Benefit of January's heavier rains short-lived

January's heavy rainfalls and thunderstorms may have meant some of the region exceeded its average rainfall, but it was a short-lived benefit for most, hydrologist Dave Stewart says.

"However, Central Otago people were pleased to see the rain as conditions had become extremely dry and the rainfalls in January were the first significant falls in that region for many months," he said.

While the month started off unseasonably cool and wet, it became fine by January 15-19 before the thunderstorms rolled in on January 20-21.

"Unfortunately, such falls are often not very beneficial to the areas they affect, even if those areas are dry because the rain is usually so intense and short-lived that most of it runs off and does not get a chance to soak into the soils."

Rivers showed very sharp rises and usually fell just as quickly, he said.

The last 10 days of the month saw east-northeasterly quarter winds prevailing and while conditions along the east coast were often cloudy and cool, conditions were very hot inland.

Farmers reported more rain was needed urgently to ease the dry conditions that had developed since the rain, which was preventing grass growth, Mr Stewart said.

In North Otago, areas upstream of about Kurow received more rain than areas downstream and other more eastern parts of this region.

Rainfall totals were above average in the northwest area of North Otago but well below average elsewhere.

Average monthly river flows were well below their long-term averages for January.

Irrigation restrictions were in place on the main North Otago rivers by the end of January.

Total rainfalls at the indicator sites (average monthly totals in brackets) were: Tara Hills 63mm (47mm) 36% above average; Waikoura 20mm (52mm) 62% below average; Oamaru Airport 18mm (50mm) 68% below average; Oamaru Town 27mm (47mm) 42% below average; Kauru the Dasher 34mm (83mm) 52% below average; Palmerston 28mm (60mm) 53% below average.

Significant falls included: January 1-3, Tara Hills 17mm; January 9-10, Tara Hills 16mm, Oamaru Town 18mm, Islay Downs 22mm, Palmerston 15mm.

Central Otago received its first significant rainfall in months during January with two good falls.

As a result, rainfall totals were mostly above average except for the Maniototo and Roxburgh.

Average monthly river flows were well below their long-term averages for January although they did improve for a time as a result of the two good rainfalls during the month, he said.