking and the last time Dunedin recorded a similar rainfall amount in one day at Musselburgh was in June, when 31.6mm fell.

As of yesterday, Musselburgh had recorded 36mm this month, 47% of the December average rainfall of 80mm.

Oamaru also experienced heavy rain, with 22mm falling between 7pm and midnight, 11.8mm of that between 7pm and 8pm.

Mr Stewart said the steady rain the city experienced after the 4.30pm storm was good for the dry grounds and gardens.

''Most of that will have soaked into the ground, giving a boost to the grass.''

The rain was quite localised, hitting Dunedin, as far inland as the Manuherikia Valley and into North Otago.

While Ranfurly received 22mm, Middlemarch only got 2.6mm.

''Any more than 5mm is of some use and no wind is a bonus but lots more is needed,'' Mr Stewart said.

Another two or three days' steady rain was needed to bring conditions back to normal.

Otago Regional Council resource science manager Matt Hickey said the rain alleviated low flow concerns in the North Otago catchment, but left much of the Central Otago district still in need of 20mm-30mm.

''The Kakanui, Shag and Waikouaiti all came up quite well.''

The Pomahaka, which was also low, received a little rain but it failed to raise its level and the upper Taieri was also very dry.

The MetService was forecasting unsettled weather for the rest of the week, including thunderstorms and rain today.

rebecca.fox@odt.co.nz

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