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Less Water In Rivers, Groundwater Levels Drop

Cool Dry Weather And Easterlies: Less Water In Rivers, Groundwater Levels Drop

The sustained cool but dry weather in November in association with the onset of irrigation after a wetter October has meant that groundwater levels have dropped in the region, say Environment Canterbury water scientists.

Since May 2009, rainfall throughout much of Canterbury has been at or close to a five to ten-year low, driven by the dominating easterly winds characteristic of the intensifying La Niña weather pattern.

“Groundwater levels are generally lower than at the same time last year and are continuing their steady decline since the wet part of the winter in May,” says Environment Canterbury groundwater scientist Dr Howard Williams.

River and stream flows have also been adversely affected by the low rainfall as have soil moisture levels which are considerably lower than mean values in some areas, he said.

“Mid December, only a few catchments within the Canterbury region are experiencing partial restrictions on some of their tributaries,” he says. So far, only one part of the West Melton area near Christchurch is under restriction.

Rainfall
November rainfall was slightly below average to well below average across the region. December rainfall throughout the region so far in December is ranging from half of long-term average to around average. Some areas of inland Canterbury, such as Waimate and Tekapo have received much less rainfall than average.

General state of river and streams
November river flows in North Canterbury were around average and in South Canterbury they were below average This trend continued in December with above average to average flows in the north and average to below average in the south. A small proportion of rivers throughout Canterbury have a level of restriction on them.

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General state of groundwater
In general, most groundwater levels are in the average range for the time of year.

Regional detail
North Canterbury
Kaikoura groundwater levels are within the average range. In Waipara, most wells are within or below the average range. Wells in the Ashley – Waimakariri groundwater zone display generally average levels of groundwater. In the Ashley-Waimakariri area these average groundwater levels are in part buoyed up by water discharged to ground associated with the Waimakariri Irrigation Scheme.

Mid-Canterbury
Groundwater levels within the Christchurch – West Melton area are slightly lower than average for this time of year. Only one of the Christchurch-West Melton monitoring wells indicates groundwater levels close to the 1st trigger level for restrictions; currently there are no restrictions there. Both the Selwyn-Waimakariri and the Rakaia-Selwyn groundwater zones are displaying levels generally below those of last year, but still within the range of average values. In the Ashburton-Rakaia zone groundwater measurements are generally below average levels and some are at very low levels.

South Canterbury
In the Mayfield-Hinds groundwater zone, most groundwater readings are below average levels. South of the Rangitata River, between the Rangitata River and Coopers Creek, shallow groundwater is also at low levels, as is groundwater in shallow wells close to the Opihi and Orari rivers. In the deep aquifer at Orton, groundwater levels are low. In the Pareora-Waihao zones, shallow groundwater associated with the Pareora and Otaio rivers are also lower than average. Wells in the Waihao River area are displaying lower than average water levels.

Forecast until February 2010
According to the NIWA seasonal outlook website at: http://www.niwa.co.nz/our-science/climate/publications/all/seasonal-climate-outlook/sco-dec-feb-2010,
temperatures are likely to be near or below average, seasonal rainfall is expected to be in the normal range and soil moisture levels and stream flows are most likely to be below normal.

Monitoring data
The most recent monitoring data are available at: http://ecan.govt.nz/services/online-services/monitoring/groundwater-levels/Pages/Default.aspx

Irrigation restrictions
For an updated list of irrigation restrictions go to http://ecan.govt.nz/services/online-services/monitoring/irrigation-restrictions/pages/default.aspx

EDNS

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